DNA replication and repair

Question 1

Cell Cycle

Answer 1

Cell Cycle: interphase, mitosis, and cytokinesis

Cells grow and function for most of their life

DNA replicates during the S phase of interphase

Question 2

DNA Replication

Answer 2

DNA is double-stranded so when it is passed down, it must remain two strands

There are three (3) major steps to DNA Replication:
- Strand Separation
- Building Complementary Strands
- Dealing with Errors

Question 3

Models of DNA Replication

Answer 3

1. conservative model
   - the parental double helic remains intact and a new copy is made
2. Semi conservative model
   - the two strands of the parental molecule seperate, and each functions as a template for syntehsis of a new complementary strand.
3. dispersive model
   - each strand of both daughter molecule contains a mixture of old and newly synthesized parts

Question 4

When DNA is copied, replication is…

Answer 4

Semi conservative

Question 5

Step 1: Strand Separation

Answer 5

To begin replication, the two DNA strands must be unwound from each other

Specific nucleotide sequences (replication origins) act as starting points

The enzyme helicase binds to the origins and unwinds the DNA strands by breaking hydrogen bonds between bases

As the strands unwind, a Y-shaped replication fork is formed.

If 2 helicases are working at different ends of the same origin of replication, a replication bubble is formed

Question 6

Step 1 continued… (DNA replication)

Answer 6

Single strand binding (SSB’s) proteins help prevent DNA from reattaching to itself – keeps strands separate

DNA gyrase and topoisomerase – helps to relieve/releases tension from unwinding of double helix

Question 7

Step 2: Building the Complementary Strand (Elongation)

Answer 7

Replication occurs as soon as the strand is unwound …in “replication bubbles”

DNA polymerases join nucleotides to build the daughter strand

DNA polymerase can only add nucleotides onto the 3’ end of parent strand (can only add onto the 3 prime end)

read 3’ to 5’
building: 5’ to 3’ –> new strand built is anti parallel

Question 8

Step 2 continued… (dna replication)

Answer 8

dna polymerase cannot start the process

RNA primase begins replication by building small complementary
- RNA (primers) at the replication fork
- places dowm markers

DNA polymerase III starts at the RNA primer and builds in the 5’ to 3’ direction

Building the daughter strand is known as elongation

Question 9

Step 2 part 3 (dna replication)

Answer 9

Synthesis occurs antiparallel as the strands run opposite each other (5ʹ-3ʹ and 3ʹ-5ʹ)

Leading strand- built toward the replication fork in one complete section

Lagging strand- built away from the replication fork

Okazaki fragments- the short segments of DNA which DNA polymerase III is acting on as it replicates the 3ʹ-5ʹ end

DNA ligase joins the gaps between Okazaki fragments by forming phosphodiester bonds

DNA polymerase I will replace the primers with complementary base pairs

Question 10

Step 3: Correcting Errors

Answer 10

There are ~150 x 106 nucleotide pairs to be copied

DNA replication occurs at 50 base pairs per second

When errors occur, DNA polymerase I and II proofread and correct mismatches in the daughter strand