molecular basis Flashcards
Who first discovered the phenomenon of bacterial transformation?
Frederick Griffith in 1928.
How is transformation now defined in molecular biology?
A change in genotype and phenotype due to assimilation of foreign DNA.
What did Griffith observe when he mixed heat-killed pathogenic bacteria with living harmless bacteria?
Some living harmless bacteria became pathogenic — a process he called transformation.
Who demonstrated that DNA is the genetic material in viruses?
Alfred Hershey and Martha Chase in 1952.
What key structural insights did Franklin’s images provide?
DNA is a helical molecule with a width of 2 nm and a base pair spacing of 0.34 nm; one full turn every 3.4 nm (10 base pairs).
What are Chargaff’s rules?
DNA composition varies between species; 2) In any species, A = T and G = C.
What did Hershey and Chase’s experiment with T2 phage show?
Only DNA, not protein, entered E. coli during infection and provided genetic information
What technique did Rosalind Franklin use to study DNA?
X-ray crystallography.
What was Watson and Crick’s main structural contribution?
DNA is a double helix with antiparallel sugar-phosphate backbones and complementary base pairing.
Why must a purine pair with a pyrimidine in DNA?
: To maintain a uniform width of the double helix.
What did the Watson-Crick model explain about Chargaff’s rules?
The specific base pairing accounts for equal A=T and G=C ratios
Which bases pair together in DNA?
Adenine with Thymine (A-T), Guanine with Cytosine (G-C).
Who is Linus Pauling and what were his contributions?
Nobel laureate; proposed protein secondary structures like the alpha helix, and studied molecular disease like sickle cell anemia.
How does replication proceed in DNA?
Bidirectionally from each origin until the molecule is fully copied.
What were the competing models of DNA replication?
Conservative model and dispersive model.
What does the semiconservative model of DNA replication state?
Each daughter DNA molecule has one old (parent) strand and one newly synthesized strand.
Where does DNA replication begin?
At origins of replication, forming replication “bubbles”.
What is the role of helicase in DNA replication?
It untwists the DNA double helix at replication forks.
What do single-strand binding proteins do?
Stabilize unwound DNA strands during replication
What does topoisomerase do during replication?
: Relieves strain ahead of the replication fork by cutting and rejoining DNA.
What is the role of DNA polymerase?
: It adds nucleotides to a primer to synthesize a new DNA strand.
Compare prokaryotic and eukaryotic DNA replication.
- Prokaryotic: One origin, fewer polymerases, faster, no telomeres.
- Eukaryotic: Many origins, more polymerases, slower, telomere handling
Why does eukaryotic DNA replicate faster than expected?
Multiple origins of replication allow simultaneous copying
What does primase do?
Synthesizes RNA primers by adding RNA nucleotides using parental DNA as a template
: What is the C-value paradox?
Genome size doesn’t correlate with organism complexity due to noncoding DNA like introns.
What is the initial nucleotide strand in DNA replication?
: A short RNA primer synthesized by the enzyme primase
Which end of the primer serves as the starting point for DNA synthesis?
The 3′ end
What do DNA polymerases do?
Catalyze the synthesis of DNA at a replication fork using a DNA template and primer
What is the rate of elongation in bacteria and humans?
~500 nucleotides/sec in bacteria, ~50 nucleotides/sec in humans.
What are nucleoside triphosphates?
Nucleotides with three phosphate groups (e.g., dATP) that lose two phosphates when incorporated into DNA.
What is the difference between the leading and lagging strands?
: Leading strand is continuous; lagging strand is made in Okazaki fragments (~150–200 bp), joined by DNA ligase.
Why can DNA elongate only in the 5′ → 3′ direction?
DNA polymerases can only add to the free 3′ end of the growing strand
What is the function of helicase?
Unwinds the parental double helix.
What do single-strand binding proteins do?
Stabilize single-stranded DNA.
Function of topoisomerase?
Relieves strain ahead of the replication fork.
What does DNA pol III do?
Synthesizes new DNA by adding nucleotides to RNA primer or existing strand.
What does DNA pol I do?
Removes RNA primers and replaces them with DNA.
What does DNA ligase do?
Joins Okazaki fragments and seals gaps on the leading strand.
Function of sliding clamp?
Holds DNA polymerase in place during replication.
What is the trombone model of replication?
: A loop in the lagging strand keeps replication enzymes in contact and coordinates synthesis
: How does DNA proofreading work?
: DNA polymerases detect and replace mismatched bases.
: What is nucleotide excision repair?
Endonuclease removes damaged DNA; gap filled by DNA polymerase and sealed by ligase.
What are telomeres?
Non-coding DNA sequences (TTAGGG) at chromosome ends that prevent gene loss.
What is the Hayflick limit?
Normal human cells divide ~50–70 times before telomere shortening leads to cell death.
What does telomerase do?
Adds telomeric repeats to 3′ ends in germ cells using its RNA component (e.g., CCCAAUCCC in vertebrates).
How is telomerase linked to cancer?
Cancer cells often have high telomerase activity, allowing them to avoid apoptosis.
what is a source of genetic variation?
when incorrect sequences of DNA are not caught and they become permanent in the DNA and passed on the next generation
difference between dATP and ATP?
dATP has deoxyribose and supplies adenine to DNA while ATP has ribose
what are the special nucleotides sequence at the end of chromosomes?
teleomeres that repeat 2500 times, slows the erosion of genes near the end of DNA
what do teleomeres not do?
prevent shortening of DNA
what is telomerase?
catalyzes the lengthening of telomeres in germ cells
