Title: LIFE SCIENCE TEXT: Cell Division, Reproduction, DNA
Course: Biology (A)

Lesson 5.1: Cell Division

Lesson Objectives

- Explain why cells need to divide.
- List the stages of the cell cycle and explain what happens at each stage.
- List the stages of mitosis and explain what happens at each stage.

Check Your Understanding

- What is the cell theory?
- In what part of your cells is the genetic information located?

Introduction

Imagine the first stages of a life. In humans, a sperm fertilizes an egg, forming the first cell. From that one cell, an entire baby with trillions of cells will develop. How does a new life go from one cell to so many? The cell divides in half, creating two cells. Then those two cells divide. The new cells continue to divide and divide. One cell becomes two, then four, then eight, and so on (Figure below ). Rapid cell division allows the development of new life, but cell division must be tightly regulated. If the body’s close regulation of cell division is disrupted later in life, diseases such as cancer can develop. Cancer involves cells that divide in an uncontrolled manner. Therefore, much research into cell division is underway across the globe in effort to further understand this process and find a cure for cancer.

Cells divide repeatedly to produce an embryo. Previously the one-celled zygote divided to make two cells (a). Each of the two cells divides to yield four cells (b), then the four cells divide to make eight cells(c), and so on. Through cell division, an entire embryo forms from one initial cell.

Why Cells Divide

Besides the development of a fetus, there are many other reasons that cell division is necessary to life. To grow and develop, you must form new cells. Imagine how often your cells must divide during a growth spurt. Growing just an inch requires countless cell divisions.

Another reason for cell division is to repair damaged cells. Imagine you cut your finger. After the scab forms, it will eventually disappear and new skin cells will grow to repair the wound. Where do these cells come from? Remember that according to the cell theory, all cells must come from preexisting cells. In order to make new skin cells, some of your existing skin cells had to undergo cell division.

Besides suffering physical damage, your cells can simply wear out. Over time you must replace old and worn-out cells. Again, cell division is essential to this process. You can only make new cells by dividing similar preexisting cells.

The Cell Cycle

The process of cell division in eukaryotic cells is carefully regulated. The cell cycle which in essence is the lifecycle of a cell, is composed of a series of steps that lead to cell division (Figure below ). These steps can be divided into two main components: interphase and mitosis. Interphase is when the cell mainly performs its “everyday” functions; for example, it is when a kidney cell does what a kidney cell is supposed to do. On the other hand, mitosis is when the cell prepares to become two cells. Some cells, like nerve cells, do not complete the cell cycle and divide, while others divide repeatedly.

Most of the cell cycle consists of interphase, the time between cell divisions. During this time the cell carries out its normal functions and prepares for the next stage. Interphase can be divided into three stages: the first growth phase (G1), the synthesis phase (S), and the second growth phase (G2). During the G1 stage, the cell doubles in size and doubles the number of organelles. Next, during the S stage, the DNA is replicated. In other words, an identical copy of all the cell’s DNA is made. This ensures that each new cell that results after cell division has a set of genetic material identical to that of the parental cell. DNA replication will be further discussed in lesson 3. Finally, in the G2 stage proteins are synthesized that will aid in cell division. In the end of interphase, the cell is ready to enter the mitotic phase.

The cell cycle is the repeated process of growth and division. Notice that most of the cell cycle is spent in interphase (G1, S, and G2) (I).

During the mitotic phase, nuclear division occurs, which is known as mitosis. Also cytokinesis, the division of the cytoplasm, occurs. After cytokinesis, cell division is complete and two genetically identical daughter cells have been produced from one parent cell. The term "genetically identical" refers to the fact that each resulting cell has an identical set of DNA, and this DNA is also identical to that of the parent cell.

Mitosis and Chromosomes

During cell division, two nuclei must form during the process of mitosis, so that one nucleus can be given to each of cells that form from cytokinesis. In the nucleus, the genetic information of the cell, DNA, is stored. The copied DNA needs to be moved into a new nucleus for the new cell to have a correct set of genetic instructions.

The DNA in the nucleus is condensed into chromosomes, structures composed of DNA wrapped around proteins. Each organism has a unique number of chromosomes; in human cells our DNA is divided up into 23 pairs of chromosomes. When a cell is not undergoing division, such as during interphase, the complex of DNA and proteins is a tangled mass of threads known as chromatin. As mitosis begins, however, the DNA becomes tightly coiled into the chromosomes which become visible under a microscope.

The DNA double helix wraps around histone proteins (2) and tightly coils a number of times to form a condensed chromosome (5). The chromosomes contains millions of nucleotide bases. This figure illustrates the complexity of the coiling process. The red dot shows the location of the centromere, where the microtubules attach during mitosis and meiosis.

As mentioned previously, the DNA is replicated during the S stage of interphase. Each chromosome now has two identical molecules of DNA, called sister chromatids, forming the "X" shaped molecule depicted in Figure above. During mitosis, the two sister chromatids must be split apart to give rise to two identical chromosomes (in essence, each resulting chromosome is made of 1/2 of the "X"). Through this process, each daughter cell receives one copy of each chromosome.

Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase. During prophase, the chromosomes become tightly wound and become visible under the microscope. Also, the nuclear envelop dissolves, and the spindle begins to form. The spindle is a structure containing many fibers that helps to move the chromosomes. By late prophase, the chromosomes are attached to the spindle fibers. The spindle fibers will later pull the chromosomes into alignment.

During metaphase, the chromosomes line up across the center of the cell. The chromosomes line up in a row, one on top of the next. During anaphase, the two sister chromatids of each chromosome separate, resulting in two sets of identical chromosomes. During telophase, the spindle dissolves and nuclear envelopes form around the chromosomes. The drawings of Figure below show this process. This is further shown in Figure below. Each new nucleus contains the exact same number and types of chromosomes as the original cell. The cell is now ready for cytokinesis, producing two genetically identical cells, each with its own nucleus.

An overview of mitosis: during prophase (I and II) the chromosomes condense, during metaphase the chromosomes line up (III and IV), during anaphase the sister chromatids are pulled to opposite sides of the cell (V and VI), during telophase the nuclear envelope forms ( VII and VIII).

This is a picture of dividing plant cells. Cell division in plant cells differs slightly from animal cells as a cell wall must form. Note that most of the cells are in interphase. Can you find examples of the different stages of mitosis?

Lesson Summary

- Cells divide for growth, development, reproduction and replacement of injured or worn-out cells.
- The cell cycle is a series of regulated steps by which a cell divides.
- During mitosis, the newly duplicated chromosomes are divided into two daughter nuclei.

Review Questions

1. In what phase of mitosis are chromosomes moving toward opposite sides of the cell?
2. In what phase of mitosis do the duplicated chromosomes condense?
3. What step of the cell cycle is the longest?
4. What is the term for the division of the cytoplasm?
5. What happens during the S stage of interphase?
6. Interphase used to be considered the “resting” stage of the cell cycle. Why is this not correct?
7. What are some reasons that cells divide?
8. During what stage of the cell cycle does the cell double in size?
9. Why must cell division be tightly regulated?
10. What is the purpose of mitosis?

Further Reading / Supplemental Links

- http://en.wikipedia.org/wiki/Mitosis
- http://www.biology.arizona.edu/Cell_bio/tutorials/cell_cycle/cells3.html
- http://biology.clc.uc.edu/courses/bio104/mitosis.htm
- http://en.wikipedia.org/wiki/Cell_cycle
- http://www.cellsalive.com/mitosis.htm
- http://www.wisc-online.com/objects/index_tj.asp?objID=AP13604

Vocabulary

anaphase
Third phase of mitosis where sister chromatids separate and move to opposite sides of the cell.

cell cycle
Sequence of steps in eukaryotic cells that leads to cell division.

chromatin
Complex of DNA and proteins that is visible when a cell is not dividing.

chromosomes
DNA wound around proteins; forms during prophase of mitosis and meiosis.

cytokinesis
Division of the cytoplasm after mitosis or meiosis.

interphase
Stage of the cell cycle when DNA is synthesized and the cell grows; composed of the first three phases of the cell cycle.

metaphase
Second phase of meiosis where the chromosomes are aligned in the center of the cell.

mitosis
Sequence of steps in which a nucleus is divided into two daughter nuclei, each with an identical set of chromosomes.

prophase
Initial phase of mitosis where chromosomes condense, the nuclear envelope dissolves and the spindle begins to form.

spindle
Fibers that move chromosomes during mitosis and meiosis.

telophase
Final phase of mitosis where a nuclear envelop forms around each of the two sets of chromosomes.

Points to Consider

- How might a cell without a nucleus divide?
- How are new cells made that incorporate the DNA of two parents?

Lesson 5.2: Reproduction

Lesson Objectives

- Name the types of asexual reproduction.
- Explain the advantage of sexual reproduction.
- List the stages of meiosis and explain what happens in each stage.

Check Your Understanding

- Can something that does not reproduce still be considered living?
- What stores the genetic information that is passed on to offspring?
- How many chromosomes are in the human nucleus?

Introduction

Can an organism be considered alive if it cannot make the next generation? For a species to survive, reproduction, the ability to make the next generation, is absolutely necessary. For a species to be successful, it not only needs to be well adapted to its environment, but also needs to be successful at reproduction. Reproduction allows a population of organisms to pass on their genetic information to the next generation. There are many different ways that organisms reproduce, and these methods are categorized as either sexual or asexual reproduction. There are advantages and disadvantages to each method, but the result is always the same: a new life begins.

Asexual Reproduction

Some organisms can reproduce asexually, meaning that the offspring have a single parent and share the exact same genetic material as the parent. The advantage of asexual reproduction is that it can be very quick and does not require the meeting of two individuals of the opposite sexes. The disadvantage of asexual reproduction is that it does not involve genetic recombination, a process that can result in an adaptive new set of traits. For example, you might inherit one advantageous trait from your maternal grandmother, another adaptive trait from your paternal grandmother, and other adaptive traits from your paternal grandfather. You have the benefit of the many genes from two lineages combining in a new way. An organism that is born through asexual reproduction, however, only has the DNA from one parent, and it is the exact copy of that parent. Therefore, no new combinations of traits can happen.

Prokaryotic organisms, which as you might recall are single-celled, reproduce asexually. Bacteria reproduce through binary fission, where they basically divide in half (Figure below). First, their chromosome replicates and the cell enlarges. After cell division, the two new cells each have one identical chromosome. Mitosis is not necessary as there is no nucleus. Then new membranes form to separate the two cells. This simple process is beneficial to the bacteria, allowing very rapid reproduction.

Bacteria reproduce by binary fission. Shown is one bacterium reproducing and becoming two bacteria.

There are also several animals that can reproduce asexually. Flatworms can divide in two, then each half regenerates into a new flatworm identical to the original. Many types of insects, fish, and lizards (Figure below)can reproduce asexually through parthenogenesis. Parthenogenesis is a process by which an unfertilized egg cell grows into a new organism. The resulting organism has half the amount of genetic material of the parent, as the starting egg cell has half the amount of DNA compared to the parent. Parthenogenesis is common in honeybees. The fertilized eggs in a hive become workers, while the unfertilized eggs become drones.

Egg cells (and also sperm cells) are produced through a cell division mechanism in which the amount of DNA is halved. This process is called meiosis and will be discussed shortly.

This Komodo dragon was born by parthenogenesis.

Sexual Reproduction

During sexual reproduction, two parents are involved. Most animals are dioecious, meaning there is a separate male and female sex, with the male producing sperm and the female producing eggs. When a sperm and egg meet, a zygote, the first cell of a new organism, is formed (Figure below). The zygote will divide and grow into the embryo.

During sexual reproduction, a sperm fertilizes an egg.

Animals often have gonads, specialized organs that produce eggs or sperm. The male gonads are the testes, which produce the sperm, and the female gonads are the ovaries, which produce the eggs. Sperm and egg, the two sex cells, are known as gametes, and unite through a variety of methods. Fish and other aquatic animals release their gametes in the water, which is called external fertilization (Figure below). Animals that live on land, however, usually practice internal fertilization. Typically males have a penis that deposits sperm into the vagina of the female. Other anatomical features can accomplish the same goal; birds, for example, have a chamber called the cloaca that they place close to another bird’s cloaca to deposit sperm. Whatever method of fertilization is used, the net result is the same: a zygote that contains DNA from both the male and female.

This fish guards her eggs, which will be fertilized externally.

Plants also can reproduce sexually, but their reproductive organs are somewhat different than animals’ gonads. Most plants are flowering plants, meaning their reproductive parts are contained in a flower. The sperm is contained in the pollen, while the egg is contained in the ovary deep within the flower. The sperm can reach the egg through several methods. In self-pollination, the egg is fertilized by the pollen of same flower. In cross-pollination, the sperm from the pollen from one flower fertilizes the egg of another flower. Cross-pollination increases the genetic diversity of the population. Like other types of sexual reproduction, cross-pollination allows new combinations of traits. Cross-pollination can be achieved when pollen is carried by the wind to another flower, or many flowers rely on animal pollinators, like honeybees, or butterflies (Figure below) to carry the pollen from flower to flower.

Butterflies receive nectar when they deposit pollen into flowers, resulting in cross-pollination.

Fungi can also reproduce sexually, but instead of female and male sexes, they have (+) and (-) strains. When the filaments of a (+) and (-) fungi meet, the zygote is formed. As with the sexual reproduction in plants and animals, each zygote receives DNA from two parent strains.

Meiosis and Gametes

The formation of gametes, the reproductive cells such as sperm and egg, is necessary for sexual reproduction. As gametes are produced, the number of chromosomes must be reduced to half. In humans, our cells have 23 pairs of chromosomes, and each chromosome within a pair is called a homologous chromosome. For each of the 23 chromosome pairs, you received one chromosome from your father and one chromosome from your mother. The homologous chromosomes have the same genes, although there might be alternate forms of each gene, called alleles, which vary between the chromosomes. These homologous chromosomes are separated during gamete formation, therefore gametes have only 23 chromosomes, not 23 pairs. This separation of chromosomes is random. The probability or chance that a particular allele will be in a gamete is 1 in 2. The gamete may receive either the paternal allele (inherited from the father) or the maternal allele (inherited from the mother). This random separation of chromosomes (and therefore alleles) occurs for each chromosome, resulting in an widely varied combination of chromosomes in each gamete. With 23 pairs of chromosomes, this results in over 8 million different combinations of chromosomes a gamete.

Haploid vs. Diploid

A cell with two sets of chromosomes is diploid, referred to as 2n, where n is the number of sets of chromosomes. A cell with one set of chromosomes, such as a gamete, is haploid, referred to as n. So when a haploid sperm and a haploid egg combine, a diploid zygote will be formed; in essence, when a zygote is formed, half of the DNA in the diploid zygote comes from each parent. The process of cell division that reduces the chromosome number by half is called meiosis.

Meiosis

Prior to meiosis, DNA replication occurs, so each chromosome contains two sister chromatids that are identical to the original chromosome. Meiosis is divided into two nuclear divisions: meiosis I and meiosis II. Each of these nuclear divisions shares many aspects of mitosis and can be divided into the same phases: prophase, metaphase, anaphase, and telophase; however, between the two divisions, DNA replication does not occur. Through this process, one diploid cell will divide into four haploid cells.

Meiosis I

During meiosis I, the pairs of homologous chromosomes are separated from each other. During prophase I, the homologous chromosomes line up together. During this time, crossing-over can occur (Figure below), the exchange of DNA between homologous chromosomes. Crossing-over increases the new allele combinations in the gametes. Without crossing-over, the offspring would always inherit all of the many alleles on one of the homologous chromosomes. Because of crossing over, the alleles on the homologous chromosomes can be scrambled to pass on unique combinations of alleles on the chromosome. Also during prophase I, the spindle forms and the chromosomes condense as they coil up tightly. The spindle has the same function as in mitosis.

During crossing-over, segments of DNA are exchanged between sister chromatids. Notice how this can result in an allele (M) on one sister chromatid being moved onto the other sister chromatid.

During metaphase I, the homologous chromosomes line up in pairs in the middle of the cell; that is, both chromosome of a pair will line up together. The maternal chromosomes or paternal chromosomes can each attach to either side of the spindle. The assignment of which side is random, so all the maternal or paternal chromosomes do not end up in one gamete. The gamete will contain some chromosomes from the mother and some chromosomes from the father. Note this is different than during metaphase of mitosis; although chromosomes still line up during mitosis, the sister chromatids are separated, and each cell obtains both the maternal and paternal chromosome of each pair.

During anaphase I, the homologous chromosomes separate. In telophase I, the spindle dissolves, but a new nuclear envelop does not need to form. That’s because after a brief resting stage, the nucleus will divide again. No DNA replication happens between meiosis I and meiosis II as the chromosomes are already duplicated, carrying sister chromatids.

Meiosis II

During meiosis II, the sister chromosomes are separated and the gametes are generated. During prophase II, the chromosomes condense. In metaphase II the chromosomes line up one on top of the next along the equator, or middle of the cell. During anaphase II, the sister chromatids separate. After telophase and cytokinesis, each cell has divided again. Therefore, meiosis results in four cells with half the DNA of the parent cell (Figure below). In our cells, the parent cell has 46 chromosomes, whereas the cells that result from meiosis have 23 chromosomes. These cells will become gametes. (See Figure below).

An overview of meiosis.

A comparison between binary fission, mitosis, and meiosis.

Lesson Summary

- Organisms can reproduce sexually or asexually.
- The gametes in sexual reproduction must have half the DNA of the parent.
- Meiosis is the process of nuclear division to form gametes.

Review Questions

1. What is parthogenesis?
2. How can organisms reproduce asexually?
3. How would sexual reproduction in a lizard be different than a fish?
4. Are the viable eggs that birds lay need to be fertilized externally?
5. How do most plants reproduce sexually?
6. What is the purpose of meiosis?
7. What is the advantage of sexual reproduction over asexual reproduction?
8. If an organism has 12 chromosomes in its cells, how many chromosomes will be in its gametes?
9. During what phase of meiosis do homologous chromosomes separate?
10. In what phase of meiosis do homologous chromosomes pair up?

Further Reading / Supplemental Links

- http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookmeiosis.html
- http://www.biology.arizona.edu/Cell_BIO/tutorials/meiosis/page3.html
- http://www.cellsalive.com/meiosis.htm
- http://www.youtube.com/watch?v=MqaJqLL49a0&#38;amp;NR=1
- http://en.wikipedia.org/

Vocabulary

allele
An alternative form of a gene.

asexual reproduction
A form of reproduction in which a new individual is created by only one parent.

binary fission
An asexual form of reproduction where a cell splits into two daughter cells.

crossing-over
Exchange of DNA segments between homologous chromosomes; occurs during prophase I of meiosis.

cross-pollination
Sexual reproduction in plants where sperm from the pollen of one flower is received by the ovary of another flower.

diploid
When a cell has two sets of chromosomes.

gametes
Cells involved in sexual reproduction; typically egg and sperm cells.

gonads
Organ that produces gametes, such as the ovaries and testes.

haploid
When a cell has only one set of chromosomes, typical of a gamete.

internal fertilization
Reproduction occurs through the internal deposit of gametes.

external fertilization
Reproduction where the eggs are fertilized outside the body.

meiosis
Nuclear division that results in haploid gametes.

ovaries
Female gonads in animals that produce eggs.

parthenogenesis
Reproduction where an unfertilized egg develops into a new individual.

sexual reproduction
Reproduction where gametes from two parents combine to make an individual with an unique set of genes.

sister chromatids
Two genetically identical chromosome segments that form after DNA replication.

testes
Male gonads in animals that produce sperm.

zygote
Single cell that is formed after the fertilization of an egg; the first cell of a new organism.

Points to Consider

- What must be replicated prior to mitosis?
- How do you think DNA might be replicated?
- What might happen if there is a mistake during DNA replication?

Lesson 5.3: DNA, RNA, and Protein Synthesis

Lesson Objectives

- Explain the chemical composition of DNA.
- Explain how DNA synthesis works.
- Explain how proteins are coded for and synthesized.
- Describe the three types of RNA and the functions of each.

Check Your Understanding

- What is the purpose of DNA?
- When is DNA replicated?

Introduction

Much research in the past fifty years has been focused on understanding the genetic material, DNA. Understanding how DNA works has brought with it many useful technologies. DNA fingerprinting allows police to match a criminal to a crime scene. Transgenic crops, or crops that contain altered DNA, have improved yields for farmers. And you can now test your DNA to find out the chance that your future children may be at risk for a rare genetic disorder. Although we can do some really complicated things with DNA, the chemical structure of DNA is remarkably simple and elegant.

What is DNA?

DNA, is the material that makes up our chromosomes and stores our genetic information. This genetic information is basically a set of instructions that tell your cells what to do. DNA is an abbreviation for deoxyribonucleic acid. As you may recall, nucleic acids are the class of chemical compounds that store information. The deoxyribo part of the name refers to the name of the sugar that is contained in DNA, deoxyribose.

The chemical composition of DNA is a polymer, or long chain, of nucleotides. Nucleotides are composed of a phosphate group, a 5-carbon sugar, and a nitrogen-containing base. The only difference between each nucleotide is the identity of the base. There are only four possible bases that make up each DNA nucleotide: adenine (A), guanine (G), thymine (T), and cytosine (C). The various sequences of these four bases make up the genetic code of your cells. It may seem strange that there are only four letters in the “alphabet” of DNA. But since your chromosomes contain millions of nucleotides, there are many, many different combinations possible with those four letters.

But how do all these pieces fit together? James Watson and Francis Crick won the Nobel Prize in 1962 for piecing together the structure of DNA. Together with the work of Rosalind Franklin and Maurice Wilkins, they determined that the structure of DNA is two strands of nucleotides in a double helix (Figure below), or a two-stranded spiral, with the sugar and phosphate groups on the outside, and the paired bases connecting the two strands on the inside of the helix (Figure below).

DNAs three-dimensional structure is a double helix. The hydrogen bonds between the bases at the center of the helix hold the helix together.

The bases do not pair randomly, however. When Erwin Chargaff looked closely at the base content in DNA, he noticed that the percentage of adenine (A) in the DNA always equaled the percentage of thymine (T), and the percentage of guanine (G) always equaled the percentage of cytosine (C). Watson and Crick’s model explained this result by suggesting that A always pairs with T and G always pairs with C in the DNA helix. Therefore A and T, and G and C, are complementary bases. If one DNA strand reads ATGCCAGT, the other strand would be made up the complementary bases: TACGGTCA. These base pairing rules state that in DNA, A always pairs with T, and G always pairs with C.

The chemical structure of DNA includes a chain of nucleotides consisting of a 5-carbon sugar, a phosphate group, and a nitrogen base. Notice how the sugar and phosphate form the backbone of DNA (one strand in blue), with the hydrogen bonds between the bases joining the two strands.

DNA Replication

The base pairing rules are crucial for the process of replication. DNA replication is the process by which DNA is copied to form an identical daughter molecule of DNA. During DNA replication, the DNA helix unwinds as the weak hydrogen bonds between the paired bases are broken. The two single strands of DNA then each serve as a template for a new stand to be synthesized. The new nucleotides are placed in the right order because of the base pairing rules. The new set of nucleotides then join together to form a new strand of DNA. The process results in two DNA molecules, each with one old strand and one new strand of DNA. Therefore, this process is known as semiconservative replication because one strand is conserved in each new DNA molecule (Figure below).

DNA replication occurs by the DNA strands unzipping, and the original strands of DNA serve as a template for new nucleotides to join and form a new strand.

Protein Synthesis

The code of DNA, stored in the base sequences, contains the instructions for the order of assembly of amino acids to make proteins. Each strand of DNA has many, many separate sequences that code for the production of a specific protein. These discrete units of DNA that contain code for the creation of one protein are called genes. Proteins are made up of units called amino acids, and the sequence of bases in DNA codes for the specific sequence of amino acids in a protein.

There are about 22,000 genes in every human cell. Does every human cell have the same genes? Yes. Does every human cell use the same genes to make the same proteins? No. In a multicellular organism, such as us, cells have specific functions because they have different proteins, and they have different proteins because different genes are expressed in different cell types. Think of gene expression as if all your genes usually are "turned off." Each cell type only "turns on" (or expresses) the genes that have the code for the proteins it needs to use. So different cell types "turn on" different genes, allowing different proteins to be made, giving different cell types different functions.

However, DNA does not directly coordinate the production of proteins. Remember that DNA is found in the nucleus of the cell, but proteins are made on the ribosomes in the cytoplasm. How do the instructions in the DNA get out to the cytoplasm so that proteins can be made? DNA sends out a message, in the form of RNA (ribonucleic acid), describing how to make the protein. There are three types of RNA directly involved in protein synthesis. Messenger RNA (mRNA) carries the instructions from the nucleus to the cytoplasm. The other two forms of RNA, ribosomal RNA (rRNA) and transfer RNA (tRNA) are involved in the process of ordering the amino acids to make the protein. This process is called translation and will be discussed below. All three RNAs are nucleic acids and are therefore made of nucleotides. The RNA nucleotide is very similar to the DNA nucleotide except for the fact that it contains a different kind of sugar, ribose, and the base uracil (U) replaces the thymine (T) found in DNA.

mRNA is created in a method very similar to DNA synthesis. mRNA is also made up of nucleotide units. The double helix unwinds and the nucleotides follow basically the same base pairing rules to form the correct sequence in the mRNA. This time, however, U pairs with each A in the DNA. In this manner, the genetic code is securely passed on to the mRNA. The process of constructing a mRNA molecule from DNA is known as transcription (Figures below and below).

Each gene (a) contains triplets of bases (b) that are transcribed into RNA (c). Every triplet, or codon, encodes for a unique amino acid.

Base-pairing ensures the accuracy of transcription. Notice how the helix must unwind for transcription to take place.

The mRNA is directly involved in the protein synthesis process and tells the ribosome (Figure below) how to assemble a protein. The base code in the mRNA dictates the order of the amino acids in the protein. But because there are only 4 bases in mRNA and 20 different amino acids, one base cannot directly code for one amino acid. The genetic code in mRNA is read in “words” of three letters (triplets), called codons. For example, GGU encodes for the amino acid glycine, while GUC encodes for valine. This genetic code is universal and used by almost all living things. These codons are read in the ribosome, the organelle responsible for protein synthesis. In the ribosome, tRNA reads the code and brings a specific amino acid to attach to the growing chain of amino acids, which is a protein in the process of being synthesized. Each tRNA carries only one type of amino acid and only recognizes one specific codon. The process of reading the mRNA code in the ribosome to synthesize a protein is called translation (Figure below). There are also three codons, UGA, UAA, and UAG, which indicate that the protein is complete. They do not have an associated amino acid. As no amino acid can be added to the growing polypeptide chain, the protein is complete. The chart in Figure below should be of use in this area of study.

Ribosomes translate RNA into a protein with a specific amino acid sequence. The tRNA binds and brings to the ribosome the amino acid encoded by the mRNA. Ribosomes are made of rRNA and proteins.

This summary of how genes are expressed shows that DNA is transcribed into RNA, which is translated in turn to protein.

This chart shows the genetic code used by all organisms. For example, an RNA codon reading GUU would encode for a valine (Val) according to this chart. Start at the center for the first base of the three base codon, and work your way out. Notice for valine, the second base is a U and the third base of the codon may be either a G, C, A, or U. Similarly, glycine (Gly) is encoded by a GGG, GGA, GGC, and GGU.

Mutations

The process of DNA replication is not always 100% accurate, and sometimes the wrong base is inserted in the new strand of DNA. A permanent change in the sequence of DNA is known as a mutation (Figure below).A mutation may have no effect on the phenotype or can cause the protein to be manufactured incorrectly, which can affect how well the protein works, or whether it works at all. Usually the loss of a protein function is detrimental to the organism.

However, in rare circumstances, the mutation can be beneficial. For example, suppose a mutation in an animal’s DNA causes the loss of an enzyme that makes a dark pigment in the animal’s skin. If the population of animals has moved to a light colored environment, the animals with the mutant gene would have a lighter skin color and be better camouflaged. So in this case, the mutation was beneficial.

There are many possible types of mutations possible in chromosomes. In the case of a point mutation, there is a change in a single nucleotide. Other mutations can be more dramatic. A large segment of DNA can be deleted, duplicated, inverted, or inserted in the wrong place. These mutations usually result in a non-functional protein, or a number of non-functional proteins. A deletion is when a segment of DNA is lost, so there is a missing segment in the chromosome. A duplication is when a segment is repeated, creating a longer chromosome. In an inversion, the segment of DNA is flipped and then reattached to the chromosome. An insertion is when a segment of DNA from one chromosome is added to another, unrelated chromosome.

Mutations can arise in DNA through deletion, duplication, inversion, insertion, and translocation within the chromosome. A deletion is when a segment of DNA is lost from the chromosome. A duplication is when a segment is repeated. In an inversion, the segment of DNA is flipped and then re-annealed. An insertion or translocation can cause DNA from one chromosome to be added onto another, unrelated chromosome.

Even if a single base is changed, it can cause a major problem. The substitution of a single base is called a point mutation. Sickle cell anemia is an example of a condition caused by a point mutation in the hemoglobin gene. In this gene, just the one base change causes a different amino acid to be inserted in the hemoglobin protein, causing the protein to fold differently and not function properly in carrying oxygen in the bloodstream.

If a single base is deleted, it can also have huge effects on the organism because this would cause a frameshift mutation. Remember that the bases are read in groups of three by the tRNA. If the reading frame gets off by one base, the resulting sequence will consist of an entirely different set of codons. The reading of an mRNA is like reading three letter words of a sentence. Imagine you wrote “big dog ate red cat”. If you take out the second letter, the frame will be shifted so now it will read “bgd oga ter edc at.” One single deletion makes the whole “sentence”, or mRNA, not read correctly.

Many mutations are not caused by errors in replication. Mutations can happen spontaneously and they can be caused by mutagens in the environment. An example of a mutagen is radiation. High levels of radiation can alter the structure of DNA. Also, some chemicals, such as those found in tobacco smoke, can be mutagens. Sometimes mutagens can also cause cancer. Tobacco smoke, for example, is often linked to lung cancer.

Lesson Summary

- DNA stores the genetic information of the cell in the sequence of its 4 bases: adenine, thymine, guanine, and cytosine.
- The information in a small segment of DNA, a gene, is sent by mRNA to the ribosome to synthesize a protein.
- Within the ribosome, tRNA reads the mRNA in sets of three bases (triplets), called codons, which encode for the specific amino acids that make up the protein.
- A mutation is a permanent change in the sequence of bases in DNA.

Review Questions

1. Translate the following segment of DNA into RNA: AGTTC
2. Write the complimentary DNA nucleotides to this strand of DNA: GGTCCA
3. What makes up a nucleotide?
4. Nucleotides are subunits of which polymers?
5. Amino acids are subunits that make up what polymer?
6. Describe the process of DNA replication
7. Name a mutagen.
8. What is made in the process of transcription?
9. What is made in the process of translation?
10. How does RNA encode for proteins?

Further Reading / Supplemental Links

- http://www.phschool.com/science/biology_place/biocoach/dnarep/intro.html
- http://nobelprize.org/educational_games/medicine/dna_double_helix/readmore.html
- http://www.biostudio.com/demo_freeman_protein_synthesis.htm
- http://learn.genetics.utah.edu/units/basics/transcribe/
- http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html
- http://learn.genetics.utah.edu/units/basics/builddna/
- http://enwikipedia.org/
- http://sickle.bwh.harvard.edu/scd_background.html

Vocabulary

amino acid
The units (monomers)that combine to make proteins.

DNA
Deoxyribonucleic acid; a nucleic acid that is the genetic material of all organisms.

DNA replication
The synthesis of new DNA; occurs during the S phase of the cell cycle.

double helix
Describes the shape of DNA as a double spiral; similar to a spiral staircase.

gene
The inherited unit of DNA that encodes for one protein (or one polypeptide).

mutagen
A chemical or physical agent that can cause changes to accumulate in DNA.

mutation
A change in the nucleotide sequence of DNA.

nucleotide
The units that make up DNA; consists of a 5-carbon sugar, a phosphate group, and a nitrogen-containing base

RNA
The nucleic acid that carries the information stored in DNA to the ribosome.

semiconservative replication
Describes how the replication of DNA results in two molecules of DNA, each with one original strand and one new strand.

transcription
The synthesis of a RNA that carries the information encoded in the DNA.

translation
The synthesis of proteins as the ribosome reads each codon in RNA, which code for a specific amino acid.

Points to Consider

- Your cells have “proofreaders” that replace mismatched pairs that occurred during DNA synthesis. How would that affect the rate of mutation in your body?
- There are many diseases due to mutations in the DNA. These are known as genetic diseases, and many can be passed onto the next generation. Think about how a single base change cause a huge medical problem like sickle cell anemia.
- Your DNA contains the instructions to make you. So is everyone’s DNA different? Can it be used to distinguish individuals, like a fingerprint?