OCR Biology A Level: Module 6.1.1 - Cellular Control

What are the two main types of gene mutation?

Point mutations and indel mutations.

What are point mutations and what are the three types of point mutation?

Point mutations are when bases are substituted. This can alter one triplet and therefore one amino acid. The three types are:

- Silent - due to the degenerate nature of the genetic code, the mutation has no impact on the protein produced.

- Missense - a base substitution causes one incorrect amino acid to be added. This produces a non-functioning protein.

-Nonsense - a base substitution occurs that causes a stop codon to be put in an incorrect place. This then codes for a shortened length of mRNA and a truncated, non-functioning protein.

What are indel mutations and what are the two main types?

Indel mutations involve the insertion or deletion of a nucleotide. The two main types are:

- Frameshift - the insertion or deletion of a base causes the triplets to be read wrong, producing a non-functioning protein.

- Expansion of triple nucleotide repeats - some gene sequences have repeating triplets, such as -CAG CAG CAG -. If the number of these repeats increases, a non-functioning protein will be produced.

What are the effects of mutations?

Mutations can be neutral (have no impact) (e.g. differently shaped earlobes), beneficial (can help an organism) (e.g. the amount of melanin in the skin) or can be harmful (e.g. tumour growth)

How is gene expression regulated at the transcriptional level?

- Lac operon - the lac operon is a set of genes that are interpreted together found in prokaryotes, such as E. coli. When glucose is present, a repressor protein binds to the operator region of the operon, preventing RNA polymerase binding to the promoter region and beginning transcription of the DNA. When all glucose is used up, and lactose is present, lactose molecules can bind to the repressor protein removing it from the DNA and allowing RNA polymerase to bind and produce enzymes necessary to respire lactose.

- transcription factors - small lengths of non-coding RNA or proteins bind to certain regions of DNA preventing the binding of RNA polymerase. This effectively allows cells to switch on/off genes and allows them to differentiate.

How is gene expression regulated at the post-transcriptional level?

Once mRNA has been produced, it is edited by enzymes in the nucleus. Primary mRNA, produced from directly transcribing DNA, contains regions that are not expressed in proteins. These regions, introns, are removed and the remaining sections, exons, are joined together to produce mature mRNA. This process is called splicing and allows the same genetic code to produce different proteins depending on the splicing locations.

How is gene expression regulated at the post-translational level?

Gene expression is regulated after proteins have been produced by the need for some proteins to be activated by cAMP (cyclic AMP). cAMP phosphorylates the proteins and is itself activated: cell signalling molecule -> receptor -> transmembrane protein -> g protein -> adenyl cyclase -> cAMP formed -> activates protein kinase A -> protein kinase A catalyses phosphorylation of proteins.

Why is mitosis important in the control of the development of the body plan?

Mitosis is the mechanism by which cells divide and allows organisms to grow. Mitosis allows cells to differentiate and divide.

Why is apoptosis important in the control of the development of the body plan?

Apoptosis is important in body form of digits (fingers and toes) as these are initially connected by cells joining them are then killed by apoptosis. It is also important in maintaining body form in adults. When the rate of mitosis is greater than the rate of apoptosis, tumours begin to grow.

What factors affect the genes that regulate the cells cycle?

These genes are affected by both internal and external factors. These include things such as nitric oxide - which triggers apoptosis; cytokines from immune system cells; thalidomide a drug that causes mutations in the hox genes; stress.

What are homeotic genes?

Regulatory genes that control body plan development

What are homeobox genes?

Homeobox genes are genes that contain a homeobox. The homeobox is a highly conserved region of DNA found in all organisms that codes for a 60-amino acid length of polypeptide capable of binding to DNA.

What is the homeodomain?

A 60-amino acid part of a protein that is highly conserved and is coded for by the homeobox. A domain is a specific conserved region of a protein that can function independently of the rest of the protein and is able to bind to DNA and so can act in gene regulation.

What are hox genes?

Hox genes are a family of homeobox genes. They are found only in animals and control the development of body plan on the anterior-posterior axis. This means that if hox genes are mutated, eyes may form on other limbs or arms and legs may grow in unusual places.

What is the mechanism of apoptosis?

- enzymes break down the cytoskeleton

- the cytoplasm becomes dense with organelles

- cell surface membrane changes and blebs form

- chromatin condenses and the nuclear envelope and DNA break down into fragments

- the cell breaks into vesicles that are ingested by phagocytic cells so that cell debris does not damage other cells.