Unit 2 - 7. DNA Structure & Replication

Question 1

Properties of DNA that make it the Ideal Genetic Material

Answer 1

1. Storage of Genetic Information
2. Replication
3. Expression of Genetic Information
4. Mutability

Question 2

Frederick Griffith (1928)

Answer 2

• injected pneumococci bacteria into mice
• observed transformation
• heredity information in the heat-killed pathogenic strain was transferred to live, nonpathogenic strain, transforming the nonpathogenic strain into pathogenic cells which caused the mice to die

Question 3

Avery, McLead, and McCarty (1944)

Answer 3

• identified the transforming substance as DNA

- used protease and DNase in an experiment with mice

Question 4

Hershey and Chase (1952)

Answer 4

• provided more conclusive evidence that DNA was the genetic material
• ran experiments on bacteriophages (a virus that infects bacteria and transfers genetic information to them, they consist of DNA surrounded by a protein coat)

Question 5

Watson and Crick (1955)

Answer 5

• discovered DNA structure

- based their model on available information from previous studies

Question 6

Previous Studies Watson and Crick used:

Answer 6

1. X-rays of pure DNA crystals (Maurice Wilkins, Linus Pauling, Rosalind Franklin)
2. Data from Chargoff on chemical composition of DNA: A=T; C=G

Question 7

Major features of Watson and Crick model

Answer 7

1. DNA is composed of two anti-parallel nucleotide chains
2. the 2 chains are coiled around a central axis to form a helix
3. bases on interior of helix w. sugar-phosphate backbone on exterior
4. the 2 chains are held together by hydrogen bonds between complementary bases in opposite chains
5. the sequences of bases along the 2 strands are complementary - i.e. the sequence of bases on the 2nd strand is determined by the 1st strand

Question 8

Meselson and Stahl (1958)

Answer 8

• found evidence to support semi-conductive method
• Grew E.Coli in Nitrogen 15 medium, then placed them in Nitrogen 14
• The DNA was sequenced buy equilibrium centrifugation in a density gradient

Question 9

Nucleoside vs. Nucleotide

Answer 9

Nucleoside = nitrogenous base + sugar
Nucleotide = nitrogenous base + sugar + phosphate

Question 10

DNA vs. RNA

Answer 10

DNA:

• double stranded
• A=T, G=C
• found/synthesized in the Nucleus
• storage of genetic information in majority of living organisms

RNA:

• single stranded
• A =U, G=C
• found/synthesized in Ribosome/cytoplasm
• stores genetic information for certain viruses
• transfers genetic info from nucleus to cytoplasm where proteins are made

Question 11

Purines

Answer 11

• 2 rings

- Adenine & Guanine

Question 12

Pyrimidine

Answer 12

• 1 ring

- Cytosine, Thymine, and Uracil

Question 13

Unique/Single Copy DNA

Answer 13

• DNA whose nucleotide sequences is represented only once in the haploid genome
• occurs in very long stretches
• comprises 50-60% of total DNA

Question 14

Moderately Repetitive DNA

Answer 14

• sequence repeated 100-1000 times in genome
• constitutes 25-40% of genome
• contains genes coding for rRNA and histones
• some can be “junk” DNA (no function)

Question 15

Highly repetitive DNA

Answer 15

• short sequences (5-10 bp) repeated thousands/millions of times in the genome
• 10-15% on human DNA
• highly localized in specific sites of the chromosomes (centromeres, telomers)

Question 16

Semi-conservative

Answer 16

• DNA replicates by separation of the two strands

- each functioning as template for new strand (complimentary)

Question 17

DNA Synthesis Requires

Answer 17

1. DNA
2. dATP, dGTP, dTTP, dCTP (deoxynucleotide triphosphates
3. DNA polymerase enzymes + additional enzymes (helicase, ss binding protein, primase, exonuclease, ligase)
4. Primer

Question 18

Steps in DNA Replication

Answer 18

1. Initiation
2. Elongation
3. Termination

Question 19

Initiation

Answer 19

• starts at replication fork / origin of replication
• DNA helicase unwinds by breaking hydrogen bonds
• single-stranded binding protein keeps strands apart

Question 20

Elongation

Answer 20

• RNA primase lays down RNA primer
• DNA polymerase recognizes primer and adds dNTPs (Reads 3’-5’; Builds 5’-3’)
• lagging strand, leading strand, okazaki fragments
• DNA polymerase proof reads
• Exonuclease removes mismatched bases

Question 21

Termination

Answer 21

• synthesis ends when DNA polymerase reaches end of strand

- DNA ligase connects last Okazaki fragments