6.1 Flashcards
what is a mutation
a mutation is a random change to the genetic material. some mutations involve changes to the structure or number of chromosomes. A gene mutation is a change to DNA
when may mutations occur
mutations may occur spontaneously during DNA replication before cell division.
-during S phase
-during transcription
name some chemicals that may be mutagenic ( increases the chance of mutation)
-tar in tobacco smoke
-ionising radiation such as UV light
- X-rays and gamma rays
mutations associated with what will not be passed onto offspring
mutations associated with mitotic division and somatic mutations will not be passed onto offspring
Though, they may be associated with cancerous tumours
mutations associated with what may be passed onto offspring
mutations associated with meiosis and gamete formation may be passed onto/inherited by offspring
what may gene mutations affect
gene mutations may affect protein production and function
what are the two main classes of DNA mutation
-point mutation- one base pair replaces another (there are 3 types)
-insertion or deletion (indel) mutation- one or more nucleotides are inserted and deleted from a length of DNA. These may cause a frameshift.
what are DNA base triplets then called when a length of DNA is converted to RNA
codons
mRNA is a copy of which strand of DNA
the coding strand
what are the 3 types of point mutations
-silent
-missense
-nonsense
whats a silent mutation
a silent mutation is a type of point mutation that involves a change to the base triplet , where that triplet still codes for the same amino acid. The primary structure, and therefore the secondary and tertiary structure is not altered.
This is due to degeneracy (most amino acids have more than one base triplet code)
what is a missense mutation
a missense mutation is a change to the base triplet sequence that leads to a change in the amino acid sequence in a protein (e.g. TTA is now TCA which codes for a different protein).
-this will have a significant affect on the protein produced because the alteration of the primary structure will lead to alterations in the secondary structure which will alter its shape and make it unable to carry out its function
give an example of a missense mutation
sickle cell anaemia results from a missense mutation on the sixth base triplet of the gene for the B-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
what is a nonsense mutation
- a nonsense mutation may alter a triplet, so that it becomes a termination (stop) triplet. This particularly disruptive point mutation results in a truncated protein that will not function. This abnormal protein is likely to be degraded within the cell.
what genetic disease is caused by a nonsense mutation?
the genetic disease Duchenne muscular dystrophy is the result of a nonsense mutation
what causes a frameshift
both insertions and deletions cause a frameshift
What are Indel Mutations? (deletions and insertion)
genetic code is non-overlapping. This means it is strictly read in groups of three bases.
a Indel mutation occurs when nucleotide base pairs (one base) are inserted in the gene or deleted from the gene.
this results in a Frameshift and all the subsequent codons will be read incorrectly, producing all different amino acids. This will result in a completely different protein. the tertiary structure is much altered and if very abnormal it will be rapidly degraded within the cell
Indel mutations can also occur when a whole codon is inserted or deleted. This results in one additional or one less amino acid being produced.
what is an example of a disorder resulting from frameshifts
some forms of thalassaemia, a haemoglobin disorder, results from frameshifts due to deletions of nucleotide bases.
What can occur from Expanding triple nucleotide repeats?
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
If the number of repeating CAG sequences goes above a certain critical number, the person will exhibit Huntington’s disease later in life.
how are alleles affected by mutation
different alleles of a particular gene are produced via mutations
what are examples of ‘good’ mutations
people having blue eyes (a mutation) allows people to see better in less bright light (e.g. in temperate regions)
- Darker skin to protect from sun damage and skin caner
- Lighter skin to allow Vitamin D synthesis (vitamin D protects us from rickets and heart disease and cancer)
what are some examples of neutral mutations (neither harmful or beneficial)
-inability to smell certain flowers, including freesias and honeysuckle
-differently shaped ear lobes
at what rate are enzymes synthesised that are needed to catalyse metabolic reactions involved in basic cellular functions
synthesised at a fairly constant rate
at what rate are enzymes synthesised that are only needed for specific conditions
synthesised at varying rates according to the needs of the cell
what are examples of bad mutations
-cancer
-genetic dieases
what does E.coli use as a respiratory substrate
The bacterium E.coli normally metabolises glucose as a respiratory substrate
However, if glucose is absent and the disaccharide lactose is present, lactose induces the production of two enzymes.
what enzymes does the presence of lactose induce in E.coli (when glucose is absent)
lactose induces the production of two enzymes:
-lactose permease, which allows lactose to enter the bacterial cell
-B-galactosidase, which hydrolyses glucose and galactose
whats an operon
an operon is a group of genes that function as a single transcription unit; first identified in prokaryotic cells
whats a lac operon
The Lac Operon is a group of genes present in E.Coli Bacteria that are needed to use Lactose as a respiratory substrate.
what is the structure of the lac operon
-a length of DNA that is about 6000 base pairs long
-a promotor region
-an operator region lacO
- structural genes lac Z and lac Y that code for the enzymes B-galactosidase and lactose permease
what is the role of the regulatory gene
the regulatory gene is a small distance from the lac operon.
The regulatory gene (I) codes for a repressor protein (LacI) when this regulatory gene is expressed, the repressor protein produced binds to the operator region.
what is the role of the promoter region
The promoter region is the region where the enzyme RNA polymerase binds to begin transcription of the structural genes lac Z and lac Y. (mRNA is made)
what is the role of lac O/ the operator region
repressor proteins can bind here which prevents RNA polymerase from binding to the promoter region.
what makes up the control sites on the lac operon
the operator region and promotor region are the control sites
what occurs in the absence of lactose for the Lac operon
The Regulator gene codes for the repressor molecule which attaches to the operator region.
RNA polymerase cant bind to the promoter region. The repressor protein thus prevents the genes lacZ and lacY from being transcribed (expressed), so the enzymes for lactose metabolism are not made. The genes are ‘off’.
What happens in the presence of Lactose for the lac operon ?
Lactose enters the cell by diffusion and binds to the LacI repressor protein molecule.
This alters the shape of the LacI repressor protein, preventing it from binding to the operator.
The RNA polymerase enzyme can then bind 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.
what is a regulatory gene
-makes a repressor protein/ transcription factor
-switches structural genes off and on
what is a structural gene
-makes a protein
-activation is controlled by a regulatory gene
what is the inducer in the lac operon
lactose
do all cells contain the same genes
yes
what are the three methods of regulating gene expression in eukaryotic cells
- transcription factors
-post-transcriptional gene regulation
-post-translational gene regulation
what are the three methods of regulating gene expression in eukaryotic cells
- transcription factors
-post-transcriptional gene regulation
-post-translational gene regulation
what are transcription factors
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 turned on or off.
how do transcription factors work
-transcription factors slide along part of the DNA molecule , seeking and binding to their specific promoter regions
-they make it easier or more difficult for RNA polymerase to bind to the gene (aid or inhibit it).
-this can then activate or supress the transcription of the gene
what is the role of transcription factors in Eukaryotic cells
-essential for regulation of gene expression in eukaryotes, making sure that different genes in different types of cells are activated or supressed.
-some transcription factors are involved in regulating the cell cycle
-play a role in cell differentiation
-tumour suppressor genes and proto-oncogenes help regulate cell division via transcription factors, mutations of these genes may lead to uncontrolled cell division or cancer.
Why are some Promoter regions some distance away from their specific gene?
Because of how the DNA bends, the promoter region may be therefore closer spatially.
what are introns
non coding regions of DNA within a gene, which are not expressed.
They separate the exons
what are exons
coding or expressed regions of DNA within a gene
how does post-transcriptional gene regulation work
- all of the DNA of a gene (both introns and exons) are transcribed
-this resulting mRNA is called primary mRNA.
-primary mRNA is then edited and the RNA introns are removed
-the remaining mRNA exons are joined together.
-endonuclease enzyme may be involved in the editing and splicing process
what happens to the resulting mRNA introns
some introns may encode proteins
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 for more than one protein
how does post-translational gene regulation work
-post-translational regulation of gene expression involves the activation of proteins.
-many enzymes are activated by being phosphorylated
-cAMP is an important second messenger involved in this activation
for post-translational gene regulation
- A signalling molecule, such as the protein hormone glucagon, binds to a receptor on the plasma membrane of the target cell.
- This activates a transmembrane protein which then activates a G protein
- The activated G protein activates adenyl cyclase enzymes
- Activated adenyl cyclase catalyses the formation of many molecules of cAMP from ATP
- cAMP activates PKA (protein kinase A)
- 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
- PKA may phosphorylate another protein ( CREB, cAMP response element blinking)
- This then enters the nucleus and acts as a transcription factor, to regulate transcription.
what is morphogenesis
morphogenesis is the anatomical development of an organism
what is a homeotic gene
homeotic genes are controlled in the anatomical development of an organism, so that all the structures develop in the correct location,
according the body plan.
several of these genes contain homeobox sequences/ genes
what is a homeobox sequence
A homeobox sequence is a sequence of 180 base pairs found within homeotic genes that are involved in regulating patterns of anatomical development in animals, fungi and plants
they encode a homeodomain sequence
what is a homeodomain sequence
A 60 amino acid section of a protein encoded by the homeobox sequence.
what is the role of a homeodomain sequence
the homeodomain sequence can fold into a particular shape and bind to DNA, regulating the transcription of adjacent genes
these proteins are transcription factors and act within a cell nucleus
what shape does the homeodomain sequence fold into
The Homeodomain sequence folds into H-T-H.
It consists of two a-helices (H) connected by one turn (T).
Where does the Homeodomain sequence bind to?
part of the homeodomain amino acid sequence recognises the TATA sequence of the enhancer region of a gene to be transcribed.
The Enhancer Region is a region that initiates or enhances transcription.
What have scientists concluded about Homeobox sequences?
Homeobox sequences are homologous / identical between different species.
They originated from a common ancestor and have been passed on. The genes have been conserved.
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what are hox genes
Hox genes are a subset of Homeobox genes, found only in animals, plants and fungi~
They are involved in the formation of anatomical features in correct locations of body plan. They control which body parts grow where.
How do Hox Genes work in early embryonic development?
Hox genes are active and are expressed in order along the anterior-posterior axis of a developing embryo.
they regulate the development of embryos along the anterior-posterior
how are hox genes arranged
hox genes are arranged in clusters and each cluster may contain up to 10 genes
humans have 4 clusters
at some stage in evolution, hox clusters have been duplicated
what is collinearity
The sequential and temporal order of the gene expressions corresponds to the sequential and temporal development of the various body parts.
what do hox genes encode for
Hox genes encode for homeodomain proteins that act in the nucleus as transcription factors.
They can switch on cascades of activation of other genes that promote mitotic cell division, apoptosis, cell migration and also help regulate the cell cycle.
(all hox genes are homeobox genes)
how do we know that hox genes are simmilar across different animals
a fly can function normally with a chicken hox gee inserted in place of its own
how are hox genes regulated
hox genes are regulated by other genes called gap genes and pair-rule genes.
In turn these genes are regulated nu maternally supplied mRNA from the egg cytoplasm
what is the role of homeobox and hox genes in mitosis
it regulates the cell cycle
it 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.
what is the Hayflick constant
-limited body cells divide a limited number of times (around 50 times ) before dying
what is apoptosis
apoptosis is regulated and programmed cell death
apoptosis is different to cell death due to trauma, which involves hydrolytic enzymes
it controls plant and animal tissue development
what is the order of events of apoptosis
- enzymes break down the cell cytoplasm
- the cytoplasm becomes dense with tightly packed organelles.
3.The cell surface membrane changes and small prostitutions called blebs form
- chromatin condenses, the nuclear envelope breaks and DNA breaks into fragments
- The cell breaks into vesicles that are indigested by phagocytic cells, so that cell debris does not damage any ither cells or tissues. The whole process happens quickly.
how is apoptosis controlled
-many cell signals help to control apoptosis, some of these signalling molecules may be released when genes that are involved in regulating the cell cycle are and apoptosis responds to internal and external stimuli such as stress.
-These signalling molecules include cytokines from cells of the immune system, hormones, growth factors and nitric oxide
What is the effect of Nitric Oxide on apoptosis?
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.
What does Apoptosis deal with?
Apoptosis removes ineffective or harmful T-lymphocytes
during limb development, apoptosis causes the digits to separate from each other
apoptosis is an integral part of plant and animal tissue development. Extensive proliferation of cell types is prevented by pruning through apoptosis , without release of any hydrolytic enzymes that could damage surrounding tissues
What happens if not enough apoptosis occurs?
Not enough apoptosis leads to the formation of tumours
What happens if too much apoptosis occurs?
Too much apoptosis leads to cell loss and degeneration.
-cell signalling plays a crucial role in maintaining the right balance
