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2.1.1 How DNA Codes for Proteins
What is a Gene?
· A gene is a length of DNA that codes for one or more polypeptides (proteins).
· The genome of an organism is the entire DNA sequence of that organism.
· Each gene occupies a specific place or locus on the chromosome.
The Genetic Code
· The sequence of nucleotide bases on a gene provides a code, with instructions for the construction of a protein. The genetic
code had a number of characteristics:
It is a triplet code: a sequence of 3 nucleotide bases codes for an amino acid.
It is a degenerate code: all amino acids have more than one code.
Some codes indicate `stop': end of the polypeptide chain.
· Proteins are assembled in the cytoplasm, at ribosomes a cop of the genetic code has to be made which can pass through a
pore in the nuclear envelop to the cytoplasm. mRNA is this copy.
1. A gene to be transcribed unwinds + unzips by dipping into the nucleolus.
2. Activated RNA nucleotides bind to complementary bases catalysed by RNA polymerase.
3. 2 extra phosphates from the activated nucleotides are released this releases energy to bind nucleotides.
4. The mRNA produced is a copy of the base sequence on the coding strand of the length of DNA.
5. The mRNA is released from the DNA and passed out of the nucleus, through a pore in the nuclear envelope, to a ribosome.…read more
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· Translation is the 2nd stage of protein synthesis (after transcription). It is when the amino acids are assembled into a
· The amino acids are assembled into the sequence dictated by the sequence of codons on the mRNA.
· This happens at ribosomes which may be free in the cytoplasm or bound to the rough endoplasmic reticulum.
· Ribosomes are made in the nucleolus from ribosomal RNA and protein. Each is made up of 2 subunits and there is a groove
where the mRNA can fit. The ribosome can then move along the mRNA, reading the code + assembling the amino acids in
the correct order to make a protein.
· tRNA is made in the nucleus and passes into the cytoplasm.
· They are lengths of RNA that fold into hairpin shapes + have 3 exposed bases at one end where a particular amino acid can
· At the other end are 3 unpaired nucleotide bases an anticodon.
· Each anticodon can bind temporarily with its complementary codon.
How the Polypeptide is Assembled
1. A molecule of mRNA binds to a ribosome 2 codons are attached to the
small subunit of the ribosome + exposed to the large subunit. Using ATP
energy + an enzyme, a tRNA forms hydrogen bonds with this codon.
2. A 2nd tRNA binds to the second exposed codon with its complementary anticodon.
3. A peptide bond forms between the 2 amino acids (using an enzyme).
4. The ribosome now moves along the mRNA, reading the next codon. A 3rd tRNA
brings another amino acid, and a peptide bond forms between it and the dipeptide.
The first tRNA leaves and is able to collect + bring another amino acid.
5. The polypeptide chain grows until a stop codon is reached.…read more
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· A mutation is a random change to the genetic material to the amount of, or arrangement of, the genetic material in a cell.
· Chromosome mutation involves a change to the structure of a chromosome.
· DNA mutation involves a change to genes due to changes in nucleotide base sequences. There are 2 main classes of DNA
POINT MUTATIONS: one base pair replaces another (substitutions).
INSERTION/DELETION MUTATIONS: one or more nucleotide pairs are inserted or deleted from a length of DNA.
Point Mutation Deletion Mutation Insertion Mutation
· Many genetic diseases are the result of DNA mutations such as sickle-cell anaemia and cystic fibrosis.
· In most cases of cystic fibrosis, the mutation is the deletion of a triplet of base pairs.
· Sickle-cell anaemia results from a point mutation on codon 6 of the gene for haemoglobin. This causes an amino acid to be
inserted instead of another one.…read more
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Mutations with Neutral Effects
· If a gene is altered by a change to its base sequence, it becomes another version of the same gene it is an allele of the
gene. It may produce no change to the organism if:
The mutation is in a non-coding region of the DNA.
It is a silent mutation: although the base triplet has changed, it still codes for the same amino so the protein is unchanged.
· If the mutation does cause a change to the structure of the protein and causes a different characteristic, but the changed
characteristic gives no particular advantage or disadvantage to the organism, the effect is neutral.
Mutations with Harmful/Beneficial Effects
· Early humans in Africa almost certainly had dark skin.
· The pigment melanin protected them from the harmful effects of UV light but they could still synthesise vitamin D.
· Any humans with mutations to some of the genes determining skin colour, producing paler skin, would have burned and
suffered from skin cancer.
· As humans migrated to more temperate climates, the sunlight was not intense enough to cause enough vitamin D to be
made by those with dark skins.
· Humans with mutations producing paler skin would have an advantage over those with dark skin as they could synthesise
more vitamin D.
· Depending on the environment, the same mutation for paler skin can be beneficial or harmful.
· Without genetic mutations there would be no evolution.…read more