DNA

Question 1

How are the two strands of DNA held together?

Answer 1

By hydrogen bonds between complementary base pairs: A-T and G-C.

Question 2

What allows DNA to be copied faithfully?

Answer 2

Base-pairing enables each strand to serve as a template for a complementary strand.

Question 3

What is the role of helicase in DNA replication?

Answer 3

It unwinds the DNA helix, introducing positive supercoils.

Question 4

How are supercoils resolved during DNA replication?

Answer 4

Topoisomerases relax torsional stress. In bacteria: DNA gyrase and Topo IV; in eukaryotes: type I and II topoisomerases.

Question 5

What does Topoisomerase IV do?

Answer 5

Relaxes positive supercoils and helps separate interlinked daughter DNA molecules post-replication.

Question 6

In which direction is DNA synthesized?

Answer 6

5’ to 3’ direction by adding nucleotides to the 3’ hydroxyl end.

Question 7

What enzyme synthesizes DNA and forms phosphodiester bonds?

Answer 7

DNA polymerase.

Question 8

How is the lagging strand synthesized?

Answer 8

In Okazaki fragments, using RNA primers, which are later replaced by DNA.

Question 9

What is transcription?

Answer 9

Synthesis of RNA (typically mRNA) from a DNA template strand.

Question 10

What strand is used for transcription?

Answer 10

The template strand; the coding strand has the same sequence as mRNA (except T → U).

Question 11

What enzyme synthesizes RNA during transcription?

Answer 11

RNA polymerase, which moves 3’ to 5’ along the DNA, synthesizing mRNA 5’ to 3’.

Question 12

What are promoters and terminators?

Answer 12

Promoters initiate transcription; terminators stop it.

Question 13

What are the major types of RNA and their functions?

Answer 13

mRNA: codes for proteins; tRNA: delivers amino acids to ribosomes; rRNA: forms ribosomes; snRNA: RNA splicing (eukaryotes); miRNA: gene regulation.

Question 14

What is meant by ‘the genetic code is degenerate’?

Answer 14

Multiple codons can encode the same amino acid, e.g., serine has 6 codons.

Question 15

What is a start codon and stop codons?

Answer 15

Start: AUG (methionine); Stop: UAA, UAG, UGA.

Question 16

What is a frameshift mutation?

Answer 16

A shift in the reading frame due to insertion/deletion, altering amino acid sequence.

Question 17

What is translation?

Answer 17

Process where ribosomes synthesize proteins based on mRNA sequence.

Question 18

What are the three stages of translation?

Answer 18

Initiation, elongation, termination.

Question 19

What is the role of tRNA?

Answer 19

Carries specific amino acids and matches mRNA codons via its anticodon.

Question 20

How is an amino acid attached to tRNA?

Answer 20

By an aminoacyl-tRNA synthetase using ATP to form a high-energy bond.

Question 21

What happens during translation elongation?

Answer 21

tRNA enters A-site, peptide bond forms, ribosome shifts: A → P → E site movement.

Question 22

How does translation terminate?

Answer 22

A release factor binds the stop codon at the A-site, releasing the polypeptide.

Question 23

What are ribosomes composed of?

Answer 23

rRNA and proteins; two subunits (e.g., 70S in prokaryotes, 80S in eukaryotes).

Question 24

What is the Svedberg unit?

Answer 24

A measure of sedimentation rate during centrifugation; indicates size/density of ribosomal subunits.

Question 25

How do ribosomes differ between prokaryotes and eukaryotes?

Answer 25

Prokaryotic: 70S (50S + 30S); Eukaryotic: 80S (60S + 40S).

Question 26

How do prokaryotic and eukaryotic mRNAs differ?

Answer 26

Prokaryotic mRNAs: polycistronic, no cap, Shine-Dalgarno sequence. Eukaryotic: monocistronic, 5’ cap, 3’ poly-A tail.

Question 27

What is the Shine-Dalgarno sequence?

Answer 27

A ribosomal binding site in prokaryotic mRNAs aligning with 16S rRNA.

Question 28

How is translation initiated in eukaryotes?

Answer 28

40S ribosomal subunit binds 5’ cap, scans to find AUG start codon.

Question 29

What are UTRs?

Answer 29

Untranslated regions at 5’ and 3’ ends of mRNA; important in regulation.

Question 30

What is epigenetics?

Answer 30

Study of heritable gene expression changes not caused by changes in DNA sequence.

Question 31

What are the major epigenetic modifications?

Answer 31

DNA methylation, histone modification (methylation/acetylation), non-coding RNAs (e.g., miRNA).

Question 32

What is the role of DNA methylation?

Answer 32

Usually suppresses gene expression; occurs mostly at CpG sites.

Question 33

How do environmental factors affect epigenetics?

Answer 33

Diet, stress, toxins can alter methylation and histone states, affecting gene expression and disease risk.

Question 34

Describe the whole process of DNA replication

Answer 34

Steps: Initiation: Origin of replication is opened Helicase unwinds DNA, forming a replication fork Single-stranded binding proteins (SSBs) prevent re-annealing Supercoiling relieved by topoisomerases (e.g., DNA gyrase in prokaryotes) Priming: Primase lays down short RNA primers on both strands Elongation: DNA polymerase adds nucleotides to 3’ end (5’→3’ synthesis) Leading strand: continuous synthesis Lagging strand: discontinuous synthesis in Okazaki fragments Each fragment initiated by new primer RNA primers replaced with DNA by polymerase DNA ligase joins fragments Termination: Forks meet; enzymes detach In circular DNA (e.g. bacteria), daughter molecules may remain linked (catenated) Topoisomerase IV separates them