cellular control

Question 1

what are the two main classes of DNA mutation

Answer 1

• Substitution (point mutation)
• Insertion and deletion(ideal) this is where one or more nucleotides are inserted or deleted from a length of DNA this causes a frameshift

Question 2

describe point mutations

Answer 2

• Genetic code consists of nucleotide base triplets within the DNA, during transcription of a gene the code is copied to a length of MRNA as codons, the sequence is therefore a copy of the sequence of base triplets on the gene

Question 3

what are the three types of point mutations

Answer 3

• silent
• missense
• non sense

Question 4

describe silent

Answer 4

• All amino acids involved in protein synthesis apart from methionine have more than one base triplet code, this reduces the effect of point mutations as they do not cause a change in the sequence of amino acids in a protein this is called the redundancy or degeneracy of the genetic code
• A point mutation involving the change to the base triplet where the triplet still codes for the same amino acids is a silent mutation, this means that the primary structure does not change so therefore the secondary and tertiary structure does not change

Question 5

describe missense mutations

Answer 5

• A change in the base triplet that leads to a change in the amino acid sequence in a protein is a missense mutation
• Have a significant effect on the protein produced, there is alteration to the primary structure that leads to change in the tertiary structure so it can not carry out its function
• Sickle cell anaemia results from a missense mutation on the 6th base triplet of the gene for the Bpolypeptide chains of haemoglobin, the amino acid valine instead of glutamic acid is inserted in this position resulting in deoxygenated haemoglobin crystallising within the erythrocytes causing them to become sickle shaped, blocking capillaries and depriving tissues of oxygen

Question 6

describe non sense mutations

Answer 6

• Point mutation may alter a base triplet so that it becomes a termination triplet, this is the disruptive point mutation results in truncated protein that will not function and this protein will be degraded within the cell
• Genetic disease of Duchenne muscular dystrophy is the result of nonsense mutation

Question 7

describe indel mutations

Answer 7

Insertions and deletions

• If nucleotide base pairs not in multiples of three are inserted in the gene or deleted from the gene, because code is non overlapping and read in groups of three bases, all subsequent base pairs are altered and this is a frameshift
• When MRNA from a mutated gene is translated the amino acid sequence after the frameshift is disrupted therefore the primary structure and secondary structure is altered so protein cannot function
• Thalassaemia – haemoglobin disorder results from the frameshifts due to deletions of nucleotide bases
• Insertions of deletions of a triplet base pair results in the addition or loss of an amino acid and not in a frame shift

Expanding triple nucleotide repeats

• In expanding triple nucleotide repeat the number of CAG triplets increase at meiosis and again from generation to generation
• Huntingtons disease results from an expanding triple nucleotide repeat, if the number of repeating CAG sequences goes above a certain critical number then the person with that genotype will develop the symptoms of Huntington disease later in life

Question 8

what are the beneficial effects of mutations

Answer 8

• Driven evolution through natural selection
• Have rise to blue eyes – may be harmful in areas where light intensity is high as the lack of iris pigmentation could lead to lens cataracts however in more temperate zone sit could enable people to see better in les slight
• Early humans in Africa had black skin – high concentration of melanin protecting them from sunburn and skin cancer – when they migrated to temperate regions a paler skin would be an advatages and enabled vitamin D to be made with lower intensity of sunlight – protects from rickets, heart disease and cancer
• Some are unharmful such as the inability to smell certain flowers and differently shaped ear lobes

Question 9

describe the regulatory mechanisms in prokaryotic cells

Answer 9

• Enzymes that catalyse the metabolic reactions involved in basic cellular functions are synthesised at a constant rate, enzymes that may only be needed under specific conditions are synthesised at varying rates according to the need of the cell

Question 10

describe transcriptional level Lac Operon

Answer 10

• The bacterium E.coli normally metabolises glucose as a respiratory substrate, however if glcose is absent and lactose is present, lactose induces the production of two enzymes, these are lactose permease which allows lactose to enter the bacterial cell and B-galactosidase which hydrolyses lactose to glucose and galactose
• The lack operon consist of a length of DNA containing an operator region lacO next to the structural genes lacZ and lacY that code for the enzyme B-galactosidase and lactose permase respectively
• Next to the operator region lacO is the promoter region P to which the enzyme RNA polymerase binds to begin transcription of the structural genes lacZ and lacY
• Operator region and promoter region are the control sites
• A small distacen away from the operon is the regulatory gene I that codes for a repressor protein LacI when the regulatory gene is expressed the repressor protein produced binds to the operator preventing RNA polymerase from binding to the promoter region
• The repressor protein therefore prevents the genes lacZ and lacY from being transcribed so the enzymes for lactose metabolism are not made and the genes are off
• When lactose is added to the culture medium, once all the glucose has been used, molecules of lactose bind to the lacI repressor protein molecules this alters the shape of the LacI repressor protein and prevents it from binding to the operator, the RNA polymerase enzyme can then bidn to the promoter region and begin transcribing the structural genes into MRNA that will then be translated into the two enzymes, thus lactose induces the enzymes needed to break it down

Question 11

describe transcription factors

Answer 11

• Every cell in a eukaryotic organism has the same genome but because different cells use it differently and function differently, this is the basis of cell differentiation
• Transcription factors are proteins or short non-coding pieces of RNA that act within the cells nucleus to control which genes in a cell are being turned off or on, transcription factors slide along a part of the DNA molecule seeking and binding to their specific promoter regions, they may then aid or inhibit the attachment of RNA polymerase to the DNA and activate or suppress transcription of gene
• Make sure that different genes are expressed or suppressed and are involved in regulating the cell cycle
• Tumour suppressor genes and proto-oncogenes help regulate cell division via transcription factors – mutations to these genes can lead to uncontrolled cell division or cancer

Question 12

what are the post transcriptional gene regulations

Answer 12

introns and exons

Question 13

describe interons and exons

Answer 13

• Within a gene there are non-coding regions of DNA called introns which are not expressed. They separate the coding or expressed regions which are called exons
• All DNA of a gene both introns or exons is transcribed, the resulting mrna is called primary mrna is then edited and the RNA introns (lengths corresponding to the DNA introns) are removed, the remaining Mrna exons corresponding to the DNA exons are joined together
• Endonuclease enzymes may be involved in the editing and splicing processes
• Some introns may themselves encode proteins and some may become short non-coding lengths of RNA involved in gene regulation
• Genes can be spliced in different ways, a length of DNA with its introns and exons can encode more than one protein according to how its spliced

Question 14

describe post translation level of gene regulation

Answer 14

• Post-translation regulation of gene expression involves the activation of proteins
• Many enzymes are activated by being phosphorylated
• Camp is important second messgender
  1. Signalling molecule such as a protein hormone binds to a receptor on the plasma membrane of the target cell
  2. This activates a transmembrane protein which then activates a G protein
  3. The activated G protein activates adenyl cyclase enzymes
  4. Activated adenyl cyclase enzymes catalyse the formation of many molecules of camp from ATP
  5. Camp activates protein kinase A (PKA)
  6. Activated PKA catalyses the phosphorylation of various proteins hydrolysing ATP in the process, this phosphorylation activates many enzymes in the cytoplasm for example those that convert glycogen to glucose
  7. PKA may phosphorylate another protein CREB, camp response element binding
  8. This then enters the nucleus and acts as a transcription factor to regulate transcription

Question 15

what is a hoemobox gene sequence

Answer 15

• A Hoemobox gene sequence is a sequence of 180 base pairs (excluding introns) found within genes that are involved in regulating patterns of anatomical development in animals, fungi and plants,

Question 16

what is the function of the hoemobox gene sequence

Answer 16

• Make sure all structures develop to the correct location according to the body plan
• They encode a 60-amino acid sequence called a homeodomain sequence within protein, this can fold into a particular shape and bind to DNA regulating the transcription of adjacent genes, these proteins are transcription factors and act within the cells nucleus
• The shape that these homeodomain containing proteins fold into is called H-T-H and consist of two alpha helices(H) connected by one turn (T)
• Part of the homeodomain amino acid sequence recognises the TAAT sequence of the enhancer region (a region that initiates or enhances transcription) of a gene to be transcripted

Question 17

describe similar and highly convserved hoemobox sequences

Answer 17

• 1984 scientists demonstrated that the homeobox sequence first identified in 1983 within the homeotic genes of the fruit fly also exsits in the mouse, moreover the base sequences in these homeobox sequences were very similar in both species, therefore this informed scientistst that these gene sequences are crucial for the regulation and development and differentiation in organisms
• There is a subset of homeobox genes called Hox genes that were found only in animals and are involved in the formation of anatomical features in correct locations of body plna
• This lead to a new branch of biology called evolutionary development
• Molecular evidence indicates that homeobox genes are present in cridaria which means that these genes arose before the Palaeozoic era and before bilaterally symmetrical organisms evolved which indicates that these genes first arose in an early ancestor which gave rise to each of these types of organisms and has since been conserve, similarity extends in all organism to date

Question 18

what is the role of Hox genes in controlling body plan development

Answer 18

• Hox genes regulate the development of embryos along the anterior posterior axis, they control which body parts grow where
• If the Hox gene is mutated abnormalities can occur such as the antennae on the head as developing legs or mammalian eyes as developing on limbs
• Hox genes are arranged into clusters and each cluster may contain up to 10 genes, in tertrapods inclduign mammals and humans there are 4 clusters, and at some stage during evolution the Hox clusters have been duplicated
• In early embryonic development Hox genes are active and expressed in order along the anterior-posterior axis of the developing embryo, sequential and temporal order of the gene expression corresponds to the sequential and temporal development of various body parts
• Hox genes encode homeodomain protein 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

Question 19

how are the regulators regulated

Answer 19

• the Hox genes are regulated by other genes called gap genes and pair rule genes, these genes are regulated by maternally supplied mrna from the egg cytoplasm

Question 20

how is mitosis regulated

Answer 20

• Mitosis is regulated with the help of homeobox and Hox genes, this ensures that each new daughter cell contains the full genome and is a clone of the parent cell
• During cell differentiation some of the genes in a particular type of cell are switched off and not expressed

Question 21

what is apoptosis

Answer 21

• this is programmed cell death and is not due to cell death due to trauma which involves hydrolytic enzymes

Question 22

describe the sequence of events during apoptosis

Answer 22

1. Enzymes breakdown the cell cytoskeleton
2. The cytoplasm becomes dense with tightly packed organelles
3. The cell surface membrane changes and small protrusions called blebs form
4. Chromatin condenses, the nuclear envelope breaks and DNA breaks into fragments
5. The cell breaks into vesicles that are ingested by phagocytic cells so that the cell debris does not damage any other cells or tissues and the whole process happens quickly

Question 23

how is apoptosis controlled

Answer 23

• Many cell signals help control apoptosis, some of these signalling molecules may be released by cells when genes that are involved in regulating the cell cycle and apoptosis respond to internal cell stimuli and external stimuli such as stress, these signalling molecules include cytokines from cells of the immune system, hormones, growth factors and nitric oxide
• Nitric oxide can induce apoptosis by making the inner mitochondrial membrane more permeable to hydrogen ions and dissipating the proton gradient, proteins are released into the cytoplasm where they bind to apoptosis inhibitor proteins allowing apoptosis to occur

Question 24

describe the role of apoptosis and development

Answer 24

• Integral part of plant and animal tissues development
• Prevents extensive proliferation of cell types by pruning through apoptosis without release of any hydrolytic enzymes that could damage surrounding tissues
• Apoptosis causes the digitis to separate from each other in limb development
• Removes ineffective or harmful T-lympocytes during the development of the immujne ystem
• Rate of cell dying should equal the rate of cells produced by mitosis not enough apoptosis leads to formation of tumours and too much apoptosis leads to cell loss and degeneration
• Cell signalling helps get the right balance