3.9- DNA Replication And The Gentetic Code

Question 1

Explain semi-conservative DNA replication?

Answer 1

1. for dna to replicate, the double helix has to unwind and then separate into 2 strands
2. so the hydrogen bonds holding the complimentary bases together must be broken.
3. free dna nucleotides will then pair with their complimentary bases, which have been exposed as the strands separate. Hydrogen bonds are formed between them.
4. finally, the new nucleotides join to their adjacent nucleotides with phosphodiester bonds.
   - 2 new DNA molecules produced. Each consists of one old dna strand and one new strand, hence the name semi-conservative (half same).

Question 2

What are the roles of enzymes in DNA replication?

Answer 2

DNA helicase - carries out the unwinding and separating of the two strands of the DNA helix before replication can occur.
Travels along the dna backbone, catalysing reactions that break the hydrogen bonds between complimentary base pairs.

DNA polymerase- catalyses the formation of phosphodiester bonds between the free nucleotides and the newly exposed bases on the template strands.

Question 3

Explain continuous and discontinuous replication. (DNA)

Answer 3

DNA polymerase can only bind to the 3’ (OH) end, so travels in the direction of 3’ to 5’. (The other strand travels 5’ to 3’ as they are anti parallel)
As DNA only unwinds in one direction, dna polymerase must replicate each of the template strands in opposite directions.
-The strand that unzips from the 3’ end can be continuously replicated as the strands unzip. It is called the leading strand and is said to undergo continuous replication.
- the other strand unzips from the 5’ end, so DNA polymerase has to wait until a section of the strand has been unzipped, then work back along the strand. This results in dna being produced in sections called okasaki fragments, which then have to be joined. Dna ligase catalyses the joining of these fragments.
This is the lagging strand which undergoes discontinuous replication.

Question 4

Explain a mutation.

Answer 4

Can occur when sequences of bases are not matched exactly.

These errors occur randomly and spontaneously.

Question 5

Explain the genetic code.

Answer 5

The sequences of bases un DNA are the instructions for the sequences of amino acids in the production of proteins.
The genetic code is universal- all organisms use this same code, although the sequences of bases coding for each individual protein will be different.

Question 6

What is triplet code and a codon?

Answer 6

The code in the base sequences is a simple triplet code.
It is a sequence of three bases, called a codon.
Each codon codes for an amino acid

There are 4 bases meaning there are 64 different base triplets/codons.

Question 7

What is a gene?

Answer 7

A section of DNA that contains the complete sequences of bases (codons) to code for an entire protein is called a gene.

Question 8

Explain how the genetic code is degenerate.

Answer 8

The same amino acid can be coded for by more than one codon.

There are 4 bases meaning there are 64 different base triplets/codons. Includes:

• 1 codon which acts as the start codon when it comes at the beginning of a gene. It signals the start of a sequence that codes for a protein.
• if it is in the middle, it codes for the amino acid methionine.
• 3 stop codons that don’t code for any amino acids and signal the end of a sequence.

Question 9

What is meant by the genetic code being non-overlapping?

Answer 9

No single base can take part in the formation of more than one codon.

The 1 codon at the start ensures that the genetic code is non-overlapping.