3.9 DNA replication and the genetic code

what are the three steps for DNA replication?

1. The double helix structure has to unwind and then seperate into two strands, which means the hydrogen bonds must be broken

2. free nucleotide in the nucleus pairs with complementary bases and hydrogen bonds are re formed

3. the phosphodiester bonds form between the adjacent nucleotide to join them together

what do codon code for?

a piece of protein called an amino acid

what the steps of semi conservative replication

1. an enzyme, helicase causes the two strands of the DNA to seperate

2. DNA helicase completes the separation of the strand.

3. free nucleotides that have been activated are attracted to their complimentary bases

4. once the activated nucleotides are lined up they are joined together by DNA polymerase (starting from the 5’ end)

5. the terminating unpaired bases continue to attract their complimentary nucleotides

6. finally, all the nucleotides are joined to form a complete polynucleotide chain using DNA polymerase. In this way, two identical molecule of DNA are formed.

7. each new molecule of DNA is composed of one original strand and one newly formed molecule- semi conservative replication

how does the enzyme DNA helicase work in DNA replication?

It travels along the backbone of DNA and catalyses reactions that break the hydrogen bonds between the bases.

it is thought of as ‘unzipping’ the DNA strand

What does the enzyme DNA polymerase do in DNA replication?

It catalyses the formation of phosphodiester bonds between the nucleotides

what’s continuous DNA replication

• DNa polymerase can only move along the DNA molecule in one direction

• The polymerase can only bind to the 3’ OH end, so it travels in the direction 3’ to 5’

• This strands is continuously replicated as the DNA unzips it is called the leading strand

what’s discontinuous DNA replication

• the other strand is unzipped from the 5’ end

• DNa polymerase has to wait until a section of the strand is unzipped and then work back along the strand

• the DNA is formed in sections called okazaki fragments

• the fragments are joined by DNA ligase

• this is the lagging strand

• it undergoes discontinuous replication

what are mutations

when sequences of bases are not always matched exactly and a newly copied strand can have an incorrect sequence. Error are random and spontaneous

what is genetic code?

DNA carries the instructions/blueprint needed to synthesise proteins needed by an organism, therefore DNA codes for the sequence of amino acids

The triplet code?

A codon is a triplet base that codes for amino acids, a section of DNA that contains the complete sequence of bases to code for an entire protein is called a gene

why are they called the degenerate code

because there are 20 amino acids, there a lore more codon than amino acids (64 codons).

Therefore many amino acids can be coded for by more than one codon