DNA, genes and chromosomes
Gene – a section of DNA on a chromosome coding for one or more
polypeptides
A gene is a section of DNA located at a particular position (locus) on a DNA
molecule. The gene is a base sequence of DNA that codes for:
- The amino acid sequence of a polypeptide which makes up a protein
- A functional RNA e.g. transfer RNA and ribosomal RNA.
The genetic code
Not all the genome codes for proteins – the non-coding sections of DNA are
called introns and the coding regions are called exons.
There are four nucleotide bases which code for 20 different amino acids.
Scientists were able to conclude that each amnio acid is coded for by one or
more combination of triplets. There are therefore 64 possible triplets (43)
meaning that each amino acid is represented by more than one triplet. Each
triplet is called a codon.
Features of the genetic code:
- The genetic code is non-overlapping meaning that each triplet is only
read once and triplets don’t share any bases.
- Genetic code is degenerate meaning that more than one triplet codes
for the same amino acid, this reduces the number of mutations which
are mistakes in the base sequence such as base deletion, insertion or
substitution.
- The genetic codes contains start and stop codons.
- The code is universal, each codon codes for the same amino acid in all
organisms
- A codon is always read in one particular direction along the DNA strand
A change in the base sequence of DNA alters the amino acid sequence and the
protein therefore it can have various effects. Some mutations are harmful such
as the mutation which leads to production of sticky mucus and causes cystic
fibrosis or sickle cell anaemia in which a mutated form of haemoglobin distorts
the shape of red blood cells.
Different types of DNA are found in eukaryotic and prokaryotic cells:
- In eukaryotic cells the DNA molecules are found in the nucleus and are
long and linear. They are associated with proteins called histones to
form structures called chromosomes. Chromosomes are visible at the
, start of cell division and is the result of the DNA being tightly coiled
around the histones. Finally in eukaryotic cells the mitochondria and
chloroplasts contain DNA.
- In prokaryotic cells, e.g. bacteria, the DNA is short and circular and not
associated with proteins, so they don’t have chromosomes.
Chromosome structure:
- They’re only visible as distinct structures when a cell is dividing
- DNA is a double helix, the helix is wound around histones to fix it in
position, the DNA-histone complex is then coiled
- The coil is then looped and further coiled before being packed into the
chromosome; therefore a lot of DNA is condensed into a single
chromosome.
- A chromosome contains one molecule of DNA which contains many
genes along its length in specific loci.
Humans have 46 chromosomes in every cell in the body that are arranged into
homologous pairs. A homologous pair consists of two chromosomes that carry
the same genes. They are not identical as they can carry different alleles of the
same gene. The homologous chromosomes come together in the fusion of the
sperm and egg to create a diploid cell. The 23rd chromosome determines the
sex of the individual with female having two X chromosomes but males having
and X and Y chromosome. An allele is described as an alternative form of the
same gene, with every gene existing in two or more possible forms. Every
individual inherits two allele of every gene from their parents.
DNA and protein synthesis
There are two stages of protein synthesis. Transcription which occurs in the
nucleus and involves DNA and mRNA and translation which involves mRNA,
tRNA and ribosomes. During transcription, a section of a DNA strand is
transcribed into mRNA which is then translated into a polypeptide chain
formed of amino acids. mRNA only includes the exons.
Genome – the complete set of genes in a cell
Proteome – the full range of proteins that a cell is able to produce
So the genome codes for the proteome.
Gene – a section of DNA on a chromosome coding for one or more
polypeptides
A gene is a section of DNA located at a particular position (locus) on a DNA
molecule. The gene is a base sequence of DNA that codes for:
- The amino acid sequence of a polypeptide which makes up a protein
- A functional RNA e.g. transfer RNA and ribosomal RNA.
The genetic code
Not all the genome codes for proteins – the non-coding sections of DNA are
called introns and the coding regions are called exons.
There are four nucleotide bases which code for 20 different amino acids.
Scientists were able to conclude that each amnio acid is coded for by one or
more combination of triplets. There are therefore 64 possible triplets (43)
meaning that each amino acid is represented by more than one triplet. Each
triplet is called a codon.
Features of the genetic code:
- The genetic code is non-overlapping meaning that each triplet is only
read once and triplets don’t share any bases.
- Genetic code is degenerate meaning that more than one triplet codes
for the same amino acid, this reduces the number of mutations which
are mistakes in the base sequence such as base deletion, insertion or
substitution.
- The genetic codes contains start and stop codons.
- The code is universal, each codon codes for the same amino acid in all
organisms
- A codon is always read in one particular direction along the DNA strand
A change in the base sequence of DNA alters the amino acid sequence and the
protein therefore it can have various effects. Some mutations are harmful such
as the mutation which leads to production of sticky mucus and causes cystic
fibrosis or sickle cell anaemia in which a mutated form of haemoglobin distorts
the shape of red blood cells.
Different types of DNA are found in eukaryotic and prokaryotic cells:
- In eukaryotic cells the DNA molecules are found in the nucleus and are
long and linear. They are associated with proteins called histones to
form structures called chromosomes. Chromosomes are visible at the
, start of cell division and is the result of the DNA being tightly coiled
around the histones. Finally in eukaryotic cells the mitochondria and
chloroplasts contain DNA.
- In prokaryotic cells, e.g. bacteria, the DNA is short and circular and not
associated with proteins, so they don’t have chromosomes.
Chromosome structure:
- They’re only visible as distinct structures when a cell is dividing
- DNA is a double helix, the helix is wound around histones to fix it in
position, the DNA-histone complex is then coiled
- The coil is then looped and further coiled before being packed into the
chromosome; therefore a lot of DNA is condensed into a single
chromosome.
- A chromosome contains one molecule of DNA which contains many
genes along its length in specific loci.
Humans have 46 chromosomes in every cell in the body that are arranged into
homologous pairs. A homologous pair consists of two chromosomes that carry
the same genes. They are not identical as they can carry different alleles of the
same gene. The homologous chromosomes come together in the fusion of the
sperm and egg to create a diploid cell. The 23rd chromosome determines the
sex of the individual with female having two X chromosomes but males having
and X and Y chromosome. An allele is described as an alternative form of the
same gene, with every gene existing in two or more possible forms. Every
individual inherits two allele of every gene from their parents.
DNA and protein synthesis
There are two stages of protein synthesis. Transcription which occurs in the
nucleus and involves DNA and mRNA and translation which involves mRNA,
tRNA and ribosomes. During transcription, a section of a DNA strand is
transcribed into mRNA which is then translated into a polypeptide chain
formed of amino acids. mRNA only includes the exons.
Genome – the complete set of genes in a cell
Proteome – the full range of proteins that a cell is able to produce
So the genome codes for the proteome.
