3.9 DNA replication and the genetic code Flashcards
Describe the semi conservative replication of DNA. (5 marks)
- Double helix DNA structure unwinds and separates into two strands (hydrogen bonds broken).
- Free DNA nucleotides pair with their complementary bases.
- Hydrogen bonds form between them.
4.The new nucleotides join to their adjacent nucleotides with phosphodiester bonds.
When the double helix DNA structure unwinds, what bonds must be broken? (1 mark)
Hydrogen bonds.
What bonds are between nucleotides? (1 mark)
Phosphodiester bonds
DNA strands are semi-conservative, what does this mean? (2 marks)
One strand is used as a template, and the other is a new strand formed from this.
What enzyme separates the two strand of DNA double helix? (1 mark)
DNA helicase
What does DNA helicase do? (2 marks)
Travel along the DNA backbone, catalysing reactions that break the hydrogen bonds between complementary base pairs as it reaches them. ‘Unzipping’.
What enzyme catalyses the formation of phosphodiester bonds between nucleotides when the new strand is forming? (1 mark)
DNA polymerase
What does DNA polymerase do? (1 mark)
Catalyses the formation of phosphodiester bonds between nucleotides when the new strand is forming
What is the leading strand? (1 mark)
The strand that is unzipped from the 3’ (OH) end.
Which strand of DNA is continuously replicated? (1 mark)
The leading strand.
The strand that is unzipped from the 3’ (OH) end.
Which strand of DNA is discontinuously replicated? (! mark)
The lagging strand
The strand of DNA that is unzipped from the 5’ end.
Why is one strand of DNA replicated continuously and the other discontinuously? (5 marks)
DNA polymerase always moves along the template strand in the same direction.
It can only bind to the 3’ end.
Because DNA only unwinds and 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 as the strand unzips,
however, the other strand is unzipped from the 5’ end so DNA polymerase has to wait until a section of the strand has unzipped and then work back along the strand.
What is the lagging strand? (1 mark)
The strand of DNA that is unzipped from the 5’ end.
Describe discontinuous replication. (4 marks)
Because DNA only unwinds and unzips in one direction, DNA polymerase has to replicate each of the template strands in opposite directions, from 3’ to 5’.
Because one of the strands is unzipped from the 5’ end, DNA polymerase has to wait until a section of the strand has unzipped and then work back along the strand.
What happens if an error spontaneously occurs in the sequence of DNA nucleotides? (1 mark)
A mutation
DNA codes for a sequence of amino acids. This is called the _____________________. (1 mark)
DNA codes for a sequence of amino acids that makes a protein. This is called the genetic code.
What is a codon? (1 mark)
A sequence of three bases.
What does a codon do? (! mark)
Each codon codes for an amino acid.
What is a gene? (1 mark)
A section of DNA that contains the complete sequence of bases to code for an entire protein.
‘Genetic code is universal.’ What does this statement mean? (1 mark)
All organisms use the same code.
How many different codons possible are there? (1 mark)
4 x 4 x 4
64
What is start codon? (1 mark)
The codon that comes at the beginning of a gene to signal the start of a sequence that codes for a protein.
What is a stop codon? (1 mark)
They signal the end of the sequence.
How many different amino acids regularly occur in biological processes? (1 mark)
20
