DNA replication

Question 1

semiconservative replication

Answer 1

each DNA strand serves as a template for the synthesis of a new DNA molecule
the original nucleotide strands remains intact (conserved), despite their no longer being combined in the same molecule; thus, the original DNA molecule is half (semi-)conserved during replication

Question 2

leading strand

Answer 2

continuous replication
new strand synthesized continuously in the 5′→3′ direction.
Synthesized toward the replication fork.
Moves in the same direction as unwinding.
Synthesized continuously (smooth, no stops).

Question 3

lagging strand

Answer 3

discontinuous replication
synthesized in the 5′→3′ direction, in the direction opposite of unwinding
the replication machinery soon runs out of template
more DNA unwound, providing new template
(Synthesized away from the replication fork.
Moves opposite the unwinding direction.
DNA polymerase keeps running into the end of available template.
Made in short fragments called Okazaki fragments.
Later joined together.)

Question 4

what are the stages of replication in bacteria

Answer 4

Four stages:

Initiation
Unwinding
Elongation
Termination

Question 5

Replication in Bacteria: (1) Initiation

Answer 5

Single origin of replication (oriC)

Initiator proteins bind to oriC and cause a short section of DNA to unwind

This unwinding allows helicase and other single-strand-binding proteins to attach to the polynucleotide strand

Question 6

Replication in Bacteria: (2) Unwinding

Answer 6

DNA Helicase Breaks hydrogen bonds
Single-Strand-Binding Proteins Stabilize the separated strands
Bind to any single stranded DNA
DNA Gyrase A type II topoisomerase
Creates (and reseals) double-strand breaks
Reduces torsional strain

Question 7

Replication in Bacteria: (3) Elongation

Answer 7

DNA polymerases require a nucleotide with a 3′-OH group to which a new nucleotide can be added.
PRIMASE synthesises short stretches (about 10–12 nucleotides long) of RNA nucleotides = primers
DNA polymerase III synthesise DNA in the 5’ -3’ direction by adding new nucleotides to the 3′ end of a growing nucleotide strand

Question 8

what does helices do?

Answer 8

Helicase to unwind the DNA

Question 9

what does single strand binding proteins do

Answer 9

Single-strand-binding proteins to protect the single nucleotide strands and prevent secondary structures

Question 10

what does DNA gyrase do

Answer 10

DNA gyrase to remove strain ahead of the replication fork

Question 11

what does primase do?

Answer 11

Primase to synthesize primers with a 3′-OH group at the beginning of each DNA fragment

Question 12

what does DNA polymerase III do

Answer 12

DNA polymerase III to synthesize the leading and lagging nucleotide strands

Question 13

DNA Polymerase I

Answer 13

Replication in Bacteria: (3) Elongation - last part
Using its 5′→3′ exonuclease activity, removes the DNA primer
Using its 5′→3′ polymerase activity, replaces the RNA nucleotides with DNA nucleotides.

Question 14

DNA ligase

Answer 14

Replication in Bacteria: (3) Elongation - last part
Catalyses the formation of a phosphodiester bond between adjacent 3′-OH and 5′-phosphate groups in a DNA molecule without adding another nucleotide to the strand.

Question 15

Replication in Bacteria: (4) Termination

Answer 15

The high level of accuracy in DNA replication is produced by:

Precise nucleotide selection

Proofreading DNA polymerases remove and replace incorrectly paired nucleotides during replication.

Mismatch repair Enzymes excise incorrectly paired nucleotides from the newly synthesised strand and use the original nucleotide strand as a template for replacing them

Question 16

DNA replication is extremely accurate, with less than…

Answer 16

one error per billion nucleotides!

Question 17

How to copy this immense amount of genetic information whenever a cell divides?

Answer 17

Up to 5000 bp of DNA copied per minute
If only a single starting point would require a week or more!
In eukaryotic cells replication does not initiate at a single point (which is what happens in bacteria) but instead starts at thousands of replication origins simultaneously

Question 18

Copy of eukaryotic DNA: two steps the for the initiation of replication

Answer 18

1. The origins are licensed - approved for replication - when licensing factors attach to each origin (G1)
2. The replication machinery initiates replication at each licensed origin (S)

Question 19

Eukaryotic DNA Polymerases

Answer 19

Eukaryotic cells contain many more different DNA polymerases than do bacteria, which function in replication, recombination, and DNA repair

Question 20

Telomeres and Telomerase

Answer 20

In single-celled eukaryotes, germ cells, and early embryonic cells chromosomes do not shorten!

Question 21

what is mutation

Answer 21

inherited change in the DNA sequence of genetic information

Question 22

The importance of mutations

Answer 22

Source of genetic variation
Raw material of Evolution
Detrimental effects
Source of diseases and disorders
useful for examining fundamental biological processes

Question 23

Genetic Stability

Answer 23

Effective mechanisms of DNA Repair
accurate DNA replication

Question 24

DNA repair: examples of types of DNA damage

Answer 24

depurination and deamination
deprivation is release of guanine as well as adenine
deamination
deamination converts cytosine to altered dna base uracil
thymine dimer -when exposed to ultraviolet irradiation (sunligh)

Question 25

What’s the difference between DNA Polymerase I vs. DNA Polymerase III?

Answer 25

DNA Polymerase III Think of it as the main builder. Adds most of the new nucleotides Builds both leading + lagging strands Very fast Main job: extend DNA from the primers. DNA Polymerase I Think of it as the finishing carpenter. It has two jobs: Remove RNA primers (using 5′→3′ exonuclease activity) Fill the gaps with DNA nucleotides It cleans up the lagging strand so it can become continuous.

Question 26

Okazaki fragments

Answer 26

The short lengths of DNA produced by discontinuous replication of the lagging strand

Question 27

What are Single-Strand Binding Proteins (SSBs)?

Answer 27

When DNA is unwound, each strand can: Stick back together Fold into weird shapes (hairpins) Single-strand binding proteins prevent this. Think of them as “strand guards”: They physically sit on the separated strands and: ✅ prevent re-pairing ✅ stop tangles / loops ✅ protect exposed nucleotides

Question 28

What are RNA primers — and why are they needed?

Answer 28

DNA polymerase has a problem: 👉 It cannot start building a new strand by itself. It can only add to something that already exists. So we need a little starter piece. Primase Makes a short piece of RNA (10–12 bases) with a free 3′-OH. That 3′-OH is like: “Okay DNA polymerase, you can start adding here!” Once DNA polymerase III starts building: On the leading strand → only 1 primer needed On the lagging strand → many primers (one for each fragment) Later, DNA polymerase I removes RNA primers and replaces them with DNA.

Question 29

Proofreading is done by:

Answer 29

C. DNA polymerase

Question 30

DNA ligase’s main job is to:

Answer 30

Join fragments by forming phosphodiester bonds

Question 31

Mismatch repair uses which strand as the template?

Answer 31

The older (original) strand

Question 32

What is the reason DNA replication is so accurate?

Answer 32

Precise selection + proofreading + mismatch repair

Question 33

1. Why do eukaryotes have multiple origins of replication?

Answer 33

A. Their DNA polymerase is slower Eukaryotic cells (like human cells) have: Much more DNA Linear chromosomes (not circular) Slower polymerases So if replication started at only ONE place (like bacteria), it would take weeks to finish. Their chromosomes are large and linear

Question 34

2. When are replication origins licensed?

Answer 34

A. G1 phase

Question 35

What happens to chromosomes without telomerase activity?

Answer 35

They shorten over time

Question 36

Telomeres are best described as:

Answer 36

Repetitive DNA that protects chromosome ends

Question 37

DNA synthesis is always which direction

Answer 37

5′→3′

Question 38

Ligase seals

Answer 38

nicks

Question 39

eukaryotes have more polymerases = more specialization While bacteria has

Answer 39

While bacteria mainly use DNA Pol III and I

Question 40

Why is the lagging strand discontinuous?

Answer 40

DNA polymerase can only build 5′→3′ and fork moves opposite

Question 41

Which enzyme catalyzes phosphodiester bond formation between Okazaki fragments?

Answer 41

DNA ligase

Question 42

Proofreading by DNA polymerase III occurs in which direction?

Answer 42

3′ → 5′ exonuclease

Question 43

Nucleotide excision repair (NER) corrects:

Answer 43

Thymine dimers caused by UV light

Question 44

The error rate after replication and repair is approximately:

Answer 44

. 1 per 10⁹ nucleotides

Question 45

Telomerase is active in:

Answer 45

Germ cells, early embryonic cells, some single-celled eukaryotes

Question 46

The “end-replication problem” refers to:

Answer 46

B. DNA polymerase’s inability to fully replicate the ends of linear chromosomes

Question 47

During which phase of the cell cycle is DNA replicated

Answer 47

S phase

Question 48

Replication of a single bacterial chromosome involves

Answer 48

One replication origin and two replication forks

Question 49

The haploid human genome is approx

Answer 49

3.2 billion base pairs of DNA

Question 50

The main enzymes involved in nuclear excision repair are

Answer 50

Nuclease telomerase ligase

Question 51

The meselson stahl experiment showed that

Answer 51

DNA replication is semi conservative