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DNA, genes and protein synthesis - A-Level Biology notes
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DNA, genes and protein synthesis - A-Level Biology notes
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Chapter 8: DNA, genes and protein synthesis8.1 Genes and the genetic code
Gene: a section of DNA that contains the coded information for making polypeptides and functional learning
Locus: a gene located at a specific position on the DNA
Features of the genetic code:
Genetic code – there must be a minimum of three bases - A few amino acids are only coded by a single triplet
that code for each amino acid because: - A degenerate code – as most amino acids are coded
- Only 20 different amino acids regularly occur in proteins for by more than one triplet
- Each amino acid has its own code - Start and stop triplets
- Only 4 different bases ( TACG ) - Code is non-overlapping meaning each base is read
- Using one base only 4 amino acids will be created only once in the sequence
- Using a pair of bases only 16 amino acids will be created - Code is universal meaning each triplet code is for the
- Using three bases 64 amino acids will be created, which same amino acid in all organisms (indirect evidence of
is enough to satisfy the 20 amino acids evolution)
8.2 DNA and chromosomes
- Prokaryotic cells: Circular, shorter DNA which is not associated with proteins therefore they don’t have
chromosomes
- Eukaryotic cells: linear, longer DNA which is associated with proteins called histones that form chromosomes.
The mitochondria and chloroplasts of eukaryotic cells have shorter DNA and don’t associate with proteins
Homologous chromosomes:
Alleles: an alternative form of a gene that occurs at the same locus
- Sexually produce organisms are the result of
on homologous chromosomes
the fertilisation of a sperm and an egg cell.
- Any changes in the base sequence of a gene causes a mutation (a
- The maternal and paternal chromosomes
new allele of that gene). This results in production of a different
contribute to a full set
polypeptide which may or may not be functional
o Diploid – total number of chromosomes
- Ex. If the mutated protein is an enzyme and it isn’t
o Haploid – half the total number of
complementary to the substrate it will cause a problem
chromosomes
8.3 Structure of ribonucleic acid
Transferring the coded information:
- Sections of DNA are transcribed onto mRNA
- mRNA is small enough to leave the nucleus into the cytoplasm where proteins are synthesised
- A codon: the sequence of three bases on mRNA that code for an amino acid
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