cellular control Flashcards
what is a mutation
is a random change to the genetic material
what is a gene mutation
a change to the DNA
how might mutations occur
- spontaneously during DNA replication before cell division
- certain chemicals such as tar in tabacco smoke
- ionising radiation such as UV light , x rays and gamma rays
mutations associated with mitotic divisions are ….
somatic mutations and are not passed to offspring . they may be associated with the development of cancerous tumours
mutations associated with meiosis and gamete formation may be ….
inherited by offspring
what are the two main classes of DNA mutation
- point mutation
- insertion or deletion
what is a point mutation
one base pair replaces (is substituted for ) another
what is an insertion or deletion mutation
one or more nucleotides are inserted or deleted from a length of DNA , these may cause a frame shift
what are the tree types of point mutations
silent
missense
nonsense
describe silent mutations
when a change to the base triplet, where that triplet still codes for the same amino acids. the primary structure of the protein , and therefore the secondary and tertiary structure is not altered
describe missense mutations
a change to the base triplet sequence that leads to a change in the amino acid sequence in a protein
within a gene , such a point mutation may have a significant effect on the protein produced . the alteration to the primary structure leads to a change to the tertiary structure of the protein , altering its shape and preventing it from its usual function
what does sickle cell anaemia result from
from a missense mutation on the sixth base triplet of the gene for the b-polypeptide chain of haemoglobin , the amino acid valine , instead of glutamic acid , is inserted at this position . this results in deoxygenated haemoglobin crystallising within erythrocytes , causing them to become sickle shaped , blocking capillaries and depriving tissue of oxygen
what are nonsense mutations
a point mutation may alter a base triplet, so that it becomes a termination (stop) triplet. this particularly disruptive point mutation results in a truncated protein that will ot function . this abnormal protein will most likely be degraded withn the cell.
what genetic disease can be a result of nonsense mutation
duchenne muscular dystrophy
describe insertions or deletions
if nucleotide base pairs , not in multiples of three , are inserted into the gene or deleted from the gene , because the code is non over lapping and read in groups of three bases , all the subsequent base triplets are altered. this is a frame shift . when the mRNA from such a mutated gene is translated , the amino acid sequence after the frame shift is severely disrupted . the primary sequence of the protein and subsequently the tertiary structure , is much altered . consequently the protein cannot carry out its normal function . if the protein is very abnormal , it will be rapidly degraded within the cell
insertions or deletions of a triplet of base pairs results in
the addition or loss of an amino acid , and not a frame shift
what results from frameshifts due to the deletion of nucleotide bases
some forms of thalassaemia , a haemoglobin disorder
describe expanding triplet nucleotide repeats
some genes contain a repeating triplet ( eg CAG CAG CAG) in an expanding triple nucleotide repeat , the number of CAG triplets increases at meiosis and again from generation to generation
what disease results from an expanding triple nucleotide repeat , and how
huntington disease
if the number of repeating triplets sequence goes above a certain critical number , then the person with that genotype will develop the symptoms of huntingtons disease later in life .
are mutations always harmful
no , many are beneficial and have helped to drive evolution through natural selection
describe 2 useful mutations
- a mutation that gave rise to blue eyes , could be harmful in areas where sunlight intensity is high , due to lack of pigments . however in more temperate zones it could enable people to see better in less bright light
- humans in Africa would have black skin , the high concentrations of melanin protecting them from the sunburn and skin caner. when humans migrated to temperate regions , a paler skin would be an advantage , enabling vitamin D to be made with a lower light intensity of sunlight
some mutations may be neutral , what does this mean
being neither beneficial nor harmful
what are examples of neural mutations
- inability to smell certain flowers
- differently shaped ear lobes
define exon
the coding , or expressed region of DNA
define intron
the non coding region of DNA
define operon
a group of genes that function as a single transcription unit, first identified in prokaryotic cells
define transcription factor
protein or short non coding RNA that can combine with a specific site on a length of DNA and inhibit or activate transcription of the gene
regulation of gene expression at the transcriptional level in prokaryotic cells include
the lac operon
the lac operon
- the bacterium E. coli normally metabolises what
glucose
lac operon
- if glucose is absent and the disaccharide lactose is present , lactose induces the production of what two enzymes and what do they allow to do
- lactose permease = allows lactose to enter the bacterial cell
- b-galactosidase = which hydrolyses lactose to glucose and galactose
the lac operon consists of ….
- of a length of DNA , about 6000 pairs long
- containing an operator region lac0
- next to the structural genes lacZ and lacy that code for enzymes b galactosidase and lactose permease
- next to the operator region , lac0 , is the promotor region , p , to which the enxyme RNA polymerase binds to begin transcription of the structural genes lacZ and lacy
describe how the lac operon works in the absence of lactose (glucose )
- repressor protein binds to operator
- RNA polymerase cant bind to promotor
- no transcription
- no genes turned on
describe how the lac operon works in the presence of lactose
- lactose binds to repressor protein
- changes shape and cant bind to the operator
- RNA polymerase binds to promotor
- genes switched on
- allows for transcription and translation
- proteins are coded for
describe the regulation of gene expression at the transcriptional level of eukaryotic cells
transcription factors slide along a part of the DNA molecule , seeking and binding to their specific promotor regions . they may then aid or inhibit the attachment of RNA polymerase to the DNA , and activate or supress transcription of the gene
what does post transcriptional gene regulation involve
introns and exons
describe what does post transcriptional gene regulation
all the DNA of a gene , both introns and exons, is transcribed . the resulting mRNA is called primary mRNA . 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 enzyme 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 . some genes can be spliced in different ways . a length of DNA with its introns and exons can , according to how its spliced , encode more than one protein
post translational level of gene regulation involves
- the activation of proteins
- many enzymes are activated by being phosphorylated
describe post translational level of gene regulation steps
1- a signalling molecule , such as the protein hormone glucagon , 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 KPA
6- activated KPA catalyses the phosphorylation of various proteins , hydrolysing ATP in the process . this phosphorylation activates man enzymes in the cytoplasm
- KPA may phosphorylate another protein ( CREB, cAMP response element binding )
-this then enters the nucleus and act as a transcription factor , to regulate transcription
define apoptosis
programmed cell death
how is apoptosis different to nercrosis
apoptosis is different to cell death due to trauma , which involves hydrolytic enzymes
define homeobox sequence
sequence of 180 base pairs found within genes that are involved in regulating patterns of anatomical development in animals , fungi and plants
hox genes are found in ….
homeobox genes are found in ….
- animals
- animals , plants ans fungi
describe homeobox gene sequences
are involved in controlling the anatomical development so that all the structure develop in correct location , according to the body plan
what is the homeodomain
- transcription factors which switch genes on or off
describe the homeobox sequence
- is a stretch of 180 DNA base pairs encoding a 60 amino acid sequence called a homeodomain sequence, within a protein
describe homeodomain structure
the homeodomain sequence can fold into a particular shape and bind to the DNA called H-T-H
- it consists of two alpha helices connected by one turn .
- part of the homeodomain amino acid sequence recognise that TAAT sequence of the enhancer region of gene to be transcribed
what did scientists discover in 1984
demonstrated that the homeobox sequence , first identified in 1983 within the homeotic genes of the fruit fly , also exist in the fruit fly . the base sequences in these homeobox sequences were very similar in both species . this informed scientists that these gene sequences are crucial for the regulation of development and differentiation in organisms
describe how hox genes control body plan development in animals
the hox genes regulates the development of embryos along the anterior posterior axis. they control which body parts grow where . if hox genes are mutated , abnormalities can occur such as antennae on the head developing as legs or mammalian eyes developing on limbs
hox genes code
homeodomain proteins that act in the nucleus as transcription factors and can swith on a cascade of other genes that promote mitotic cell division
how are regulators regulated
hox genes are regulated by other genes called gap genes and pair rule genes. in turn, these genes are regulated by maternally supplied mRNA from the egg cytoplasm
describe the sequence of events during apoptosis
1- enzymes break down the 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 the DNA breaks into fragments
5- the cell breaks into vesicles that are ingested by phagocytic cells, so that cell debris does not damage any other cells or tissues . the whole process happens very quickly
describe the control of apoptosis
many cell signals help to control apoptosis . some of these signalling molecules may be released by cells when genes 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
apoptosis and development
- apoptosis is an integral part of plant and animal tissue development . extensive proliferation of cell types is prevented by pruning through apoptosis , without releasing of any hydrolytic enzymes that could damage surrounding tissues
- during limb development , apoptosis causes the digits to separate from each other
- apoptosis removes ineffective or harmful lymphocytes during the development of the immune system
- the rate of cells dying should equal the rate of cells produced by mitosis
- not enough apoptosis leads to the formation of tumours
- too much apoptosis leads to cell loss and degeneration
