GENE AND GENOME:
a gene is a sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain. Together all genes in an individual are known as the genome.
DNA is a chain of nucleotides:
DNA is one type of nucleic acid, called deoxyribonucleic acid. It is a long chain molecule made up of many units called nucleotides or mononucleotides a mononucleotide consists of:
• a deoxyribose sugars
• a phosphate
• a base
A mononucleotide contains three molecules linked together by condensation reactions. They link together by condensation reactions between the sugar of one and the phosphate of the next.
THERE ARE FOUR BASES:
• adenine (A)
• cytosine (C)
• guanine (G)
• thymine (T)
In DNA there are two long strands of nucleotides twisted around to form a double helix. The sugars and phosphates are on the outside, and the bases point inwards and are held together by hydrogen bonds. The bases pair up along the two nucleotides A pairs up with G and T with C.
The code carried by the DNA is a three-base or triplet code, each group of three bases codes for an amino acid there are also start and stop signals.
DNA AND RNA:
RNA has a single strand made of a string of RNA nucleotides.
• RNA contains a ribose sugar instead of a deoxyribose sugar
• In RNA U (uracil) replaces T (the RNA never contains thymine
When a cell divides an exact copy of the DNA must be produced so that each of the daughter cells receives a copy
This process of copying the DNA is called replication.
There are three possible ways DNA could replicate:
• fragmentary replication – all DNA strands are made up of a mixture of original parent DNA nucleotides and new nucleotides
• conservative replication – one DNA molecule has two original parent DNA strands, the other molecule has two new strands
• semi conservative replication – each DNA molecule contains one original parent strand and one new strand
If a test tube containing DNA dissolved in a special density gradient solution is centrifuged. Heavy DNA containing N-15 sinks to the bottom, light DNA containing N-14 collects in a band near the top and DNA of a medium density (containing heave and light nucleotides) is in the middle.
TRANSCRIPTION: (IN THE NUCLEUS)
• DNA double helix unwinds and hydrogen bonds break
• the template strand is used in the production of a messenger RNA molecule
• every triplet code on DNA gives rise to a complimentary codon on messenger RNA
• the completed messenger RNA molecules now leave the nucleus through a pore in the nuclear
envelope and enters the cytoplasm
TRANSLATION: (ON RIBOSOMES)
• ribosomes are small organelles made of ribosomal RNA and protein
• ribosomes are found free in the cytoplasm or attached to endoplasmic reticulum
• a transfer RNA molecule carrying an amino acid has three bases called an anticodon
• these pair with complimentary bases on the mRNA codon
• then the amino acids that the tRNA carry are joined by peptide bonds
MISTAKES IN REPLICATION:
Sometimes DNA replication does not work properly. As the new strand of DNA is being built and incorrect base may slip into place.
SUBSTITUTION
a nucleotide base is replaced with another
INSERTION
an extra nucleotide base is inserted into the sequence causing ‘frameshift’ where all the subsequent bases are shifted down 1 place relative to the twin DNA strand
DELETION
the absence of a nucleotide, causing ‘frameshift’ where all the subsequent bases are shifted back 1 place relative to the twin DNA strand
THE EFFECTS OF THESE MUTATIONS CAN BE NEUTRAL
May occur in phenotypically insignificant strand of DNA
May not result in change of polypeptide primary sequence (because DNA is degenerate)
May result in a change of polypeptide primary sequence that does not affect secondary/tertiary/quaternary structure of protein therefore protein function unaffected
May results in a change of polypeptide secondary/tertiary/quaternary structure but where active site of protein remains the same therefore function still unaffected
HARMFUL
May result in change in final protein shape where protein and active site is deformed and therefore cannot fulfil function
BENEFICIAL
May result in change in final protein shape where the protein performs its function better than it would have without the mutation
This is the basis of natural selection and evolution
The individual is better suited to survival and will pass on the mutation to its offspring E.g. eye colour
Blue eyes was a mutation that occurred about 7000 years ago
In sunny areas, this would be harmful as the retina is more exposed
However in cloudy regions this was beneficial as it enabled people to see better
So the mutation was carried down generations and became widespread
POINT MUTATION
Mutations can affect 1 nucleotide base, or more than one adjacent bases A point mutation is where only one base is affected There are 3 types: silent, nonsense, missense
SILENT MUTATION
No change in amino acid sequence of polypeptide
MISSENSE MUTATION
The mutation changes the code for 1 amino acid
1 amino acid in the sequence is changes
NONSENSE MUTATION
The mutation changes the code turning the triplet into a stop codon
Instructs the end of polypeptide synthesis
The polypeptide is shorter than it would normally be
CHROMOSOMAL NON-DISJUNCTION
one pair of chromosomes fails to separate, so the gamete and zygote has an extra chromosome (e.g. Down’s syndrome).
SICKLE CELL ANEMIA:
mutation in the gene that codes for one of the polypeptide chains in hemoglobin (the pigment in red blood cells which carries oxygen around the body. The base adenine replaces thymine at one position along the chain. A protein valine is made instead of glutamic acid. The red blood cells are distorted, and
the sickle shaped cells carry less oxygen and can block blood vessels.
CYSTIC FIBROSIS:
Mutations affect CFTR protein in different ways.