1. DNA, genes and protein synthesis
Genes and the triplet code
A gene is a base sequence of DNA that codes for:
• the amino acid sequence of a polypeptide
• a functional RNA (including ribosomal RNA and tRNAs).
A gene occupies a fixed position, called a locus, on a particular DNA
molecule. A sequence of three DNA bases, called a triplet, codes for
a specific amino acid. The genetic code is universal, non-overlapping
and degenerate. In eukaryotes, much of the nuclear DNA does not
code for polypeptides. There are, for example, non-coding multiple
repeats of base sequences between genes. Even within a gene only
some sequences, called exons, code for amino acid sequences.
Within the gene, these exons are separated by one or more non-
coding sequences, called introns.
Features of genetic code
A few amino acids coded for by one single triplet
The remaining amino acids are coded for between 2 and 6 triplets each
- Degenerate code – most amino acids are coded for by more than
one triplet
Stop codes don’t code for an amino acids – they mark the end of a polypeptide
chain
The code is non overlapping – each base is only read once
The code is universal – each triplet codes for the same amino acids in all
organisms – indirect evidence for evolution
Exons – coding sequences
Introns – non coding sequences
DNA and chromosomes
In prokaryotic cells, DNA molecules are short, circular and not
associated with proteins. In the nucleus of eukaryotic cells, DNA
molecules are very long, linear and associated with proteins, called
histones. Together a DNA molecule and its associated proteins form
a chromosome. The mitochondria and chloroplasts of eukaryotic cells
also contain DNA which, like the DNA of prokaryotes, is short,
circular and not associated with protein.
Prokaryotes
, Shorter
Circular
Not associated with histone proteins
No introns
Like the DNA found in chloroplasts and mitochondria
Eukaryotes
Longer
Linear
Associated with histones
Contains introns
Chromosome structure
Each thread is called a chromatid- DNA has already replicated to give 2 identical
DNA molecules
DNA molecule
Combined with histones
DNA histone complex coiled
Coils fold into loops
Loops coil and pack together to form the chromosome
Homologous chromosomes – 2 chromosomes that carry the same genes but not
necessarily the same alleles
The structure of ribonucleic acid
Codon – 3 bases on mRNA that codes for a single amino acids
Genome – complete set of genes in a cell including those in mitochondria/
chloroplasts
Proteome – full range of proteins produced by the genome
RNA
Ribose
Uracil or other 3 bases
Phosphate
mRNA- single helix, base sequence depends on transcription – sequence of
codons determines amino acid sequence
tRNA – smaller than mRNA, single stranded chain folded into a clover leaf shape
Genes and the triplet code
A gene is a base sequence of DNA that codes for:
• the amino acid sequence of a polypeptide
• a functional RNA (including ribosomal RNA and tRNAs).
A gene occupies a fixed position, called a locus, on a particular DNA
molecule. A sequence of three DNA bases, called a triplet, codes for
a specific amino acid. The genetic code is universal, non-overlapping
and degenerate. In eukaryotes, much of the nuclear DNA does not
code for polypeptides. There are, for example, non-coding multiple
repeats of base sequences between genes. Even within a gene only
some sequences, called exons, code for amino acid sequences.
Within the gene, these exons are separated by one or more non-
coding sequences, called introns.
Features of genetic code
A few amino acids coded for by one single triplet
The remaining amino acids are coded for between 2 and 6 triplets each
- Degenerate code – most amino acids are coded for by more than
one triplet
Stop codes don’t code for an amino acids – they mark the end of a polypeptide
chain
The code is non overlapping – each base is only read once
The code is universal – each triplet codes for the same amino acids in all
organisms – indirect evidence for evolution
Exons – coding sequences
Introns – non coding sequences
DNA and chromosomes
In prokaryotic cells, DNA molecules are short, circular and not
associated with proteins. In the nucleus of eukaryotic cells, DNA
molecules are very long, linear and associated with proteins, called
histones. Together a DNA molecule and its associated proteins form
a chromosome. The mitochondria and chloroplasts of eukaryotic cells
also contain DNA which, like the DNA of prokaryotes, is short,
circular and not associated with protein.
Prokaryotes
, Shorter
Circular
Not associated with histone proteins
No introns
Like the DNA found in chloroplasts and mitochondria
Eukaryotes
Longer
Linear
Associated with histones
Contains introns
Chromosome structure
Each thread is called a chromatid- DNA has already replicated to give 2 identical
DNA molecules
DNA molecule
Combined with histones
DNA histone complex coiled
Coils fold into loops
Loops coil and pack together to form the chromosome
Homologous chromosomes – 2 chromosomes that carry the same genes but not
necessarily the same alleles
The structure of ribonucleic acid
Codon – 3 bases on mRNA that codes for a single amino acids
Genome – complete set of genes in a cell including those in mitochondria/
chloroplasts
Proteome – full range of proteins produced by the genome
RNA
Ribose
Uracil or other 3 bases
Phosphate
mRNA- single helix, base sequence depends on transcription – sequence of
codons determines amino acid sequence
tRNA – smaller than mRNA, single stranded chain folded into a clover leaf shape
