2.1.2j DNA Replication Flashcards
Why does DNA need to replicate
- DNA copies itself before cell division so that each new cell has the full amount of DNA
This is important for: - making new cells fro growth & repair
- for passing genetic info from generation to generation (reproduction)
Why must DNA replication be accurate
The DNA replicate must be an exact copy to form 2 sister chromatids, and so the process must be accurate
What are enzymes
DNA replication is controlled by enzymes, a class of proteins that act as catalyst for biochemical reactions
4 enzymes involved in DNA replication
- DNA ligase
- Helicase
- DNA polymerase
- Single-stranded binding protein
What does DNA ligase do
Joins together short sections of the lagging strand
What is Helicase also known as
The unzipping enzyme
What does Helicase do
- Enzyme that catalyses the breaking of hydrogen bonds between the nitrogenous pairs of bases in DNA molecule.
- The result is 2 single strands of DNA w exposed nucleotide bases
What is DNA polymerase also known as
The builder enzyme
What does the DNA polymerase do
- Enzyme that catalyses the formation of DNA from activated deoxyribose nucleotides using single stranded DNA as a template
- Catalyses the formation of phosphodiester bonds between the nucleotides.
- This addition of nucleotide bases happens in the 5’ ti 3’ direction
What do Single-strand binding proteins do
Separates the 2 DNA strands before replication
Process of Semi-conservative replication
(pg37 dia)
- DNA helicase breaks the hydrogen bonds between the two polynucleotide DNA strands. The helix unzips to form 2 single strands
- Each og single strand acts as a template for a new strand. Free-floating DNA nucleotides join to the exposed bases on each og template strand by complementary base pairing (A with T, C with G)
- The nucleotides of new strands are joined tg by the DNA polymerase. This forms the sugar-phosphate backbone.
- Hydrogen bonds form between the bases on the og & new strand. The strands twist to form a double-helix
- Each new DNA molecule contains one strand from the og DNA molecule & one new strand
What is Semi-conservative replication
Each new DNA molecule is formed of one conserved strand from the og molecule, which acts as the template, & one strand of new nucleotides
Uses of semi-conservative replication
The loops of DNA in prokaryotes, & inside mitochondria & chloroplast, also replicate semi-conservatively.
A bubble sprouts from the loop & this unwinds and unzips. The complementary nucleotides join to the exposed nucleotides. Eventually, the whole loop is copied
Process of Continuous & Discontinuous Replication
- DNA polymerase always moves along the template strand in the same direction. It can only bind to the 3’ (OH) end, so travels in the direction of 3’ to 5’
- As DNA only unwinds & unzips in one direction, DNA polymerase has to replicate each of the template strands in opposite directions
- The strand that is unzipped from the 3’ end can be continuously replicated. This strand is called the leading strand & is said to undergo continuous replication
- The other strand is unzipped from the 5’ end, so DNA polymerase has to wait until a section of the strand has been unzipped & then work back along.
- This results in DNA being produced in sections, called Okazaki fragments. These fragments then have to be joined. This strand is called the lagging strand & is said to undergo discontinuous replication
see showbie for diagrams
Why must DNA replication be really accurate
To ensure genetic info is conserved each time the DNA in a cell is replicated
What are mutations
A mutation is any change to the DNA base sequence
How do mutations occur
- Every so often, a random, spontaneous mutation occurs during semi-conservative replication.
- Mutations dont always have an effect, but they can alter the sequence of amino acids in a protein.
- This can cause an abnormal protein to be produced. The abnormal protein might function better than the normal protein - or might not work at all
What causes the double-helix in DNA
The twisting of DNA produces its double-helix shape
