DNA REPLICATION AND PROTEIN

SYNTHESIS

The DNA double helix unwinds and unzips, using an enzyme, to make two individual strands of DNA

In the nucleus, there are nucleotides to which two extra phosphate groups have been added

These extra phosphates “activate” the nucleotides, enabling them to take part in reactions

The bases of the activated nucleotides pair up with their complementary base on each of the old DNA strands. An enzyme, DNA polymerase links the sugar and innermost phosphate groups of next door nucleotides together.The two extra phosphate groups are broken off and recycled.

DNA polymerase will only link an incoming nucleotide to the growing new chain if it is complementary to the base on the old strand. Very few mistakes are made, perhaps one in every 108

base pairs (1:1,000,000,000)

How do we know the mechanism of DNA replication is as described? There are three possible ways that it could actually happen.

Conservative Replication, where one completely new double helix is made from an old one.

Semi Conservative Replication, where each new molecule would contain one new strand and one old strand.

Dispersive replication, in which each new molecule would be made of old bits and new bits scattered randomly through the molecules.

The Genetic Code

A gene is a sequence of bases in DNA that codes for the sequence of amino acids in a polypeptide (protein)

The ‘language’ of a gene has only 4 letters- these are?

A T C and G

The Genetic Code

The ‘language’ of a protein has 20 letters- these are?

The 20 different amino acids that make up proteins

The Genetic Code

If 1 base coded for one amino acid in a protein then, only 4 amino acids could be coded for

If 2 bases coded for one amino acid in a protein then, only 16 amino acids could be coded for

If 3 bases coded for one amino acid in a protein then, 64 amino acids could be coded for – more than enough

41 = 4

42 = 16

43 = 64

The genetic code is a triplet code

The Genetic Code

There are 20 amino acids to be coded for and 64 base triplets to use to code them

Each amino acid has more than one code word – that is the genetic code is degenerate.

The Genetic Code

The genetic code is non-overlapping

ATTCGAGGCGGT is ‘read’ as

ATT CGA GGC GGT

Each base is a part of only one triplet.

The Genetic Code is:

A triplet code Degenerate Non-overlapping Universal

Protein synthesis

2 major processes involvedTranscriptionTranslation

Transcription

The relevant gene in the DNA in the nucleus is ‘copied’ into a molecule of RNA called mRNA or messenger RNA

Transcription

DNA double helix unzips as hydrogen bonds between complementary bases break and the two polynucleotide strands separate

A

G

C

T A

G

C

T

Transcription

One strand called the sense strand acts as a template, free RNA nucleotides complementary base pair to the exposed bases on this strand by forming hydrogen bonds

RNA polymerase forms sugar-phosphate bonds between nucleotides

A

G

C

TA

G

C

U

A

G

C

T

Transcription

The mRNA detaches from the sense strand The two DNA strands join together by complementary

base pairing The DNA molecules winds back up into a helix

A

G

C

TA

G

C

U

A

G

C

T

Transcription

The sequences of 3 bases on the mRNA coding for amino acids are called CODONS.

Not all the bases in the DNA code for amino acids so the mRNA just transcribed contains non-coding regions known as INTRONS

Transcription

exon intronintron exon exon

enzymes

These introns are removed by enzymes before the mRNA leaves the nucleusThis leaves just EXONS or coding regions of mRNA

Transcription

exon exon exon

enzymes

intron intron

These introns are removed by enzymes before the mRNA leaves the nucleusThis leaves just EXONS or coding regions of mRNA

nucleus

Transcription to translation

mRNA

ribosome

Following the removal of introns the mRNA moves out through a nuclear pore and attaches to a ribosome

tRNA

GGG

aa2Translation

AUG CCC GGG CGC ACA CGU UUC UGA

tRNA

UAC

aa1

start codon

anticodon

stop codon

tRNA

GGG

aa2

AUG CCC GGG CGC ACA CGU UUC UGA

tRNA

UAC

aa1

peptide bond formed

tRNA

GGG

aa2

AUG CCC GGG CGC ACA CGU UUC UGA

tRNA

UAC

aa1

‘empty’ tRNA leaves to pick up another specific amino acid

tRNA

CCC

aa3

tRNA

GGG

aa2

AUG CCC GGG CGC ACA CGU UUC UGA

aa1

Ribosome moves along mRNA by one codon

tRNA

CCC

aa3

tRNA

GGG

aa2

AUG CCC GGG CGC ACA CGU UUC UGA

aa1

peptide bond formed

‘empty’ tRNA leaves to pick up another specific amino acid

tRNA

ACUAUG CCC GGG CGC ACA CGU UUC UGA

aa2aa1

This process is repeated until the ribosome reads a stop codon

aa4aa3 aa6aa5 aa8aa7