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Rachel Parlour

The genetic code and cell function
DNA’s function is to carry the genetic code for an organism
...

DNA= deoxyribonucleic acid
RNA= ribonucleic acid
They’re both macromolecules and polymers of nucleotides
...

There are 5 different bases
Bases in DNA: adenine, guanine, cytosine, thymine
Bases in RNA: adenine, guanine, cytosine, uracil
Purines: adenine, guanine
Pyrimidines: cytosine, thymine, uracil
Polynucleotides are made of many nucleotides joined together in a long chain
...

DNA
RNA
Longer
Shorter
Deoxyribose sugar
Ribose sugar
Double stranded
Single stranded
A, C, G, T
A, C, G, U
Complementary base pairing; adenine with thymine by 2 hydrogen bonds, cytosine
with guanine by 3 hydrogen bonds
The bonds can be broken relatively easily meaning the strands can be separated
for replication to take place by mitosis
...

Helicase= unwinds and unzips the two strands of the DNA double helix
...

RNA polymerase= attaches to and moves along the coding strand of DNA adding ribonucleotides
(sugar) to the growing mRNA strand
...

DNA ligase= facilitates the joining of sections of polynucleotides strands together by catalysing the
formation of a phosphodiester bond
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Rachel Parlour
DNA replication
Differences between protein synthesis and replication
Protein synthesis
End result is protein
RNA is involved
Could be initiated from DNA or RNA
A new protein chain is formed
Three main steps are involved
Takes place in the nucleus, mitochondria and
cytoplasm

DNA replication
End result is DNA
Only DNA involved
Initiated from only DNA
A new DNA strand is formed
Highly synonymous to the first of those three
main steps
Occurs in the nucleus and sometimes
mitochondria

Mechanism of replication:
o

o
o

o
o

DNA double helix unwinds controlled by the enzyme helicase, which unwinds and unzips 2
polynucleotide strands
...

This results in two polynucleotide strands
...

Each time the DNA polymerase reaches a base, a nucleotide with a complementary base is
brought into position and joins into the base by formation of a hydrogen bond
...

The free nucleotide is held in place by the enzyme until it binds into the preceding
nucleotide, extending the new strand of DNA
...


This replication is semi conservative replication because one parent strand is always passed on to
the daughter helix of DNA
...

Semi conservative replication
Semi conservative replication= two genetically identical molecules of DNA are formed from the
original molecule
...
Therefore each new molecule of DNA has one strand from the original molecule and
one newly synthesised strand
...
Meselson and Stahl designed as
experiment to determine how DNA replicates
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Rachel Parlour

Meselsohn and Stahl used the bacterium E coli, a common and harmless bacteria
...
The cells were then broken open
and centrifuged
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The bacteria grown in heavy nitrogen were transferred to a medium containing 14N
...
The cells from each generation were broken open and centrifuged
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After second generation there were 50% light,
50% intermediate
...
A hybrid banding
pattern could not have been achieved with conservative replication
...


Protein synthesis
All chemical reactions in a cell, and therefore all cell activities, are controlled by enzymes (proteins)
...
Protein molecules are made up of amino acids and the shape and
behaviour of a protein molecule depends of the sequence of amino acids (primary structure)
...

The triplet code
The sequence of bases or nucleotides in a DNA molecule is a code for the amino acid sequence in a
polypeptide
...
This is known as the degenerate
code
...

Code read downwards
Coding strand

C
A
A
T
T
A
G
A
A

G
T
T
A
A
T
C
T
T

Non Coding strand

Rachel Parlour

The code is a three letter- triplet code
...
The sequence is read in one direction
...
This DNA carries instructions to the cell to make a chain of
amino acids in the sequence valine, lysine, leucine, glycine
...
If the code was 2 letters, 16 amino acids could be coded (4^2)
...

The genetic code
o
o
o
o

Universal- same for all organisms
Degenerate- more than triplet for most amino acids
Stop or nonsense codes- signal the end of translation
Non-overlapping- each triplet is read separately

Genes and genomes
o
o
o
o

Gene is the part of the DNA molecule which codes for one polypeptide
One DNA molecule contains many genes
A total set of genes is the genome
The genome is the total information in one cell
...


Protein synthesis




DNA acts as a template providing instructions in the form of a long nucleotide sequence
A complementary part of this sequence is made into mRNA by transcription
mRNA acts as a template which complementary tRNA molecules attach to and the amino
acids they carry are linked forming a polypeptide by translation
...
mRNA is transcribed from the coding strand of the region of the DNA molecule making up the
gene carrying the code for the particular polypeptide required
...
mRNA attaches to the
ribosomes in the cytoplasm and protein is assembled
...

The enzyme RNA polymerase recognises and binds to a promoter region on the coding
strand, or template strand
...

The RNA polymerase unzips the two DNA strands by breaking hydrogen bonds and moves
along one of them, adding nucleotides to the 3carbon end of the RNA
...

RNA polymerase continues moving along until it reaches a stop code
...

mRNA leaves the nucleus via a nuclear pore and passes into the cytoplasm where it attaches
itself to a ribosome consisting of ribosomal RNA and protein and translation can begin
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Translation
This occurs in the cytoplasm, on a ribosome (2 subunits different sizes with 2 sites for tRNA
attachment)
...
tRNA and rRNA are required
...
This energy is used to link amino acids
forming polypeptides
...
Each of the tRNA molecules has a
specific anticodon which can combine with the codon on the mRNA molecule by
complementary base pairing
...
This allows the anticodon of
tRNA to combine with the first codon of the mRNA
...

The ribosome moves along one codon, allowing the appropriate tRNA to combine with the
next codon
...
This is catalysed by a ribosomal enzyme
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The ribosome moves along one codon and a new tRNA with its appropriate amino acid
attaches to the next mRNA codon
...

Each time the ribosome moves along one codon, one more amino acid is added to the
polypeptide chain
...
The mRNA and ribosomes complex is referred to as a polysome or
polyribosome
...
If the anticodon sequence is CCC then amino acid glycine will
attach to the other end of the tRNA molecule
...
The mRNA molecule translates into
amino acid glycine
...
Energy from ATP is
needed for amino acid to attach to the tRNA- activation
...
Proteins carry out a range of biological
activities- enzymes, antibodies, hormones, carrier and transport proteins
and structural proteins
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Rachel Parlour
Definitions
Transcription
Translation
DNA
mRNA
rRNA
tRNA
mRNA polymerase
codon
anticodon
axon
promoter
intron
RNA nucleotide
Template strand
Ribosome
Binding site
Amino acid binding site
Amino acid activation
Ribosomal enzyme
Peptide bond
Stop codon
Golgi body



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