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A2 2 - DNA as the Genetic Code - Summary

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Summarises the DNA as the Genetic Code topic within A2 Biology Unit 2. Good luck!

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September 5, 2025
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DNA as the 'Genetic Code
Genetic code transcription mRNA
* Three consecutive bases can form 64 combos 1) DNA helicase breaks hydrogen bonds btwn bases linking 2 strands Advantages :
Mod
There are only 20 A. A 's that need codes but this allows for :
in relevant section of DNA to be copied causing separation /unzip ) * Only 2 copies of each genes .
However , many A D
↳ Initiation codons 1 beginning or genes 2) One of DNA strands acts as template for mRNA complementary copies or mRNA can be made → can
f
↳ Stop codons 1 gene termination
3) Complementary RNA nucleotides 1in nucleoplasm ) pair WI exposed each form template →
production 01 Polypeptides
↳ Alternative codons for AA 's DNA nucleotides + join by H bonds mRNA bases
.
will be in * By remaining in nucleus DNA less
,
likely to be $
features 01 look :
complementary sequence to template DNA damaged than in cytoplasm
A non-overlapping 4) RNA polymerase enzymes moves along RNA nucleotides joining * B
A degenerate / more than / possible code ) sugar phosphate backbone
-

structure : r
* code tor A. A. but terminate A single stranded
some don't 5) Approx 20 base pain exposed on template strand at anyone time , w/ -




* DNA code is a universal code double helix directly rejoining immediately behind A ribose replaces deoxyribose in backbone A fu
6) When RNA polymerase reaches a stop triplet Lode → detaches * Uracil replaces thy amine n


synthesis ing Proteins Ribosomes
* Shorter than DNA / only codes for gene )


* Assembling primary polypeptide structure by reading
Epigenetics
I




mRNA code * found free in cytoplasm larracned to ER


* Requires transfer DNA + ribosomes fiytoplasml A two subunits made of protein 1- ribosomal RNA in nucleolus 01 nucleus +
A study of heritable modifications of the genome that don't inv

* tRNA single stranded
-



molecule of 70-80 nucleotides transported into alto Plasm through nuclear pores
* affects activity of a gene :
promoting / suppressing it 11^1,1, like
cloverleaf shape One end there's A large 1- small subunits attach to start codon on mRNA at start of translation
1- twists poform .


* mutations →
change of DNA sequence 1 NOT epigenetic modifications )

sequence of 3 bases / anticodon ) form complementary base
-


$ larger section has spaces where tRNA molecules carrying AAS held * allows environment I e. g. nutrition ,
hormones age )
,
to influence

pairs w/ mRNA 10dm Other end is exposed
.
section of mRNA Beach sire can cover one mRNA codon
for many generations
→ attaches to specific amino acid w/ help from enzymes A first site -
aminoacyl / A site → where anticodon on tRNA molecule

links
DNA Methylation
pole of tRNA : to complementary codon on mRNA

* second peptidyl IP site where adjacent AAS linked together by
.


* transport A. A 's to site →
within cytoplasm site of protein synthesis
-




A ensure each mRNA codon codes for particular A. A. Peptide bonds (energy from ATD A methyl group may be added to cytosine when immediate
followed by guanine in DNA sequence

* "" """" " """""" "" " " " " "" " " " " " "

Translation ←
AUG
One gene / one polypeptide Cpa islands

1) Ribosome air aches to mRNA start codon in A site As If A Critical .
islands methylated DNA
no 01
,
in region switched
reverse
a%i aches be transcribed
-
-
" '
2) tRNA w/ complimentary anticodon to ribosomes in A site using base
i
transcription cannot
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