1
Q
What are genetic mutations
A
- Mutation is a random change to the genetic material
- Some involve changes to structure or number of chromosomes
- Gene mutation is change to DNA
2
Q
What can cause genetic mutations
A
- May occur spontaneously during DNA replication before cell division
- Certain chemicals e.g. Tar in tobacco smoke, and ionising radiation e.g. UV light, X-rays and Gamma rays, may be mutagenic
3
Q
When do mutations occur
A
- Structure of DNA makes it stable and fairly resistant to corruption of genetic information stores within it
- Errors may occur, however, during DNA replication (where they are most likely top occur in cell cycle
4
Q
Describe the passing on of mutations to offspring
A
- Mutations associated with mitotic division are somatic mutations- not passed int offspring. May be associated with development of cancerous tumours
- Mutations associated with meiosis and gamete formation may be inherited by offspring
5
Q
Name 2 categories of genetic mutation
A
- point
- indel
6
Q
Outline point mutations
A
The genetic code consists of nucleotide base triplets within the DNA. During transcription of a gene, this code is copied to a length of mRNA as codons, complementary to the base triplets on the template strand of the length of DNA .The sequence of codons on the mRNA is therefore a copy of the sequence of base triplets on the gene (coding strand of the DNA).
Point mutations are a base substitution.
7
Q
name 3 types of point mutation
A
Silent, Missense, Nonsense
8
Q
Describe silent mutations
A
- All amino acids involved in proteins synthesis apart from methionine) have more than one base triplet code
- This reduces the effect of point mutations, as they do not always cause a change to the sequence of amino acids in a protein
- Often called the ‘redundancy’ or ‘degeneracy’ of the genetic code
- Involves change to base triplet where that triplet still codes for the same amino acid
- Primary structure of the protein, and therefore the secondary and tertiary structure, is not altered
9
Q
Describe missense mutations
A
- Change to base triplet sequence that leads to change in amino acid sequence in a protein
- Within a gene, point mutation may have significant effect on protein produced- alteration to primary structure leads to change to tertiary structure of the protein- altering its shape and preventing it from carrying out its function
- May not make large difference if in non-essential part of protein or substitutes amino acid with similar properties
- Example- Sickle cell anemia results from missense mutation on the sixth base triplet of the gene for the beta-polypeptide chains 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 tissues of oxygen
10
Q
Describe missense mutations
A
- Change to base triplet sequence that leads to change in amino acid sequence in a protein
- Within a gene, point mutation may have significant effect on protein produced- alteration to primary structure leads to change to tertiary structure of the protein- altering its shape and preventing it from carrying out its function
- May not make large difference if in non-essential part of protein or substitutes amino acid with similar properties
- Example- Sickle cell anemia results from missense mutation on the sixth base triplet of the gene for the beta-polypeptide chains 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 tissues of oxygen
11
Q
Describe nonsense mutations
A
- Point mutation may alter a base triplet so it becomes a termination (stop) triplet
- Particularly disruptive point mutation
- Results in truncated protein that won’t function
- This abnormal protein will most likely be degraded within the cell
- Example- genetic disease Duchenne muscular dystrophy
12
Q
Outline indel mutations
A
Cause a frameshift in sequence of amino acids
13
Q
Name 2 types of indel mutations
A
Insertions (incl. expanding triple nucleotide repeats), Deletions
14
Q
Describe insertions and deletions
A
- If nucleotide base pairs, not in multiples of 3, are inserted in the gene or deleted from the gene, because the code is non-overlapping and read in groups of three bases, all the subsequent base triplets are altered
- This is a frameshift
- When the mRNA from such a mutated gene is translated, the amino acid sequence after the frameshift 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 will result in the addition or loss of an amino acid, not in a frameshift
- Example- some forms of thalassaemia, a haemoglobin disorder, result from frameshifts due to deletions of nucleotide bases
15
Q
Describe expanding triple nucleotide repeats
A
- Some genes contain a repeating triplet such as -CAG CAG CAG-
- In an expanding triple nucleotide repeat, the number of CAG triplets increases at meiosis and again from generation to generation.
- Example- Huntington 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
16
Q
Describe he harmfulness of genetic mutations
A
- Many mutations are beneficial- help drive evolution through natural selection e.g. mutation for blue eyes may enable people to see better in less bright light in temperate zones - Different alleles of a particular gene are produced via mutation
- Some mutations can appear to be neutral- neither beneficial nor harmful- such as those in humans that cause inability to smell certain flowers (incl. freesias and honeysuckle), and differently shaped earlobes
17
Q
Describe the lac operon (prokaryotic cells)
A
- contains P and lacO (control sites, and lacZ and lacY (structural genes)
18
Q
Describe control sites on the lac operon
A
- P- promoter region- RNA polymerase binds here which beings transcription of structural genes lacZ and lacY
- lacO- operator region- binds to repressor protein
19
Q
Describe structural genes on the lac operon
A
- lacZ- codes for β-Galactosidase
- LacY- codes for lactose permease
20
Q
Describe the regulatory gene (regulation of gene expression at transcriptional level in prokaryotic cells)
A
- Codes for repressor protein (LacI)
- When expressed, repressor protein produced binds to lacO
21
Q
Describe what occurs when e.coli is grown on glucose, and why this is advantageous
A
1) I (regulatory gene) codes for repressor protein
2) Repressor protein binds to lacO
3) Means RNA polymerase can’t bring to promotor region
4) Prevents transcription and thus translation- lacZ and lacY can’t be expressed-genes are off- β-Galactosidase and lactose permease not made
This is advantageous because E.coli can directly respire glucose- would be waste of amino acids and energy to produce enzymes for respiring lactose
22
Q
Describe what happens when E.coli are grown on lactose
A
Lactose is the inducer:
1) Lactose binds to repressor protein
2) Changes shape of repressor protein- prevents it from binding to lacO
3) Means RNA polymerase can bind to promoter
4) Means mRNA can be transcribed from lacZ and lacY- can be translated into β.Galactosidase and lactose permease
23
Q
Describe regulation of gene expression and the transcriptional level in eukaryotic cells
A
- For a gene to be transcribed, the enzyme RNA polymerase (needed to produce the mRNA) must attach to the control site for that gene- attaches to the promoter.
- In eukaryotes, RNA polymerase can only attach to the promoter with the help of a transcription factor
- Transcription factors slide along a part of the DNA molecule, seeking and binding to their specific promotor regions
- Once it’s bound to the promoter it makes it either easier or harder for RNA polymerase to then also bind to the promoter of the gene
- The transcription factor (or factors as there can be more than 1) therefore either activates or supresses transcription of the structural gene
24
Q
Describe transcription factors
A
- a protein or short non-coding piece of RNA
- within the cells nucleus
- controll which genes in a cell are turned on/off
- some are involved in regulating the cell cycle
- tumour suppressant genes and porto-oncogenes help regulate cell division via transcription factors
- mutations to these genes can lead to uncontrolled cell division or cancer
- around 8% of the genes in the human genome encode transcription factors
- many genes have their promoter regions some distance away, along the unwound length of DNA, but, because of how the DNA can bend, the promoter region may not be too far away spatially
25
Define introns and exons
- Introns- non-coding regions of DNA, which are not expressed
- Exons- coding units of DNA which are expressed
26
Describe Post-transcriptional gene regulation
- All the DNA of a gene (including introns and exons) are transcribed
- This results in primary mRNA
- Primary RNA s 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 process
- Some genes can be spliced in different wats- a length of DNA with its introns and exons can, according to how it is spliced, encode more than one protein
27
What happens to some inions that have been spilled out during post-transcriptional gene regulation
- may themselves encode proteins
- some may become short non-coding lengths of RNA involved in gene regulation (or transcription factors)
28
Describe Post-translational level of gene regulation
- Involves the activation of proteins
- Many enzymes are activated by being phosphorylated
29
Process of post-translational gene regulation
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 PKA (protein kinase A).
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.
30
What is apoptosis
- programmed cell death
- different from cell death due to trauma (necrosis), which involves hydrolytic enzymes
31
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 DNA breaks into fragments
5) the cell breaks into vesicles tat are ingested by phagocytic cells, so that cell debris doesn't damage any other cells pr tissues
- the whole process happens quickly
32
Describe how apoptosis is controlled
- controlled by many cell signals
- 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 and external cell stimuli e.g. stress
- 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 inti the cytoplasm where they bind to apoptosis inhibitor proteins, allowing apoptosis to occur
33
Describe the importance of apoptosis
- integral part of plant and animal tissue development
- extensive proliferation of cell types is prevented pruning any apoptosis, without release of any hydrolytic enzymes that could damage surrounding tissues
- during limb development, apoptosis causes the digits to separate from each other
- removes ineffective or harmful T-lympohocytes during the development of the immune system
- in children between 8-14 years, 20-30 billion cells per day apoptoses, and around 50-70 million in adults
34
Describe what the rate of apoptosis should be like
- Should equal the rate of mitosis
- not enough leads to formation of tumours
- too much leads to cell loss and degeneration
- cell signalling pays a crucial role in maintaining the right balance
35
Term for the anatomical development of organisms
Morphogenesis
36
What controls morphogenesis
Homeotic genes
37
Describe the contents of homeotic genes
- Homeotic genes contain a subset of homeobox genes
- The homeobox gene contains a homeobox sequence
38
Describe the homeobox sequence
- 180 base pair length of DNA
- The homeobox sequence codes for a homeodomain
- highly conserved in plants, animals and fungi- same base pairs- conserved genes are genes that have remained unchanged throughout the evolution of different descendent species- crucial for the regulation of development an differentiation in organisms- discovery led to development of evolutionary development
39
Describe the homeodomain
- Coded for by the homeobox sequence
- specific sequence of 60 amino acids within the synthesised protein
- The homeodomain folds into a specific shape- can bind to DNA
- 3 alpha helices
- The 2nd and 3rd helix create a helix-turn-helix (H-T-H)- two alpha helixes connected by a short loop of amino acids
- H-T-H allows the protein to bind to DNA- regulates the transcription of nearby genes
- The proteins that contain a homeodomain are therefore transcription factors
- 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 transcribed
40
What is the name of homeobox genes only found in animals
Hox genes
41
Describe hox genes
- Homeobox genes only found in animals
- involved in the correct positioning of body parts in an organism
- Hox genes are found in all bilaterian animals- suggests that Hox gens existed in the common ancestor of all bilaterians
- similar across different classes of animals- a fly can function properly with a chicken hox gene inserted in place of its own
42
Describe the arrangement of hox genes
- Several hox genes are found next to each other on a chromosome- arranged in clusters
- each cluster may contain up to 10 genes
- in tetrapods (4 limbed vertebrates) including mammals, there are 4 clusters
- At some stage during evolution, the hox genes have been duplicated
- In some animal lineages including vertebrates, Hox genes have been duplicated- resulting in multiple Hox clusters
43
Describe muttaions in hox genes
- Homeotic mutations- leads to body parts developing in wrong place on body
44
Describe the expression of hox genes
- Hox genes get expressed in early embryonic development along the anterior-posterior (head-tail) axis of the organism
- Order of hox genes on the chromosomes matches the expression patterns along the embryo- spatial linearity
- Expression occurs intemporal order- expression starts with anterior Hox genes
- Called co-linearity- the sequential and temporal order of the gene expressions corresponds to the sequential and temporal development of the various body parts
- When each hox gene is expressed, it codes for a specific hox protein- acts as a transcription factor
45
Descripbe the action of the transcription factor (coded for by hox gene)
- act in the nucleus
- binds to specific regions of DNA- switches on a specific set of genes in each segment
- can switch on cascades of activation of other genes that promote mitotic cell division, apoptosis, cell migration and differentiation, and also help to regulate the cell cycle
- These activated genes promote correct development of each body segment by regulating mitosis, apoptosis, and cell differentiation
46
Describe what regulates hox genes
- Regulated by other genes called gap genes and pair-rule genes
- In turn, these are regulated by maternally supplied mRNA from the egg cytoplasm
47
Describe the role of the homeobox/hox gene in mitosis
- regulates mitotic divison
- ensures each new daughter cell contains the full genome and is a clone of the parent cell
