CK-12 Life Science
Page 11
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).
Figure 5.12
An overview of meiosis.
Figure 5.13
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
What is parthogenesis?
How can organisms reproduce asexually?
How would sexual reproduction in a lizard be different than a fish?
Are the viable eggs that birds lay need to be fertilized externally?
How do most plants reproduce sexually?
What is the purpose of meiosis?
What is the advantage of sexual reproduction over asexual reproduction?
If an organism has 12 chromosomes in its cells, how many chromosomes will be in its gametes?
During what phase of meiosis do homologous chromosomes separate?
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&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).
Figure 5.14
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.
Figure 5.15
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 r
eplication, 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).
Figure 5.16
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).
Figure 5.17
Each gene (a) contains triplets of bases (b) that are transcribed into RNA (c). Every triplet, or codon, encodes for a unique amino acid.
Figure 5.18
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.
Figure 5.19
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.
Figure 5.20
This summary of how genes are expressed shows that DNA is transcribed into RNA, which is translated in turn to protein.
Figure 5.21
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.
Figure 5.22
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.