Download Structure of Proteins

Higher Biology
Unit 1: DNA and the Genome
3. Genome and Mutations
Notes
0
The Genome
1. What is an organism's genome?
2. What are genes?
3. Is all of the genome made of genes?
Role of non-protein-coding sequences
1. Regulation of transcription
Some non-coding sections of DNA are used to regulate transcription. This means
they can bind proteins which promote or prevent transcription of a gene. The
diagram below illustrates how a sequence of DNA can regulate transcription of a
gene.
aka
Transcription
factor
Regulation of transcription
1
2. Protection
Telomere
The telomere
prevents fraying at
the ends of the
chromosome.
3. Transcription of non-translated RNA
- tRNA (transfer RNA, which carries specific amino acids to the
-
ribosome during translation.
rRNA (ribosome RNA), which together with protein forms the
ribosome.
2
Mutations
A mutation is the change in the structure or amount of an organism’s genetic material
Mutations can have various effects on the individual:
Lethal – many mutations are lethal and embryos are non – viable( results in
spontaneous abortion)
Harmful –non lethal mutations are expressed and cause deleterious effects eg
Downs Syndrome, Cystic Fibrosis
Useful –occasionally mutations arise which are useful, particularly in a new
environment e.g antibiotic resistance in bacteria
(THIS CAN BECOME THE RAW MATERIAL FOR EVOLUTION)
Mutations occur randomly and spontaneously
They can be increased by ( induced) by other factors called mutagenic agents
X-rays
UV rays
Chemical agents – benzene, carbon tetra chloride, mustard gas
3
GENE MUTATIONS
Point mutations
Substitutions result in a change that only affect one codon. The resulting mutation can be missense,
nonsense or splice-site mutations:



Missense – these substitutions change the codon from one amino acid to another
amino acid. This may result in the shape of the protein changing and not being
able to carry out its function. However, some changes in amino acid may not have
any effect.
Nonsense – these substitutions change the codon from an amino acid to a stop
codon. The shortened protein is generally non-functional or its function is
affected.
Splice-site – these substitutions affect the boundaries between exons and introns
(splice sites). A mutation here can prevent splicing at that site. This will result in a
very different protein being made due to the post transcriptional processing.
4
5
Frameshift mutation
Insertions and deletions result in what are called frame shift mutations.
This means that when the groups of three nucleotides (codons) are read at the ribosome, the codon
reading frame is shifted by one codon and so completely different amino acids are coded for.
6
The resulting protein is usually non-functional. Insertions may also increase the number of times a
nucleotide sequence is repeated.
Mutations to regulatory sequences
Mutations may occur at regions other than those coding for proteins.
The expression of genes can be affected if mutations occur in the sequences involved in regulating gene
expression.
7
Research Topic
With reference to gene mutations write notes on Lactose tolerance
With reference to gene mutations write notes on Sickle Cell Trait
8
Gene mutation questions
9
10
CHROMOSOME STRUCTURE MUTATIONS
Deletion
A break in two places can result in a shorted chromosome which lacks certain genes.
e.g Cri du chat syndrome





Deletion on chromosome 5
Causes mental retardation
Small head and
Widely spaced eyes
Cry resembles a cat -hence name
11
Duplication
Caused by the insertion of a segment of genes causing a set of genes to be repeated.
Detrimental effect – duplication of oncogenes could cause cancer
Beneficial effect – the extra copy has freedom to undergo point mutations and could
result in an improvement.
Research Topic
With reference to duplication mutations write notes on alpha and beta globins in
haemoglobin
12
Inversion
A break in two places and the reversal of the sequence when re-joining.
Often results in nonviable gametes
Translocation
A section of one chromosome breaks off and attaches to another chromosome.
Often results in nonviable gametes
13
Polyploidy

A change in the number of chromosomes where polyploid individual has three or
more times the haploid number of chromosomes.

Fairly common in plants, but rare in animals.

Polyploidy occurs through spindle failure during mitosis (so that a 2n individual
becomes 4n), or when all the chromosomes undergo non-disjunction
simultaneously during meiosis.

Polyploid plants usually show an increase in size and resistance to disease. Many
modern crop plants are polyploid. They give greater yields that their diploid
relatives.

Polyploidy can be induced by a chemical called colchicine.
14
Questions on polyplody
1
2
3
Which of the following statements regarding polyploid
is correct?
A
B
C
D
15
It is more common in animals than in
plants
It is the term used to describe the four
haploid cells formed at the end of meiosis
It can produce individuals with increased
vigour
It always results from non-disjunction of
chromosomes
Problem Solving Question
16