6.1 Cellular control Flashcards
What is a mutation?
A MUTATION is a random change to the genetic material.
A mutation changes the DNA, involving difference to the structure of number of chromosomes. Mutations occur spontaneously during DNA replication before cell division.
What may cause a mutataion?
Certain chemicals, such as tar in tobacco smoke, and ionising radiation such as UV lights X-rays and gamma rays, may be mutagenic.
What are the different types of genetic mutation?
- Point mutations (one base pair substitutes another), which include, silent, missense and nonsense mutations.
- Indel mutations (one+ nucleotides inserted or deleted causing a frame shift) which include an insertion and a deletion muation.
The structure of a DNA molecule makes it stable and fairly resistant to corruption of the genetic information stored within it. But when may any errors occur?
Errors may occur during replication of a DNA molecule:
- Mutations during mitotic division are somatic mutations so are not inherited by offspring - yet they can cause cancerous tumours to develop
- Mutations during meiotic division are gamete mutations so are inherited by offspring.
What may a mutation effect?
A mutation may affect protein production and function.
What is a silent mutation?
- A point mutation involving a change to the base triplet, where that triplet still codes for the same amino acid is a silent mutation.
Why might a silent muatation occur?
- This can occur as every amino acid involved in protein synthesis, apart from methionine, have more than one base triplet code. This means there is not a change to the sequence of amino acids in a protein: known as the ‘redundancy’ or ‘degeneracy’ of the genetic code.
- This consequently means there is no change in the primary structure of the protein, and therefore the secondary and tertiary structures.
What is a missense mutation?
- A point mutation involving a change to the base triplet sequence, where that triplet codes for a different amino acid is a missense mutation.
- This consequently means there is a change in the primary structure of the protein, and therefore the secondary and tertiary structures, altering its shape and preventing the protein from carrying out its usual function.
Give an example of an disease resulting from a missense mutation.
An example of a disease resulting from a missense mutation is sickle cell anaemia. There is a mutation on the sixth base triplet of the gene for the β-polypeptide chains of haemoglobin: glutamic acid is substituted for valine. This results in deoxygenated haemoglobin crystallising within erythrocytes, causing them to become sickle shaped, blocking capillaries and depriving tissues of oxygen.
Describe a nonsence mutation.
- A point mutation involving a change to the base triplet, where that triplet codes for a termination codon is a nonsense mutation.
- This consequently means a truncated protein is produced which will not function, so it will be degraded within the cell.
Give an example of a disease caused by a nonsense muation.
An example of a disease resulting from a nonsense mutation is the genetic disease Duchenne muscular dystrophy.
Describe an indel mutation.
- If nucleotide base pairs, not in multiple of three, are inserted in or deleted from the gene, all subsequent base triplets will be altered - this is called a frameshift. This is because the code is non-overlapping and read in groups of three bases.
- This consequently means there is a change in the primary structure of the protein, and therefore the secondary and tertiary structures, altering its shape and preventing the protein from carrying out its usual function.
What may an indel mutation cause within the cell?
- If the protein is very abnormal, it will rapidly degrade within the cell.
- An example of a disease resulting from a deletion mutation are some forms of thalassaemia, a haemoglobin disorder.
Why might a frameshift not be caused by an indel mutation?
Insertions or deletions of a triplet of base pairs result in the addition of loss of an amino acid, and not a frameshift.
What is it called when a base triplet is repeated?
Expanding triple nucleotide repeates
Describe an expanding triple nucleotide repeate.
In an expanding triple nucleotide repeat, the number of CAG triplets in the gene with the repeating triplet -CAG CAG CAG- can increase at meiosis from generation to generation. For example, Huntington disease genotype is caused by the number of the CAG triplet repeats rising above a critical number so that the individual will develop the symptoms later in life.
Not all mutations are harmful, give an example of an mutation that could be benifical, or harmful.
- A mutation that is beneficial and non-beneficial is the mutation that gave rise to blue eyes 6000-8000:
- In temperature zones, it may enable people to see better in less bright light
- In areas of high sunlight intensity, the lack of iris pigmentation may lead to lens cataracts.
Why might a mutation be beneficial?
Many mutations have helped to drive evolution through natural selection as different alleles of a particular gene are produced via mutation.
Give two examples of benefical mutations.
- A mutation that is beneficial is the mutation that gave rise to black skin:
- Early humans in Africa would have high concentration of melanin protecting them from sunburn and skin cancer.
- Another mutation that is beneficial is the mutation that gave rise to light skin:
- When humans migrated to colder regions pale skin allowed vitamin D to be made with a lower intensity of sunlight.
Give an example of a muation that is neutral.
Examples of mutation that are neutral are:
- Inability to smell certain flower, including freesias and honeysuckle
- Differently shaped ear lobes.
How is energy and resources conserved by transcriptional gene regulation in prokaryotic cells?
- Enzymes that catalyse the metabolic reactions involved in basic cellular functions are synthesise at a fairly constant rate.
- However, enzymes that may only be needed under specific conditions are synthesised at varying rates, according to the cell’s needs.
- This conserves energy and resources.
Where are the controll sites of the Lac Operon and their functions?
Promotor region (P)
Enzyme RNA polymerase binds to begin transcription of the structural genes
Operating region (lacO)
Repressor protein LacI binds to prevent RNA polymerase from binding to promoter region
What are the structual genes od the Lac Operon and their function?
lacZ
Codes for β-galactosidase
lacY
Codes for lactose permease
What is the regualatory gene, and its function on the lac operon?
I
Codes for repressor protein LacI
What is an example of the effect of the Lac Operon on an E.coli bacterial cell?
- glucose is used as a respiratory substrate.
- If glucose is absent and lactose is present, lactose induces the production of two enzymes:
- Lactose permease: allows lactose to enter the bacterial cell
- -galactosidase: hydrolyses lactose to glucose and galactose.
- The presence of lactose induces the production of the enzyme, not the absence of glucose.
What is the lac Operon?
The lac operon is of a length of DNA, about 6000 base pairs long.
Describe how the lac Operon mechanism prevents constant transcription of the structural proteins.
- The regulatory gene is expressed causing the production of protein repressor molecule LacI
- This molecule binds to the operator preventing RNA polymerase from binding to the promoter region, stopping transcription of the structural genes
- When lactose is added to the culture medium, once there is an absence of glucose, molecules of lactose induce conformational change to the LacI repressor molecules, preventing them from binding to the operator
Describe post-translational gene regualtion.
Activation of proteins by cAMP
Post-translational regulation of gene expression involves the activation of proteins by, for example, phosphorylation.
Cyclic adenosine monophosphate (cAMP) is a secondary messenger involved in activating enzymes, stimulating transcription, by phosphorylation.
Draw or note the process of activating a protein in post-translational gene regulation.
What do Hox genes regulate?
The Hox genes regulate the development of embryos along the anterior-posterior (head-tail) axis by controlling which body parts grow where.
What is caused by the Hox genes misfunctioning?
If Hox genes are mutated, abnormalities can occur such as the antennae on the head of Drosophila developing legs, or mammalian eyes developing on limbs.
Why have hox clusters formed?
At some stage during evolution, the Hox clusters have been duplicated: Hox genes are arranged into clusters and each cluster may contain up to 10 genes. In tetrapods (four-limbed vertebrates), for example, there are four clusters.
How do Hox genes create the desired function?
- Hox* genes encode homeodomain proteins that act in the nucleus as transcription factors and can switch on cascades of activation of other genes that promote mitotic cell division, apoptosis, cell migration and also help to regulate the cell cycle.
- Hox* genes are similar across different classes of animals; a fly can function properly with a chicken Hox gene inserted in place of its own.
What are the stages of apoptosis?
The stages of apoptosis, which is a quick process, are as follows:
- Enzymes break down the cell cytoskeleton
- The cytoplasm becomes dense with tightly packed organelles
- The cell surface membrane changes and small protrusions called blebs form
- Pyknosis occurs which is the condensing of the chromatin
- Karyorrhexis occurs which is the fragmentation of the DNA after the nuclear envelope breaks
- The cells breaks into vesicles
- The debris is ingested by phagocytic cells so that the cell debris does not damage any other cells or tissues.
