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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
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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