CHAPTER 16
THE MOLECULAR BASIS
OF INHERITANCE
1. List the three components of a nucleotide.
Phosphate, pentose sugar, and a base make up nucleotides.
2. Distinguish between deoxyribose and ribose.
Deoxyribose, the sugar component of DNA, has one less hydroxyl group than ribose,the sugar component of RNA.
3. List the nitrogen bases found in DNA, and distinguish between pyrimidine and purine.
The nitrogenous bases found in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T). Adenine and guanine are the larger bases and are called purines. Cytosine and thymine are the smaller bases and are called pyrimidines.
4. Explain the "base-pairing rule" and describe its significance.
The base pairing rule dictates the combination of nitrogenous bases that
form the “rungs” of the double-helix. The specific pairing suggests a copying mechanism for the genetic material.
5. Describe the structure of DNA, and explain what kind of chemical bond connects the nucleotides of each strand and what type of bond holds the two strands together.
DNA is in the form of a double helix. It has a sugar-phosphate backbone of two strands of DNA. The two strands are held together by hydrogen bonds between the nitrogenous bases, which are paired in the interior of the double helix.
6. Explain, in their own words, semiconservative replication.
During semi-conservative replication the parent molecule remains intact (is conserved)) and the new molecule is formed entirely from scratch.
7. Describe the process of DNA replication, and explain the role of helicase, single strand binding protein, DNA polymerase, ligase, and primase.
(Figure 15.7)
8. Define antiparallel.
The 5` 3` direction of one strand runs counter t the other strand.
9. Distinguish between the leading strand and the lagging strand.
The leading strand can elongate continually in the 5` 3` direction as the replication fork progresses in DNA replication. The lagging strand must grow in an overall 3` 5` direction by the addition of short segments, Okazaki fragments, that individually grow 5` 3`.
10. Explain how the lagging strand is synthesized when DNA polymerase can add nucleotides only to the 3’ end.
As a replication bubble opens, polymerase can work its way away from a replication fork and synthesize a short segment of DNA. As the bubble widens, another short segment of the lagging strand can be made by a polymerase working away from the fork. Another enzyme, DNA ligase, joins the fragments into a single DNA strand.
11. Explain the role of DNA polymerase, ligase, and repair enzymes in DNA proofreading and repair.
CHAPTER 17
FROM GENE TO PROTEIN
1. Explain how RNA differs from DNA.
Genes (DNA) are the instructions for making specific proteins. But a gene does not build a protein directly. The bridge between genetic information and protein synthesis is RNA. DNA and RNA both contain adenine, guanine, and cytosine. The nitrogenous base uracil is unique to RNA, and the base thymine is unique to DNA.
2. In your own words, briefly explain how information flows from gene to protein.
Nucleic acids have specific sequences of monomers that are like bits of information – much like the letters of the alphabet. In DNA or RNA, the monomers are the four types of nucleotides, which differ in their nitrogenous bases. Genes are hundreds of thousands of nucleotides long – each gene with a different specific sequence. A protein also has monomers arranged in a particular linear order, but its monomers are the twenty amino acids. Therefore the nucleic acids and proteins contain information written in two “languages”. To go from one language to another requires transcription and translation.
3. Distinguish between transcription and translation.
Transcription is the synthesis of RNA under the direction of DNA. Both nucleic acids use the same language and so the information is simply copied from one molecule to the other. Translation is the actual synthesis of a polypeptide, which occurs under the direction of mRNA. There is a change in language: The cell must translate the base sequence of an mRNA molecule into the amino acid sequence of the polypetide.
4. Describe where transcription and translation occur in prokaryotes and in eukaryotes.
In a prokaryoutic cell, which lacks a nucleus, mRNA produced by transcription is immediately translated without additional processing. In a eukarytic cell transcription occurs in the nucleus and translation in the cytoplasm.
5. Define codon, and explain what relationship exists between the linear sequence of codons on mRNA and the linear sequence of amino acids in a polypeptide.
Codons are mRNA base triplets. For each gene, one of the two strands of DNA functions as a template fro transcription – the synthesis of an mRNA molecule of complementary sequence. The same base-pairing rule that apply to DNA synthesis also guide transcription, but the base uracil (U) takes the place of thymine (T) in RNA. During the translation, the genetic message, mRNA, is read as a sequence of base triplets – a codon.
6. Explain the process of transcription including the three major steps of initiation, elongation, and termination.
As an mRNA polymerase molecule moves along a gene from the initiation site to the termination site, it synthesizes and RNA molecule that consists of the nucleotide sequence determined by the template strand of the gene. The entire stretch of DNA that is transcribed is called and transcription unit. Transcription begins at the initiation site when the polymerase separates the two DNA strands and exposes the template strand for base pairing with RNA nucleotides. The RNA polymerase works it way “downstream” from the initiation site, priying apart the two strands of DNA and elongating the mRNA in the 5` 3` direction. All the while, the two strands of DNA reform the double helix. The RNA polymerase continues to elongate the RNA molecule until it reaches the termination site, a specific sequence of nucleotides along the DNA that signals the end of the transcription unit. The mRNA, a transcript of that gene, is released, and the polymearase dissociates from the DNA.
7. Describe the general role of RNA polymerase in transcription.
RNA polymerase pry the two strands of DNA apart and hook together the RNA nucleotides as they base-pair along tha DNA template. Thay can add nucleotides only to the 3` end of the growing polymer and thus elongates in its 5` 3`direction.
8. Distinguish among mRNA, tRNA, and rRNA.
mRNA functions as a genetic message from DNA to the protein synthesizing machinery of the cell. tRNA translates the message received from mRNA. A functional ribosome subunit forms after attaching to mRNA is rRNA.
9. Describe the structure of tRNA and explain how the structure is related to function.
A tRNA molecule consists of a single strand of RNA that is only about 80 nucleotides long. This strand folds back on itself to form a molecule with a secondary structure, reinforced by interactions between different parts of the nucleotide chain. It has a clover-leaf shape. The function of tRNA is to transfer amino acids from the cytoplasm`s amino acids pool to a ribosome.
10. Given a sequence of bases in DNA, predict the corresponding codons transcribed on mRNA and the corresponding anticodons of tRNA.
(Figure 16.4)
11. Describe the structure of a ribosome, and explain how this structure relates to function.
Ribosomes facilitate the specific coupling of tRNA anti-codons with mRNA codons during protein synthesis. A ribosome is made up of two subunits, called large and small subunits. The large and small subunits merge to form a functional ribosome only when they attach to an mRNA molecule.
12. Describe the process of translation including initiation, elongation, and termination.
(Figures 16.3, 16.4, 16.5)
13. Describe the difference between prokaryotic and eukaryotic mRNA.
In a prokaryotic cell, transcription and translation are coupled with ribosome attaching to the “leading” end of an MRNA molecule while transcription is still in proess. In a eukaryotic cell the nuclear envelope separates transcriptions from traslation. Trascription occurs in the nucleus, and mRNA is dispatched to the cytoplasm, where translation occurs.
14. Explain how eukaryotic mRNA is processed before it leaves the nucleus.
Eukaryotic Mrna goes through Rna processing before leaving the nucleus. Both ends of the Mrna are altered. In some cases the molecule is then cut apart, and parts of it are spliced together again.
15. Explain why base-pair insertions or deletions usually have a greater effect than base-pair substitutions.
Base – pair substitutions are called silent mutations because, due to the reduridancy of the genetic code, they have no effect on the protein coded for. Base – pair insertions or deletions are disastrous because mRNA is read as a series of nucleotide triplets, an insertion or deletion, may alter the reading frame of the genetic message.
CHAPTER 18
MICROBIAL MODELS: THE GENETICS OF
VIRUSES AND BACTERIA
1. List and describe structural components of viruses.
A virus is a small nucleic acid genome enclosed in a protein capsid and sometimes a membranous envelope.
2. Explain why viruses are obligate parasites.
Viruses can reproduce only within a host cell. Viruses use enzymes, ribosome’s, and small molecules of host cells to synthesize progency viruses.
3. Explain the role of reverse transcriptase in retroviruses.
4. Describe how viruses recognize host cells.
Each type of virus can infect and pasasatize only a limit range of host cells called its “host” range. Viruses identify their host cells by a “lock – and – key” fit between proteins on the outside of the virus and specific receptor molecule on the surface of the cell.
5. Distinguish between lytic and lysogenic reproductive cycles using phage T4 and phage l as examples.
The reproduction cycle of a virus that culinates in the death of the host is the lytic cycle. The lysogenic cycle reproduces the viral genome with out destroying the host. (Figure 17.4, 17.5)
6. Explain how viruses may cause disease symptoms, and describe some medical weapons used to fight viral infections.
Some viruses damage or kill cells by causing the release of hydrolytic enzymes from lysosomes. Some cause the infected cells to produce toxins the lead to disease symptoms, and some have toxic components themselves vaccines are harmless variants or derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen.
7. List some viruses that have been implicated in human cancers, and explain how tumor viruses transform cells.
The viruses responsible for hepatitis B also seem to cause liver cancer in individuals with chronic hepatitis. Papilloma viruses have been associated with cancer of the cervix. Oncogens code for cellular growth factors or for proteins involved in growth factors actions. In some cases, the tumor virus lack oncogens and transform the cell simply by turning on or increasing, the expression of one or more of the cells own oncogenes.
8. List some characteristics that viruses share with living organisms, and explain why viruses do not fit our usual definition of life.
Viruses reproduce – but they can only do so in a host cell, as a parasite does. They passon their genetics (heredity). They have DNA – but sometimes they contain RNA.
9. Describe the structure of a bacterial chromosome.
Bacteria’s genome is one double – stranded DNA molecule arranged in a circle. The chromosomes so tightly packed within this genome that it does not have even fill the whole cell but forms a structure something like a long loop of yarn tangled into a ball.
10. List and describe the three natural processes of genetic recombination in bacteria.
The bacterial mechanisms of genetic recombination are three processes called transformation, transduction and conjugation. Transformation is the alteration of a bacterial cell’s genotype by the uptake of naked, foreign DNA from the surrounding environment. In transduction, phages (the bacteria that infect viruses) transfer bacterial genes form one host cell to another. Conjugation is the direct transfer of genetic material between two bacterial cells that are temporarily joined.
11. Explain how the F plasmid controls conjugation in bacteria.
The “male” cell has the ability to form sex pili and donate DNA during conjugation. This requires the presence of F plasmid.
12. Briefly describe two main strategies cells use to control metabolism.
Cells can vary the number of specific enzyme molecules; that is they can regulate the ezpression of a gene. Cells can also vary the activities of enzymes already present.
13. Distinguish between structural and regulatory genes.
Structural genes are genes that code for polypeptides. Regulatory genes are genes that produce repressors.
CHAPTER 1 9
THE ORGANIZATION AND CONTROL OF
EUKARYOTIC GENOMES
1. Compare the organization of prokaryotic and eukaryotic genomes.
Prokaryotic DNA is usually circular, and the nucleoticle is so small it can only be seen with an electron microscope. Eukaryotic DNA is a double-helix.
2. Describe the current model for progressive levels of DNA packing.
In beaded string (DNA) can coil tightly to make a cylinder 30 nm in diameter, known as the 30-nm chromatin fiber. The 30-nm fiber forms loops called looped domains, which are attached to a nonhistone protein scaffoiled. The looped domains, themselves, coil and forld, further compacting all the chromatin.
3. Distinguish between heterochromatin and euchromatin.
Hecterochromatin is more compact than euchromatin, and is visible with a light microscope.