3.9 Dna Replication And The Genetic Code

Front 1

When two daughter cells are genetically identical to the original cell what does this mean?

Back 1

The daughter cells contain DNA with a base sequence identical to the original parent cell.

Front 2

In order for DNA to replicate what must happen first?

Back 2

The double helix strand must unwind and then separate into two strands.

Front 3

For the the DNA strand to unwind and separate what must happen?

Back 3

The hydrogen bonds between the complimentary base pairs must first be broken.

Front 4

What will free nucleotides do?

Back 4

Pair with their complimentary bases, which have been exposed as the strands separate.

Front 5

What forms between the new nucleotides that join onto their complimentary base pairs once the strands have opened up?

Back 5

Hydrogen bonds.

Front 6

Once the new nucleotides have joined to their complimentary base pairs and hydrogen bonds have formed between the bases what happens next?

Back 6

The new nucleotides join to their adjacent nucleotides with phosphodiester bonds.

Front 7

What does the term semi-conservative replication mean?

Back 7

Two new molecules of DNA are produced from the replication, each one consisting of one old strand of DNA and one new strand.

Front 8

What is DNA replication controlled by?

Back 8

Enzymes.

Front 9

Which enzyme carries out the process of unwinding and separating of the two strands of the DNA double helix?

Back 9

DNA helicase.

Front 10

What does the DNA helicase do?

What can this be seen as?

Back 10

It travels along the DNA backbone, catalysing the reactions that break the hydrogen bonds between complimentary base pairs as it reaches them.

This can be seen as the strand 'unzipping'.

Front 11

Free nucleotides pair with the newly exposed bases on the template strands during the 'unzipping process'. A second enzyme catalyses the formation of phosphodiester bonds between these nucleotides , what is this enzyme?

Back 11

DNA polymerase.

Front 12

How does DNA polymerase always move along the template strand?

Back 12

In the same direction.

Front 13

Which end is the only end of which DNA polymerase can bind to and so what direction do they go in?

Back 13

It can only bind to the 3' (OH) end so travels in the direction of 3' to 5'.

Front 14

Why does DNA polymerase have to replicate each of the template strands in opposite directions?

Back 14

Because DNA only unwinds and unzips in one direction.

Front 15

Which of the template strands can be continuously replicated as the strands unzip?

Back 15

The strand that is unzipped from the 3' end.

Front 16

As the strand that is unzipped from the 3' end can be continuously replicated what name is it given and what does it undergo?

Back 16

It is called the leading strand and undergoes continuous replication.

Front 17

The strand being unzipped from the 5' end means that DNA polymerase has to wait until what?

Back 17

Until a section of the strand has been unzipped and then work back along the strand.

Front 18

The strand unzipped from the 5' end has DNA being produced in sections. What are these section called?

Back 18

Okozaki fragments.

Front 19

The strand which undergoes discontinuous replication is known as what?

Back 19

The lagging strand.

Front 20

Sequencing of bases are not always matched properly, and an incorrect sequence may occur in the newly-copied strand. These errors occur randomly and spontaneously and lead to a change in the sequence of bases. What is this known as?

Back 20

A mutation.

Front 21

What is the genetic code?

Back 21

DNA carries the instructions needed to synthesise the different proteins needed by organisms - proteins are made up of a sequence of amino acids - folded into complex structures - so DNA must code for the sequence of amino acids.

Front 22

Where are the instructions that DNA carries contained?

Back 22

In the sequence of bases along the chain of nucleotides that make up the two strands of DNA.

Front 23

The code in the base sequences is what?

Back 23

Triplet code.

Front 24

What is a triplet code?

Back 24

A sequence of three bases called a codon.

Front 25

What does a codon do?

Back 25

Codes for an amino acid.

Front 26

What is a codon?

Back 26

A sequence of three bases.

Front 27

What is a gene?

Back 27

A section of DNA that contains the complete sequence of bases (codons) to code for an entire protein.

Front 28

The genetic code is universal - all organisms use the same code, but what is different?

Back 28

The sequence of bases coding for each individual protein.

Front 29

How many different bases are there?

Back 29

4

Front 30

If there are four bases, how many different base triples or codons are possible?

What does this include?

Back 30

64 (4 cubed - 4x4x)

This includes the start codon.

Front 31

What is the function of a start codon?

Back 31

It signals the start of a sequence that codes for a protein.

Front 32

If the start codon is in the middle of the gene, what amino acid does it code for?

Back 32

Methionine.

Front 33

How many stop codons are there?

Back 33

Three.

Front 34

Do stop codons code for any amino acids?

Back 34

No.

Front 35

What is the function of stop codons?

Back 35

To signal the end of the sequence.

Front 36

What ensure that the genetic code is non-overlapping?

Back 36

By having a single codon to signal the start of a sequence it is ensured that the codons (bases) are read in frame, which means the DNA base sequence is read from base 1, not 2 or three.

Front 37

Why are there a lot more codons than amino acids?

Back 37

There are only 20 different amino acids that regularly occur in biological proteins.

Front 38

Why is the genetic code known as degenerate?

Back 38

There are a lot more codons than amino acids, therefore many amino acids can be coded for by more than one codon.