DNA Structure And Replication Flashcards
Nucleic acids are huge polymers composed of what monomers?
Nucleotides.
What are nucleotides composed of?
A phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base.
What are the four nitrogenous bases in DNA and RNA?
DNA: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).
RNA: Adenine (A), Uracil (U), Cytosine (C), Guanine (G).
What are the two types of nucleic acids?
DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
What are the base-pairing rules in DNA?
Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G).
What determines the direction of DNA?
The sugar-phosphate backbone: the 5’ end has a phosphate group, and the 3’ end has a hydroxyl group.
What holds the two strands of a DNA molecule together?
Hydrogen bonds between complementary nitrogenous bases.
What does ‘anti-parallel’ mean in DNA?
The two strands run in opposite directions: one strand runs 5’ to 3’, the other 3’ to 5’.
If adenine is 26% of a DNA sample, what is the percentage of guanine?
24%.
(A = 26%, T = 26%, G and C together = 48%, so G = 24%).
If one DNA strand is 5’-AGGTCCG-3’, what is the complementary strand?
3’-TCCAGGC-5’.
How is RNA different from DNA?
- RNA is single-stranded; DNA is double-stranded.
- RNA contains uracil (U) instead of thymine (T).
- RNA has ribose sugar; DNA has deoxyribose.
Why does adenine bond only with thymine/uracil, and cytosine only with guanine?
- Their shapes and sizes allow proper hydrogen bonding.
- Purines (A, G) always pair with pyrimidines (T, C, U).
How does DNA control all traits in an organism?
Through proteins, which perform critical roles such as enzymes, structural components, motor proteins, and membrane transport.
When does DNA replication occur?
During the S phase of interphase in the cell cycle.
What are the three proposed models of DNA replication?
- Conservative: Entire original DNA molecule remains intact, and a completely new copy is made.
- Semiconservative: Each new DNA molecule has one parental strand and one new strand.
- Dispersive: DNA is broken into fragments, and each fragment is a mix of old and new DNA.
How was the semiconservative model proven using ^{15}N and ^{14}N?
After one replication, DNA was intermediate in density (one strand heavy, one light). After two replications, half was intermediate and half was light.
Why were ^{15}N and ^{14}N used in the DNA replication experiment?
To distinguish old (heavier) DNA strands from newly synthesized (lighter) strands.
What enzyme breaks hydrogen bonds to separate DNA strands?
Helicase.
What enzyme adds new nucleotides during DNA replication?
DNA polymerase.
What is the significance of multiple origins of replication in eukaryotic DNA?
It allows faster replication of large eukaryotic genomes.
What is the function of topoisomerase and helicase in DNA replication?
Topoisomerase: Prevents DNA from over-twisting ahead of the replication fork.
Helicase: Unwinds the DNA double helix.
What is the function of primase?
Synthesizes RNA primers to initiate DNA synthesis.
What is the function of DNA polymerase III and I?
Polymerase III: Synthesizes new DNA strands.
Polymerase I: Replaces RNA primers with DNA.
Could DNA polymerase work without a primer?
No, it requires a free 3’ hydroxyl group to add nucleotides.
What environmental factors increase DNA replication errors?
Radiation, chemicals, and UV light can damage DNA, leading to mutations.
Why do prokaryotes not have telomere shortening?
Their DNA is circular and does not lose ends during replication.
What enzyme prevents telomere shortening in germ cells?
Telomerase.
What are histones and nucleosomes?
Histones: Proteins that help package DNA.
Nucleosomes: DNA wrapped around histones.
Why do DNA and histones interact?
DNA is negatively charged (phosphate groups), and histones are positively charged, allowing tight binding.
