AQA A2 Biology unit 5 - Genetics

revision notes for genetics topic



2 types:

1) mRNA -> made in nucleus during transcription, carries genetic code from nucleus to the cytoplasm

2) tRNA -> found in the cytoplasm where its involved in translation, carries amino acids that are used to make proteins

DNA and RNA are similar but there are a few differences:

DNA                                        mRNA                                       tRNA

double stranded                    single stranded                          single stranded

Deoxyribose sugar               ribose sugar                               ribose sugar

A,T,C,G                                 A,U,G,C                                       A,U,G,C

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Protein Synthesis

Transcription (mRNA copy of gene made)

  • RNA polymerase attaches to start gene
  • hydrogen bonds between strands break -> one strand acts as a template
  • RNA polymerase lines up free nucleotides by specific base pairing = complemetary copy
  • bases joined together forming a mRNA molecule
  • RNA polymerase separates strands until STOP signal reached


pre mRNA contain introns (non coding) and exons (coding), introns removed during splicing leaving just the coding sections of DNA.


  • ribosome attches to mRNA strand
  • tRNA molecules carry amino acids to ribosome -> match with complementary codon
  • second tRNA molecule attaches, amino acids from a chain
  • process continues unil STOP codon reached = polypeptide chain (protein)
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Regulation of transcription and translation

transcriptional factors control the transcription of target genes

- they move from the cytoplasm to the nucleus

- bind to specific DNA sites near to target gene

- some act as acivators (speed up) or repressors (slow down) 

e.g. Oestrogen:

binds to a receptor called an oestrogen receptor forming a oestrogen receptor complex

complex moves from cytoplasm to nucleus 

binds to specific DNA site close to target gene

acts as a repressor or activator


short, double stranded RNA molecule that interfers with gene expression, affects translation, binds to bases and cuts up sections of DNA = prevents expression as it can't be translated

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Mutation, Genetic Disorders and Cancer

Mutations are changes in the base sequence of DNA

2 main types:

1) substitution = one base sunsituted for another

2) Deletion = one base is deleted, causes a 'frame shift'

not all mutations affects the order of amino acids

- genetic code is degenerate -> amino acids coded by more than one DNA triplet,  therefore substitutions won't always cause a mutation, however deletions will

Mutagenic agents

increase the rate of mutations by:

- acting as a base -> substitute for a base during DNA replication

- altering bases -> delete or alter bases causing a change in structure

- changing the structure of DNA -> causes problems during replication

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hereditary mutations can cause genetic disorders and some cancers

if a gamete containing a mutation for a genetic disorder is fertilised then the mutation will be present in the offspring = called a hereditary mutation

 e.g Cystic Fibrosis

acquired mutations can cause cancer (mutations that occurr after fertilisation)

if mutations occurr in the genes that control cell division it can cause uncontrolled cell division:

1) Tumour suppressor gene -> normally they slow down cell division by producing proteins that stop cells dividing, mutation prevents the protein from being produced = uncontrolled cell division

2) Proto oncogene -> normally they stimulate cell division, mutation causes them to be over active = uncontrolled cell division

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Stem Cells

stem cells can mature into any type of body cell = totipotent

found in embryos and bone marrow

in mature animals on a small number of stem cells are present and can only divide into a few types of cells = multipotent

stem cells become specialised during protein synthesis, only certain genes are transcribed and translated


found in places where the plant is growing e.g. roots and shoots

all stem cells are totipotent

means you can grow a whole plant from just a small cutting

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Stem Cells and Medicine

Stem cells can be used to replace cells damaged by illness or injury

e.g. SCID (disorder that effects the immune system)

- white blood cells don't work properly

- stem cells taken from bone marrow used to replace other cells

- don't contain faulty gene so they will differentiate into functional white blood cells

other diseases:

spinal cord injuries, heart disease, bladder conditions, respiratory diseases and organ transplants

benefits of using stem cells:

1) save lives

2) Improve quality of life

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