Higher Biology

Unit 1: DNA and the Genome

3. Genome and Mutations

Notes

The Genome

  1. What is an organism's genome?
  1. What are genes?
  1. 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.


Regulation of transcription

  1. Protection

  1. 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.

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


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.

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.

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.

Research Topic

With reference to gene mutations write notes on Lactose tolerance

With reference to gene mutations write notes on Sickle Cell Trait

Gene mutation questions

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

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

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

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.

Questions on polyplody

Problem Solving Question

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