Topic 8.1

What is a gene mutation?

A change in the base sequence of DNA. It can occur spontaneously during DNA replication (interphase).

What is a mutagenic agent?

A factor that increases the rate of mutation, e.g. UV light or alpha particles.

How can a gene mutation lead to a non-functional protein or enzyme?

• Alters DNA base triplet sequence → mRNA codons change

• Changes amino acid sequence in polypeptide

• Affects hydrogen/ionic/disulphide bonds between amino acids

• Changes tertiary structure (shape)

• For enzymes: active site changes → substrate can't bind → no enzyme-substrate complex

What is a substitution mutation?

One base/nucleotide is replaced with another in the DNA sequence.

What is an addition mutation?

One or more bases/nucleotides are added to the DNA base sequence.

What is a deletion mutation?

One or more bases/nucleotides are removed from the DNA base sequence.

What is a duplication mutation?

A sequence of DNA bases is repeated or copied in the DNA.

What is an inversion mutation?

A sequence of DNA detaches, flips around, and reattaches in reverse order at the same location.

What is a translocation mutation?

A sequence of DNA detaches and inserts into a different location, possibly on another chromosome.

Why might a gene mutation not affect the order of amino acids?

• The genetic code is degenerate, so different codons can code for the same amino acid.

• Some mutations occur in introns, which do not code for amino acids.

Why might a change in amino acid sequence not be harmful?

• May not alter the tertiary structure if bonding isn't affected

• Could improve protein properties, giving a selective advantage

What is a frameshift mutation?

• Caused by mutations that add/remove nucleotides not in multiples of 3

• Changes reading frame → shifts codons → changes all downstream amino acids

• Can produce a stop codon → premature termination → shorter polypeptide

What are stem cells?

Undifferentiated cells that:

1. Divide by mitosis to replace themselves indefinitely

2. Differentiate into specialised cell types

How do stem cells become specialised during development?

• Stimuli activate some genes (via transcription factors)

• Only certain genes are transcribed → mRNA → proteins

• These proteins permanently alter the cell’s structure and function

What are totipotent cells?

• Found in early embryos

• Can divide and differentiate into any type of body or extra-embryonic cell (e.g. placenta)

What are pluripotent cells?

• Found in embryos (after initial divisions)

• Can divide and differentiate into most cell types (not placenta)

What are multipotent cells?

• Found in mature mammals

• Can become a limited number of cell types

• E.g. bone marrow cells → different types of blood cells

What are unipotent cells? Give an example.

• Found in mature mammals

• Can become one specific cell type only

• E.g. heart unipotent cells → cardiomyocytes

How can stem cells be used to treat human disorders?

Transplanted to divide & differentiate into healthy cells to replace faulty ones

Examples of how stem cells can be used to treat human disorders?

• Type 1 diabetes: create insulin-producing islet cells

• Bone marrow transplant: for sickle cell disease or leukaemia

  1. Destroy patient’s faulty bone marrow

  2. Transplant healthy stem cells → produce healthy blood cells

What are induced pluripotent stem (iPS) cells and how are they made?
A:

• Adult somatic cells (e.g. fibroblasts) are reprogrammed using transcription factors

• These factors stimulate pluripotency genes

• Cells then divide and can differentiate like embryonic stem cells

: Why are iPS cells useful in treatment?

• Can produce healthy cells for same patient → no immune rejection

• Avoids use of embryos (fewer ethical concerns)

Give 4 arguments for the use of stem cells in treatment

Can save lives / improve quality of life
IVF embryos would be discarded anyway
iPS cells avoid immune rejection
iPS cells don’t require embryo destruction; adult can give can give consent

Give 3 arguments against the use of stem cells in treatment

Ethical issues: embryonic stem cells require destruction of a potential life
Immune rejection risk; requires immunosuppressants
Stem cells could divide uncontrollably → cancer/tumours