Flashcards in Chapter 16 - The Molecular Basis of Inheritance Deck (55):
Who are the two men credited for proposing the double helical model for DNA?
James Watson and Francis Crick
Who is the man that showed that genes exist as parts of chromosomes?
What are the two chemical components of chromosomes?
DNA and protein
What is encoded in DNA and what does DNA direct?
Hereditary information; DNA directs the development of biochemical, anatomical, physiological, and (to some extent) behavioral traits
*DNA is reproduced in all cells of the body
Who is responsible for the initial discovery of the genetic role of DNA?
Frederick Griffith in 1928
- Griffith worked with two strains of bacteria, one pathogenic (disease-causing) and one nonpathogenic (harmless)
- When he mixed heat killed remains of the pathogenic strain with the living cells of the harmless strain, some living cells became pathogenic
- He called this phenomenon transformation
What is transformation?
A change in genotype and phenotype due to the assimilation of external DNA by a cell
What is a virus?
A virus is NA, and little more than DNA (or sometimes RNA) enclosed by a protective coat, which is often simply protein. To produces more viruses, a virus must infect a cell and take over the cell's metabolic machinery
What are bacteriaphages?
Meaning "bacteria-eaters"; viruses that infect bacteria
*Phages for short
In 1944, who announced that the transforming substance was DNA?
Oswald Avery, Maclyn McCarty, Colin MacLeod
Who performed experiments showing that DNA is the genetic material of a phage?
1952; Alfred Hershey and Martha Chase
- Hershey and Chase concluded that the DNA injected by the phage must be the molecule carrying the genetic information that makes the cells produce new viral DNA and proteins
- The Hershey-Chase experiment was a landmark study because it provided powerful evidence that nucleic acids, rather than proteins, are the hereditary material, at least for certain viruses
*Radioactive sulfur in phage protein, radioactive phosphorus in phage DNA; conclusion - injected NA of the phage provided genetic information
What is DNA?
A polymer of nucleotides, each consisting of three components; a nitrogenous base, a pentose sugar called deoxyribose, and a phosphate group
The base can be adenine (A), thymine (T), guanine (G), or cytosine (C)
*The base composition varies from one species to another
Who reported that DNA composition varies from species to species?
Erwin Chargaff; 1950
What are Chargaff's rules?
1. The base composition of DNA varies between species
2. For each species, the percentages of A and T bases are roughly equal and the percentages of G and C bases are roughly equal
Who used a technique called x-ray crystallography to study the molecule structure of DNA?
Maurice Wilkins and Rosalind Franklin
Franklin produced a picture of DNA using this technique, which indicated that;
- The helix makes one full turn every 3.4 nm along its length
- The bases are stacked just .34 nm apart
- There are ten layers of base pairs in each full turn of the helix
- The double helix alone is 2nm across
What is antiparallel?
The two sugar-phosphate backbones run in opposite directions
What are purines and pyrimidines? Which nitrogenous bases are which?
Purines - nitrogenous bases with two organic rings (adenine and guanine)
Pyrimidines - nitrogenous bases with one organic ring (cytosine and thymine)
What did Watson and Crick reason about the pairing of nitrogenous bases in DNA?
A purine and pyrimidine pair always results in a uniform diameter;
Adenine (A) can form two hydrogen bonds with thymine (T)
Guanine (G) can form three hydrogen bonds with cytosine (C)
*The base pairs are held together by hydrogen bonds
*The Watson-Crick model explains Chargaff's rules: in any organism the amount of A = T and the amount of G = C
Who was awarded the nobel prize for their molecular model of DNA?
1953; Watson, Crick, and Maurice Williams
What happens when a cell copies a DNA molecule?
Each strand serves as a template for ordering nucleotides into a new, complementary strand; nucleotides line up along the template strand according to the base pairing rules and are linked to form the new strands
What is the semiconservative model of DNA replication that Watson and Crick predicted?
The two strands of the parental molecule separate, and each functions as a template for synthesis of a new, complementary strand
What are the other two completing models of DNA replication?
- Conservative model; the two parental strands reassociate after acting as templates for new strands, thus restoring the parental double helix
- Dispersive model; each strand of both daughter molecules contains a mixture of old and newly synthesized DNA
Whose experiments supported the semiconservative model?
Matthew Meselson and Franklin Stahl
They labeled the nucleotides of the old strands with a heavy isotope of nitrogen, while any new nucleotides were labeled with a lighter isotope
What are origins of replication?
Replication begins at particular sites called origins of replication, where the two DNA strands are separated and open up a replication bubble; replication then proceeds in both directions until the entire molecule is copied
Prokaryotic (bacterial) chromosomes have one origin or replication while eukaryotic chromosomes may have hundreds or even a few thousand replication origins
What is the replication fork?
At each end of a replication bubble is a replication fork, a Y-shaped region where the parental strands of DNA are being unwound (several kinds of proteins participate in the unwinding)
What are helicases?
Enzymes that untwist the double helix at the replication forks, separating the two parental strands and making them available as template strands
What are single-strand binding proteins?
After the parental strands separate, single-strand binding proteins bind to the unpaired DNA strands and stabalize them, keeping them from re-pairing
Bind to and stabalize single stranded DNA
What is topoisomerase?
Corrects "overwinding" ahead of replication forks by breaking, swiveling, and rejoining DNA strands
What is the initial nucleotide strand?
A short stretch of RNA; this RNA chain is called a primer and is synthesized by the enzyme primase
What is primase?
An enzyme that synthesizes RNA primers, using the parental DNA as a template; primase starts a complementary RNA chain from a single RNA nucleotide, adding more RNA nucleotides one at a time, using the parental DNA strand as a template; the completed primer, generally 5-10 nucleotides long is then base paired to the template strand; the new DNA strand will start from the 3' end of the RNA primer
What are DNA polymerases?
Enzymes that catalyze the synthesis of new DNA strands by adding nucleotides to a preexisting chain; most DNA polymerases require a primer and a DNA template strand
What is the rate of elongation in bacteria as well as human cells?
500 nucleotides per second in bacteria; 50 nucleotides per second in human cells
Discuss the synthesizing of a new DNA strand?
- Each nucleotide that is added to a growing DNA strand is nucleoside triphosphate
- dATP supplies adenine to DNA and is similar to the ATP of energy metabolism
- The nucleotides used for DNA synthesis are chemically reactive, partly because their triphosphate tail have an unstable cluster of negative charge
- As each monomer of dATP joins the DNA strand, it loses two phosphate groups as a molecule of pyrophosphate
How does the antiparallel arrangement of the double helix affect replication?
DNA polymerases can add nucleotides only to the free 3' end of a primer or growing DNA strand; thus, a new DNA strand can elongate only in the 5' ---> 3' direction
What is the leading strand?
The new complementary DNA strand synthesized continuously along the template strand toward the replication fork in the mandatory 5' ---> 3' direction
Along one template strand of DNA, the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork
What is the lagging strand?
A discontinuously (as a series of segments) synthesized DNA strand that elongates by means of Okazaki fragments, each synthesized in the 5' ---> 3' direction away from the replication fork
What polymerase is used to synthesize the leading strand?
DNA polymerase III ( DNA pol III); only one primer is required for DNA pol II to synthesize the entire leading strand
What are Okazaki fragments?
The segments of the lagging strand; the fragments are about 1,000-2,000 nucleotides long in E. coli and 100-200 nucleotides on in eukaryotes
What is the function of DNA polymerase I?
DNA pol I replaces the RNA nucleotides of the adjacent primer with DNA nucleotides
What is DNA ligase?
An enzyme that joins the sugar phosphate backbones of all the Okazaki fragments into a continuous DNA strand; forms a bond between the newest DNA
What are the steps to synthesize the lagging strand?
1. Primase joins RNA nucleotides into a primer
2. DNA pol II adds DNA nucleotides to the primer, forming Okazaki fragment 1
3. After reaching the next RNA primer to the right, DNA pol III detaches
4. Fragment 2 is primed; then DNA pol II adds DNA nucleotides, detaching when it reaches the fragment 1 primer
5. DNA pol I replaces the RNA with DNA, adding nucleotides to the 3' end of fragment 2 (and earlier fragment 1)
6. DNA ligase forms a bond between the newest DNA and the DNA of fragment 1
7. The lagging strand in this region is now complete
What is the DNA replication "complex"?
- The proteins that participate in DNA replication form a large complex, a "DNA replication machine"
- The DNA replication machine may be stationary during the replication process
- Recent studies support a process in which DNA polymerase molecules "reel in" parental DNA and "extrude" newly made daughter DNA molecules
What is mismatch repair?
DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides; in mismatch repair of DNA, repair enzymes correct errors in base pairing
What is nucleotide excision repair?
A DNA repair system;
1. Teams of enzymes detect and repair damaged DNA
2. A nuclease enzyme cuts the damaged DNA strand at two points, and the damaged DNA strand at two points, and the damaged section is removed
3. Repair synthesis by a DNA polymerase fills in the missing nucleotides
4. DNA ligase seals the free end of the new DNA to the old DNA, making the strand complete
What is nuclease?
A DNA cutting enzyme; cuts out and replaces damaged stretches of DNA
What are the sources of genetic variation upon which natural selection operates?
What are telomeres?
Special nucleotide sequences at the end of DNA molecules (does not contain genes) that hold protective functions
- Specific proteins associated with telomeric DNA prevent the staggered end of the daughter molecule from activating the cell's systems for monitoring DNA damage
- Telomeric DNA acts as a kind of buffer zone that provides some protection against the organism's genes shortening
- Telomeres do not prevent the erosion of genes near the end of chromosomes, they merely postpone it
- Telomeres become shorter during every round of replication; it has been proposed that the shortening of telomeres is connected to aging
What is telomerase?
An enzyme that catalyzes the lengthening of telomeres in eukaryotic germ cells, thus restoring their original length and compensating for the shortening that occurs during DNA replication
Telomerase is not active in most human somatic cells, but its activity varies from tissue to tissue
What does a chromosome consist of?
A DNA molecule packed together with proteins
What is chromatin?
A complex of DNA and protein
What is the bacterial chromosome?
A double-stranded circular DNA molecule associated with a small amount of protein
In bacterium, the DNA is supercoiled and found in a region of the cell called the nucleoid
What is the eukaryotic chromosome?
Linear DNA molecules associated with a large amount of protein
What is the model for the progressive levels of DNA coiling and folding?
1. DNA, the double helix
2. Histones; proteins that are responsible for the first level of DNA packing in chromatin. More than 1/5th of a histone's amino acids are positively charged (lysine or arginine) and therefore bind tightly to the negatively charged DNA
3. Nucleosomes (10nm fiber); "beads on a string" - the basic unit of DNA packing which consists of DNA wound twice around a protein core of eight histones, two each of the main histone types (H2A, H2B, H3, and H4) with the amino end extends outward from the nucleosome
4. 30nm fiber; interactions between the histone tails of one nucleosome and the linker DNA and nucleosomes on the other side cause the 10nm fiber to coil and fold, forming a chromatin fiber
5. Looped domains (300nm fiber); the 30nm fiber forms loops called looped domains attached to a chromosome scaffold composed of proteins, thus making up a 300nm fiber; rich in topoisomerase
6. Metaphase chromosome; the looped domains coil and fold, further compacting all the chromatin to produce the characteristic metaphase chromosome; the width of the chromosome is 700nm
What is euchromatin?
Loosely packed chromatin; most chromatin is loosely packed in the nucleus during interphase and condenses prior to mitosis
What is heterochromatin?
During interphase, a few areas of chromatin (centromeres and telomeres) are highly condensed into heterochromatin; dense packing of heterochromatin makes it difficult for the cell to express genetic information coded in these regions