Unit 4 - Genetic information, variation and relationships between organisms
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Summary DNA, genes and protein synthesis
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Unit 4 - Genetic information, variation and relationships between organisms
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AQA
Summary notes for AQA A-level Biology DNA genes and protein synthesis topic. Includes clear information on DNA, genes, chromosomes, RNA, transcription and translation. Summarised from class notes and the official course textbook. From an A* student.
Unit 4 - Genetic information, variation and relationships between organisms
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DNA, genes and protein synthesis
Genes and the genetic code
DNA as a genetic material
• Stores large quantities of information that codes accurately for the characteristics of an
organism
• Very stable structure that passes generation to generation without many changes
• Replicates exactly during cell division so new cells have an identical copy of genetic material
• DNA (Deoxyribonucleic acid) is the genetic material in all living organisms
• In eukaryotes, DNA is associated with histone proteins which act as a scaffold for the coiling
Genome: complete set of all the genes able to code for proteins in a cell
Proteome: All the proteins a cells genome could produce
DNA code
• There are two strands, and the template strand is used to code
• The base sequence on this strand is read 3 bases (a triplet) at a time. Each triplet codes for one
amino acid:
• Some triplets code for ‘stop’ which means the end of the amino acid chain has been reached
and these codons don’t code for amino acids
• A change in just one base in DNA can code for a different amino acid which alters the primary
and therefore tertiary structure of the protein. The protein may then be non-functional.
• A deletion/addition of a base causes a frame shift: all subsequent triplets are altered
4 key features of the DNA code
Degenerate
• Degenerate means some amino acids are coded for by more than one
triplet code
• e.g. Threonine = TGG, TGC, TGA and TGT
Non-Overlapping
• Bases are read in groups of three and each
base is read once
Universal
• Universal means the same codons code for
the same amino acids in all organisms
• This is evidence for evolution
• It allows movement of genes between species
e.g. in genetic engineering
, Ratios of bases
• The amount of A = amount of T and the
amount of C = amount of G due to
complementary base pairing
• Total of A+C+T+G = 100%
• Different organisms can have similar
percentages of bases but its the sequence of
bases that determines characteristics
Exons and introns
• In eukaryotes, some DNA base sequences don’t code for amino acids. These non-coding
regions are called introns.
• The coding regions, e.g. triplets that code for amino acids, are called exons
Genes
• A gene is a length of DNA that carries a coded base sequence to synthesise a single
polypeptide, or a functional RNA molecule
• The position of a gene on a chromosome = locus
Prokaryote vs Eukaryote DNA
Prokaryote Eukaryote
No histones DNA is associated with histone proteins:
DNA wrapped around these proteins and
then there is further coiling of the proteins
known as supercoiling
Circular Linear but it is wound up
not in the nucleus In the nucleus
no chromosomes chromosomes
Shorter and with fewer bases Longer and with more bases
Plasmids contain DNA DNA in mitochondria and chloroplasts which
is short circular and not associated with
proteins
no introns and no splicing exons and introns and splicing occurs
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