finleydoesbiology

[finleydoesbiology]

2.2 : DNA REPLICATION

Stages of cell division:

Nuclear division (nucleus divides)

Cytokinesis (cell divides)

Q: Why does the DNA have to be replicated?

A: To ensure that after division, the daughter cells have the genetic code needed to produce all required proteins needed to sustain them (eg. enzymes).

DNA replication is precise: Daughter cells are roughly genetically identical to parent cell.

4 requirements for DNA replication:

Presence of all 4 types of nucleotide

Presence of DNA polymerase

Source of chemical energy

Each DNA strand must act as a template

Process of DNA replication:

DNA gyrase untwists double helix

DNA helicase breaks hydrogen bonds between the genetic bases

Free nucleotides bind to exposed bases by complementary base pairing

DNA polymerase catalyses the bonding of the new phosphate-sugar backbone by phosphodiester bonds

Q: Why is DNA semi-conservative?

A: DNA is made up of 1 conserved polynucleotide strand from the DNA it was replicated from, and 1 new strand.

Evidence for semi-conservative replication:

Experiment was done where bacteria was grown in medium containing ¹⁵N.

The DNA in the bacteria was dense as the nitrogen in this DNA was the ¹⁵N isotope.

This bacteria was then moved to a medium with ¹⁴N.

In the next generation, the DNA became lighter because while the old strand of each DNA molecule was conserved and made with ¹⁵N, the new strand was made with ¹⁴N.

In the next generation after that, all strands were now made using ¹⁴N, and were thus lighter.

The density of the DNA at each stage was obtained by taking a sample of the bacteria at each interval, and homogenising and centrifuging it to separate the DNA.

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Disclaimer:

This is content from the AQA Biology A level course.

This is not a replacement for your own notes.

I am a student, not a teacher.

This is mainly for my own benefit, I am not responsible for your grades.

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