bio cell cycle and DNA replication

cell cycle

The series of events a cell goes through to grow and divide.

Interphase

Includes G1, S, and G2

Cell grows, copies DNA, and prepares for division

Longest part of the cell cycle

G1 Phase (Gap 1)

Cell grows in size

Produces proteins and organelles

Normal cell functions occur

S Phase (Synthesis)

DNA is replicated

Each chromosome is copied → forms sister chromatids

G2 Phase (Gap 2

Further growth

Cell prepares for mitosis

Checks DNA for errors

Mitosis (M Phase)

Division of nucleus

Sister chromatids separate into two nuclei

Followed by cytokinesis (cell splits into 2 cells)

Sister Chromatids =

Two identical copies of a chromosome attached together.

When are sister chromatids made?

During S phase of interphase

Relationship to original chromosome

They are identical copies of the original DNA molecule

How many strands in each chromatid?

Each chromatid = 1 double helix (2 strands of DNA)

DNA is always double-stranded

Orientation of DNA strands

Strands are antiparallel

One runs 5’ → 3’

Other runs 3’ → 5’

5’ end

Has a phosphate group attached to the 5’ carbon

3’ end

Has a hydroxyl (-OH) group on the 3’ carbon

Each new DNA molecule contains:

1 original (parent) strand

1 new strand

Direction of synthesis

New DNA is made 5’ → 3’

Template is read 3’ → 5’

Base pairing rules guide replication:

A pairs with T

C pairs with G

Where are sister chromatids held together?

At the centromere

Enzyme that uncoils DNA

Helicase

Opening strands

Helicase breaks hydrogen bonds between bases

stabilizing proteins

Single-strand binding proteins (SSBs)

Prevent strands from rejoining or tangling

Role of primase

Lays down RNA primers

Needed because DNA polymerase cannot start on its own

DNA polymerase III (main enzyme)

Adds nucleotides 5’ → 3’

Uses base-pairing rules

Proofreading:

3’ → 5’ exonuclease activity

Removes incorrect bases

Template:

3’ → 5’ (left → right)

New strand:

5’ → 3’ (left → right)

Leading strand =

continuous synthesis

Lagging strand =

discontinuous (Okazaki fragments)

Leading strand

Continuous

Fewer primers

Lagging strand

Made in fragments

Requires more primase activity

Because it must restart repeatedly

Second polymerase

DNA polymerase I

Has 5’ → 3’ exonuclease activity

Removes RNA primers and replaces them with DNA

Final enzyme

DNA ligase

Seals gaps between fragments

More gaps on the lagging strand

Eukaryotes note

Same process, different enzymes

Example: DNA polymerase delta

Circular Chromosome

One origin of replication

Replication moves in two directions (bidirectional)

Forms a replication “bubble”

Ends meet → complete circle

Linear Chromosome

Multiple origins of replication

More complex

Ends (telomeres) require special handling