A-Level Biology OCR Notes

Google rating

4.8

Trustpilot rating

4.7

View All Resources

Mutations

Gene mutations are changes to the base sequence or quantity of DNA within a gene or section of DNA.
Gene mutations occur spontaneously during the process of DNA replication.
The mutation rate is increased by mutagenic agents, which are chemical, physical or biological agent that causes mutations e.g. UV light

Type of Mutation	Description
Addition	Addition of one or more nucleotides
Deletion	Removal of one or more nucleotides
Substitution	A nucleotide is replaced by a different nucleotide
Inversion	A sequence of bases is separated and then reattached in the inverse order
Duplication	One or multiple bases are repeated
Translocation	A piece of DNA breaks off and doesn't reattach to itself or its homologous pair.

Some mutations may only affect a single codon, changing a single amino acid in a protein, therefore the protein may remain functional. Other may have no effect on protein structure due to the genetic code being degenerate (silent mutation).
Mutations such as insertions and deletions can cause frame shifts, changing all the codons and amino acids downstream from the mutation. This results in a unfunctional protein

Transcription Factors

In eukaryotes, transcription of target genes can be regulated by DNA-binding proteins (transcription factors). They can be help RNA polymerase bind (activators) or prevent it binding (repressors),
In prokaryotes, lactose induces the production of lactose permease and β-galactosidase. Which allows lactose to enter the cell and hydrolyses lactose to glucose and galactose, respectively.
The lac I gene in the Lac operon codes for a repressor protein, which binds to the operator sequence, preventing RNA polymerase from binding to the promoter. When lactose is present, it binds to the repressor protein, allowing RNA polymerase to carry out transcription of the enzymes.

Post-Transcriptional Regulation

In eukaryotes, gene expression can also be regulated after the gene has been transcribed but before it is translated- post-transcriptional modification
Transcription results in the synthesis of pre-mRNA which must be modified to form mature mRNA, as it contains introns and exons
Introns are sections of DNA that code for proteins
Exons are sections of DNA that do not code for proteins
Splicing occurs which involves the removal of introns and joining the exons back together. It is catalysed by an enzyme-RNA complex called a spliceosome.

The process of splicing does not always just remove the introns from the pre-mRNA – some of the exons may also be removed
Depending on which exons are removed, different combinations of mature RNA are formed (alternative splicing) This means that one gene can produce many different mRNAs and code for many different proteins.

Regulating Translation

The final level of regulating gene expression occurs after the protein has been synthesised – this is post-translational control
Proteins can be modified in many ways, including:
- Carbohydrate chains can be attached to form glycoproteins
- Lipids can be attached which target the protein to cell membranes
- Proteins can be activated by the addition of a phosphate group
Many proteins can be activated by signalling from hormone.

Control of Body Plan Development

DNA contains homeotic genes which regulate morphogenesis
A subset of homeotic genes are called homeobox genes which contain a 180 base pair length of DNA called a homeobox
The homeobox sequence is highly conserved in plants, animals and fungi
The homeobox sequence codes for a specific sequence of 60 amino acids within the synthesised protein called a homeodomain
The homeodomain sequence folds into a specific shape consisting of three α-helices
The helix-turn helix shape allows the protein to bind to DNA and regulate the transcription of nearby genes
The proteins that contain a homeodomain are therefore transcription factors
A subset of homeobox genes are called Hox genes
Hox genes are homeobox genes that are only found in animals. They involved in the correct positioning of body parts in an organism
In some animal lineages, including vertebrates, Hox genes have been duplicated, resulting in multiple Hox clusters
When a Hox gene is mutated, body parts end up developing in the wrong place on the body (e.g. legs in place of the antennae in flies) - these are called homeotic mutations
Hox genes are expressed in early embryonic development along the anterior-posterior (head-tail) axis of the organism

Hox genes show colinerality, where the order of the genes on the chromosomes matches their temporal order and spatial order of expression.

Apoptosis is the process of carrying out programmed cell death. It is important to remove old, damaged or unwanted cells.
Apoptosis involves enzymes breaking down the cytoskeleton, forming blebs. Chromatin condenses and the nuclear envelope and the DNA breaks up. The cell is broken into apoptotic bodies, which are phagocytosed by macrophages

Apoptosis plays an important role during development, removing the surplus cells allowing different body parts to be shaped e.g. allowing separation of digits of hands and feet.
Apoptosis is controlled by internal signals in response to stimuli such as cellular stress or by external signalling molecules such as cytokines which bind to the target cell to initiate apoptosis.

Google rating

4.8

Trustpilot rating

4.7