Genes and Chromosomes
What is the function of DNA?
DNA is the code for making polypeptides
Gene - A length of DNA that codes for one polypeptide
It codes for the amino acid sequence of a polypeptide and the functional RNA (rRNA and
tRNAs)
Locus - the position of the gene on a DNA molecule
How does DNA code for a protein?
A codon - a triplet of bases that each amino acid is coded by
Stop/nonsense codons - codons that do not code for amino acids
The genetic code is:
1) Non-overlapping - a specific 'start' codon ensures ribosomes 'read' the mRNA code in groups
of 3
There are also 3 'stop' codons
Essentially each specific codon is read in sequence separate from the codon before it and
the codon after it
Base triplets do not share their bases
2) Degenerate - some amino acids are coded for by more than one codon
Also not all codons code for amino acids
3) Universal - in almost all living organisms the same triplet of DNA bases codes for the same
amino acids
Non-coding DNA
In eukaryotes, a lot of nuclear DNA is non-coding e.g. non-coding multiple repeats, introns,
some code for rRNA and tRNA
Chromosomes
Only clearly visible during cell division
DNA molecule is coiled around histone proteins to make chromatin - these proteins fix the
DNA into place
This is then further coiled to form chromosomes
This means a large amount of DNA can be stored in a small space
Homologous chromosomes
Chromosomes on sexually reproducing organisms are found in pairs (one from the mother
and one from the father)
Total number = diploid number
All diploid cells have the same quantity of DNA
Homologous chromosomes carry the same genes at the same loci but may have different
alleles for the gene
Alleles - an alternative form of gene
Genome - the complete set of genes in a cell (including those in mitochondria/chloroplasts)
Proteome - the full set of proteins produced by a genome or by a given type of cell under a
certain set of conditions
, RNA
The RNA Polymer
Made up of a single nucleotide chain
The pentose ribose sugar
A, G, C and U nitrogen containing bases (uracil requires less energy to make than T, and is
less stable but that doesn’t matter as RNA is not needed for long)
A phosphate group
Two types are used in protein synthesis:
1. mRNA (messenger)
2. tRNA (transfer)
tRNA - transfer RNA
Small molecules of around 80 nucleotides
Single-stranded, folded into a clover-leaf shape
The 'stalk' end is where the amino acid attaches
The anticodon end is where 3 bases are exposed
Manufacture in the nucleus but found throughout the cell
mRNA - messenger RNA
A long strand in a single helix
It is formed in the nucleus then moves out through a nuclear pore
In the cytoplasm it associates with a ribosome
It acts as a template from which proteins are built
It is not very stable - it is easily broken down as it is only needed when proteins are being
manufactured
Protein Synthesis - Transcription
Protein Synthesis Overview
1. Transcription - pre-RNA is formed from the DNA strand
2. Pre-mRNA is spliced forming mRNA
3. Translation - mRNA is a template to which complementary tRNA molecules attach
Amino acids held by tRNA link to form the protein
Transcription - the formation of pre-mRNA using DNA as a template
1. DNA helicase (attached to RNA polymerase) breaks the hydrogen bonds between DNA as
template
2. RNA polymerase attaches to the DNA double-helix and moves along template strand and
DNA molecule uncoils, exposing some bases
Free ribonucleotides in the nucleus join to the template DNA strand
3. RNA polymerase forms phosphodiester bonds between the ribonucleotides and forms a
complementary copy of the DNA template strand (same as the nontemplate strand) - an
mRNA strand
4. Behind the forming pre-mRNA strand, the hydrogen bonds between uncoiled strands of DNA
reform, and the strands coil back into a double-helix
5. When RNA polymerase reaches a 'stop' codon, it detaches and formation of pre-mRNA is
complete
What is the function of DNA?
DNA is the code for making polypeptides
Gene - A length of DNA that codes for one polypeptide
It codes for the amino acid sequence of a polypeptide and the functional RNA (rRNA and
tRNAs)
Locus - the position of the gene on a DNA molecule
How does DNA code for a protein?
A codon - a triplet of bases that each amino acid is coded by
Stop/nonsense codons - codons that do not code for amino acids
The genetic code is:
1) Non-overlapping - a specific 'start' codon ensures ribosomes 'read' the mRNA code in groups
of 3
There are also 3 'stop' codons
Essentially each specific codon is read in sequence separate from the codon before it and
the codon after it
Base triplets do not share their bases
2) Degenerate - some amino acids are coded for by more than one codon
Also not all codons code for amino acids
3) Universal - in almost all living organisms the same triplet of DNA bases codes for the same
amino acids
Non-coding DNA
In eukaryotes, a lot of nuclear DNA is non-coding e.g. non-coding multiple repeats, introns,
some code for rRNA and tRNA
Chromosomes
Only clearly visible during cell division
DNA molecule is coiled around histone proteins to make chromatin - these proteins fix the
DNA into place
This is then further coiled to form chromosomes
This means a large amount of DNA can be stored in a small space
Homologous chromosomes
Chromosomes on sexually reproducing organisms are found in pairs (one from the mother
and one from the father)
Total number = diploid number
All diploid cells have the same quantity of DNA
Homologous chromosomes carry the same genes at the same loci but may have different
alleles for the gene
Alleles - an alternative form of gene
Genome - the complete set of genes in a cell (including those in mitochondria/chloroplasts)
Proteome - the full set of proteins produced by a genome or by a given type of cell under a
certain set of conditions
, RNA
The RNA Polymer
Made up of a single nucleotide chain
The pentose ribose sugar
A, G, C and U nitrogen containing bases (uracil requires less energy to make than T, and is
less stable but that doesn’t matter as RNA is not needed for long)
A phosphate group
Two types are used in protein synthesis:
1. mRNA (messenger)
2. tRNA (transfer)
tRNA - transfer RNA
Small molecules of around 80 nucleotides
Single-stranded, folded into a clover-leaf shape
The 'stalk' end is where the amino acid attaches
The anticodon end is where 3 bases are exposed
Manufacture in the nucleus but found throughout the cell
mRNA - messenger RNA
A long strand in a single helix
It is formed in the nucleus then moves out through a nuclear pore
In the cytoplasm it associates with a ribosome
It acts as a template from which proteins are built
It is not very stable - it is easily broken down as it is only needed when proteins are being
manufactured
Protein Synthesis - Transcription
Protein Synthesis Overview
1. Transcription - pre-RNA is formed from the DNA strand
2. Pre-mRNA is spliced forming mRNA
3. Translation - mRNA is a template to which complementary tRNA molecules attach
Amino acids held by tRNA link to form the protein
Transcription - the formation of pre-mRNA using DNA as a template
1. DNA helicase (attached to RNA polymerase) breaks the hydrogen bonds between DNA as
template
2. RNA polymerase attaches to the DNA double-helix and moves along template strand and
DNA molecule uncoils, exposing some bases
Free ribonucleotides in the nucleus join to the template DNA strand
3. RNA polymerase forms phosphodiester bonds between the ribonucleotides and forms a
complementary copy of the DNA template strand (same as the nontemplate strand) - an
mRNA strand
4. Behind the forming pre-mRNA strand, the hydrogen bonds between uncoiled strands of DNA
reform, and the strands coil back into a double-helix
5. When RNA polymerase reaches a 'stop' codon, it detaches and formation of pre-mRNA is
complete
