DNA Replication And Genetic Code

What does DNA do before cell division?

It copies itself - self-replication.

What is DNA replication controlled by?

Enzymes.

How are the enzymes able to carry out their functions?

By recognising and attaching to specific molecules or particular parts of the molecules.

What enzymes are involved in DNA replication?

DNA helicase and DNA polymerase.

What is the first step of DNA replication?

DNA helicase breaks the hydroegn bonds between the two polynucleotide DNA strands - the DNA unzips to form two single strands.

What is the second step of DNA replication?

Each original single strand acts as a template for a new strand. Free floating DNA nucleotides join to the exposed bases on each original template strand by complementary base pairing.

What is the third step of DNA replication?

The nucleotides of the new strand are joined together by DNA polymerase, forming ths sugar-phosphate backbone. Hydrogen bonds form betwwen the bases on the original and new strand. The strands twist to a double-helix.

What is the fourth step of DNA replication?

Each new DNA molecule contains one new strand from the original DNA and one new strand.

What type of replication is DNA replication?

Semi-conservative.

Why is DNA replication semi-conservative?

Half of the strands in the new DNA molecule are from the original piece of DNA and the other half is newly synthesised.

Why must DNA replication be really accurate?

To make sure genetic information is conserved (stays the same) each time DNA in a cell is replicated.

What direction does DNA polymerase move in?

It can only move in the direction of 3’ to 5’ since it it can only bind to the OH (3’).

As DNA only unwinds and unzips in one direction, what must DNA polymerase do?

Replicate each of the template strands in opposite directions.

What is the leading strand?

The strand that is unzipped from 3’ to '5’ and can continuously replicate as the strands unzip.

What is the lagging strand?

The strand that is unzipped from 5’ to 3’ so DNA polymerase has to wait until a section of the strand has unzipped and then work back along the strand.

What does the lagging strand result in?

DNA being produced in sections.

What are the DNA sections produced by the laggins strand called?

Okazaki fragments, which then have to be joined.

What are proteins?

The foundation for the different physical and biochemical characteristics of living things.

What are proteins made up of?

A sequence of amino acids, folded up into complex structures.

What must DNA code for?

A sequence of amino acids.

What is the triplet code?

The code in the base sequences.

What is a codon?

A sequence of three bases

What does a codon code for?

An amino acid.

What is a gene?

A section of DNA tat contains the complete sequence of bases to code for an entire protein.

What are the characteristics for the genetic code?

• Non-overlapping.

• Degenerate.

• Universal

How is the genetic code non-overlapping?

Each triplet is read in sequence, seperate from the triplet before and after it. Bases don’t share their bases.

How is the genetic code degenerate?

There are more possible combinations of triplets than there are amino acids (20 amino acids but 64 combinations) meaning some amino acids are coded for by more than one base triplet.

What are start codons?

A codon that comes at the beginning of the gene, signalling the start of the sequence.

What does a start codon do if it is in the middle of a gene?

It codes for the amino acid methionine.

What are stop codons?

Codons that signal the end of the sequence.

What does having a single codon to signal the start of a sequence ensure?

That the triplets of bases (codons) are read ’in frame’.

How is the genetic code universal?

All organisms use this same code although the sequences of bases coding for each individual protein will be different.

Every so often, what happens with the DNA?

A mutation occurs.

What is a mutation?

Any change to the DNA base sequence.

Do mutations always have an affect?

No, but they can alter the sequence of amino acids in a protein which can cause an abnormal protein to be produced - the abnormal protein may function better than the normal one or it might not work at all.