2.11 Flashcards
a gene is a
length of DNA that codes for one polypeptide
alleles are
different forms of a gene
changes in DNA that makes up one gene will result in
new allele
DNA is made of a double helix and is like a ‘twisted-rope-ladder’ with pairs of
organic bases forming the ‘rungs’ of the ladder
Four bases:
Adenine
Thymine
Cytosine
Guanine
The sequence of these four bases A,C,G,T carries the coded information that cells use to make
polypeptides and proteins
Whenever DNA replicates, there is a risk that a
mistake may be made
Almost all mistakes in copying DNA are corrected by the
DNA polymerase enzyme that copy the DNA
Uncorrected errors in the DNA are called
mutations
A mutation is a
permanent change in the amount or arrangement of a cell’s DNA
Some mutations will have no effect, because they occur in so-called
‘junk’ DNA
The mutation where one base pair has been deleted is known as
deletion
one base pair missed out changes the way the code is
read
deletion mutation is known as
frame-shift mutation
deletion is called frame-shift mutation as
all the triplets are changed after the point of mutation
frame-shift mutation produces a very big change in the
primary structure of the protein that is coded for by the mutated allele
as a result of frame-shift mutation the resulting protein will have a very different
tertiary structure and is likely to be non-functional
(EXAMPLE OF DELETION) most common mutation of CFTR gene is the deletion of just 3 nucleotides that code for the amino acid
As a result the CFTR protein coded for has an
phenylalanine
altered tertiary structure
the mutation where one base has been changed to another is known as
substitution
substitution will only affect a single triplet, therefore, just one
amino acid in the whole protein will be altered
Example of substitution is
sickle-cell anaemia
Sickle-cell anaemia results from haemoglobin S, which differs from normal haemoglobin by a change in just
one amino acid in 2 of its 4 polypeptide chains
Sickle-cell anaemia: the triplet code CTT (codes for glutamate) in the DNA has changed to CAT (codes for valine). These amino acids have very different R-groups, making haemoglobin S much less
soluble than normal haemoglobin
It is possible for a substitution mutation to occur, but without causing any change in the protein coded for as there are more
triplet codes than amino acids, so some amino acids have more than one DNA triplet code
Example of no change in protein due to substitution is if triplet code CTT changed to CTC, there would be no change in the amino acid coded for, this is because they both code for
glutamate
When substitution results in a base that still codes for the same amino acid as the original base it is a
silent mutation
Silent mutations are possible due to the
degenerate nature of the genetic code
Silent mutations would have no effect on the
final polypeptide
changes in the base sequence of genes are usually referred to as
point mutations
chromosome mutations may also occur. these are changes in the
number and structure of chromosomes
an example of chromosome mutation is
non-disjunction
non-disjunction is
the failure of one or more pairs of homologous chromosomes to separate normally during nuclear division, usually resulting in an abnormal distribution of chromosomes in the daughter nuclei
when a substitution of a base occurs leading to a premature ‘stop-codon’ being coded for is known as a
non-sense mutation
a non-sense mutation would lead to the
premature end to the synthesis of a polypeptide and the final protein would be almost certainly not able to function normally
stop codons are:
UAA
UAG
UGA
A mutation that occurs when a change in a base leads to a different amino acid being coded for is known as
mis-sense
a mis-sense mutation is due to
substitution
mis-sense mutation causes the polypeptide to have a single amino acid that is different and the effect is determined by the role of the
amino acid in the final polypeptide. it maybe that it was involved in the formation of bonds to structure the active site of an enzyme - if this changes so does active site, therefore substrate no longer fits.
DNA mutations occur naturally at a very
low rate
The rate at which mutations occur may be increased by environmental factors known as
mutagens
a mutagen is an
agent, such as radiation, which causes genetic mutation
Mutagens include:
Radiation - UV, X-rays, alpha and beta
Chemicals - nitrous acid
Carcinogens - diesel exhaust and cigarette smoke
