Chapter 5. Duplicating the DNA: replication

Prepared by Woojoo Choi

Replication of the DNA

1) Since each cell needs a complete set of genes, it is necessary for the original cell to duplicate its genes before dividing

2) Replication – Duplication of DNA prior to cell division

3) Semi-conservative replication – replication of DNA in which each daughter molecule gets one of the

two original strands and one new complementary strand

Replication of the DNA

4) The steps of replication – First, separate the two strands of the DNA double helix – Second, build a complementary strand

How does a double helix separate

1) Before separating the strands, both the supercoils and the double helix must be unwound

2) This is done by 2 stages – First, the supercoils are unwound by an enzyme known as DNA

gyrase

How does a double helix separate

– Second, the double helix is unwound by the enzyme DNA helicase

How are the parental strands of DNA kept apart?

1) The two separated strands are complementary to each other 2) Consequently all of their respective base are capable of pairing off and

binding to each other 3) In order to prevent the two strands from getting back together, single

strand binding protein(SSB) binds to the unpaired single stranded DNA

Making a new strand of DNA

1) The critical issue in replication is the base paring 2) Each of the separated parental strands of DNA serves as a template

strand for the synthesis of a new complementary strand 3) Although hydrogen bonding alone would match bases correctly 99

percent of the time, this is not good enough

Making a new strand of DNA

4) DNA polymerase III (PolIII), which links the nucleotide, can also sense if bases are correctly paired, If not, the mismatched base pair is rejected

Making a new strand of DNA

5) DNA polymerase has two subunit – Synthetic subunit and sliding clamp subunit

Synthesis always goes from 5’ to 3’

1) When a new nucleotide is added it is joined, via its own phosphate group on position 5’ to the 3’ position as indicated by the arrow

(Figure 5.8)

Synthesis always goes from 5’ to 3’

1) New DNA strands always start at the 5’ end and grow in the 3’ diection 2) However, DNA is normally double stranded, and it happens that the two

strands run in opposite directions; that is, if one goes 5’ to 3’ then its complementary partner will run from 3’ to 5’

3) The strands are said to be antiparallel.

The replication Fork is where the action is

1) Replication fork: region where the enzymes (DNA gyrase, helicase, SSB, DNA polymerase III) replicating a DNA molecule are bound to untwisted, single stranded DNA

2) Leading strand: the new strand of DNA that is synthesized continuously during replication

3) Lagging strand: the new strand of DNA that is synthesized in short pieces and joinded together later

Completing the lagging strand.

1) Okazaki fragments: the short pieces of DNA that make up the lagging strand

2) DNA polymerase I: enzyme that makes small stretches of DNA to fill in gaps between Okazaki fragments or during repair of damaged DNA

3) DNA ligase: an enzyme that joins DNA fragments end to end 4) Gap is where bases are missing and nick means that there is a break in

the DNA backbone although no bases are missing

Recoiling the DNA into a helix

1) The replication fork has moved past, the double stranded DNA molecule automatically rewind into a helix

How are new chromosomes shared between daughter cells?

1) In the case of bacteria, cell division is relatively simple 2) Two replication forks meet and merge. This yields two new chromosomes

How are new chromosomes shared between daughter cells?

3) These are attached to the wall of the cell and as the bacterial cell elongates 4) The chromosomes are pulled apart. By the time the cell divides, by building a cross-wall, there is one chromosome in each new cell

How do cells of higher organisms divide?

1) Since cells of higher organisms are more complex, they handle cell division differently

2) Consequently a much more elaborate process is needed 3) Mitosis: division of mother cell into two daughter cells with identical sets

of chromosomes ① Disassmebly of the nucleus of the mother cell ② Division of the chromosomes ③ Partition of the chromosomes between daughter cells ④ Division of the mother cell ⑤ Building a new nucleus around the chromosomes in each daughter

cell

Division of Eukaryotic chromosomes

1) Chromosomes are Linear and often very long 2) There are several replication forks scattered along the length of each

chromosomes 3) Each pair of replication forks start at a separate origin of replication and

then moves in opposite directions – Origin of replication: site on a DNA molecule where replication begins

The eukaryotic cell cycle

1) The cell cycle of eukaryotes may be divided into several phases 2) Mitosis is subdivided in to 4 phases: prophase, metaphase, anaphase,

telophase

The eukaryotic cell cycle