CHAPTER 19 - GENETICS OF LIVING SYSTEMS Flashcards
What is a mutation
A change in the sequence of bases in DNA
What causes a change in the DNA base sequence
Substitution, Deletion or insertion
What is a mutation called if only one nucleotide is affected
point mutation
If a base is changed, what are the outcomes
Either a different amino acid is synthesised - meaning a different protein or change in structure of the protein will occur
OR
degenerate code means that the change of codon still makes the same amino acid leading to no change
What does insertion and deletion mutations cause
Frameshift mutation - which will change the reading frame and change every codon from that point forward
What are the effects of mutations upon an organism as a whole
No effect - proteins are still functioning and fully synthesised
Damaging - Proteins are no longer synthesised or are non-functional, which can interfere with many essential processes
Beneficial - Can give a useful characteristic eg. mutation in human cells means that HIV cannot bind to cell surface membranes or enter the cells
What can increase the rate of mutations
Mutagens
What is depurination
Loss of a purine base
What is depyrimidination
loss of a pyrimidine base
Why are vitamins A, C and E known as anticarcinogens
Negates effects of free radicals which could cause mutations
What are the 3 types of mutagens
Physical, Chemical and Biological
What is an example of Physical mutagens and what do they do
Ionizing radiations such as X-Rays
Breaks one or both DNA strands, some breaks can be repaired but mutations occur in the process
What is an example of Chemical mutagens and what do they do
Deaminating agents
Chemically alter bases in DNA such as converting Cytosine to uracil in DNA, changing the base sequence
What are some examples of Biological agents and what do they do
Alkylating agents - Methyl or ethyl groups are attached to bases - leading to incorrect pairing during replication
Base analogs - incorporated into DNA in place of the usual base during replication
Viruses - Viral DNA may insert itself into a genome changing the base sequence
What is a nonsense mutation
A codon becoming a stop codon instead of coding for an amino acid - resulting in a shortened - normally non-functional - protein
What is a missense mutation
Incorporation of an incorrect amino acid(s) into the primary structure when the protein is synthesised, could have silent, beneficial or harmful effects
What is an Amorph mutation
Mutation that results in the loss of function of a protein
What is a hypomorph mutation
Mutation that results in a reduction of function of a protein
What is a hypermorph mutation
Mutation that results in a gain in function of a protein
What is sickle-cell anaemia, that type of mutation is it and why is it seen as a beneficial mutation
A blood disorder where erythrocytes develop abnormally
Mutation of just one base - T for A, substitution
Gives resistance to malaria
Evaluate the benefits of being heterozygous for sickle cell anaemia
Heterozygotes have one normal and one mutant allele (for haemoglobin)
normally healthy
except in low oxygen concentrations
part of malarial parasite life cycle is in red blood cells
red blood cells of heterozygotes are, sickled/destroyed, when parasites enter
infection reduced
being heterozygous is a clear advantage in an area where malaria is present and a disadvantage in an area where malaria is absent
Explain how the change of one amino acid in haemoglobin could reduce the oxygen-carrying ability of blood
Change in primary structure
change in secondary and tertiary structure
change in 3D shape
change in function
reduced oxygen carrying ability
How many types of chromosome mutations are there and what is involved
Deletion - A section of chromosome breaks off and is lost within the cell
Duplication - sections get duplicated on a chromosome
Translocation - a section of one chromosome breaks off and joins another non-homologous chromosome
Inversion - a section of chromosome breaks off, is reversed, and then joins back onto the chromosome
The development of lactose tolerance is thought to have spread over approximately 20000 years, which in evolutionary terms is very quick
Explain why the percentage of adults with the ability to digest lactose increased at such a rate
Ability to digest lactose is a beneficial characteristic
drinking milk prevented starvation
reduced osteoporosis
directional selection
Outline why the majority of mutations do not have an influence phenotype
Mutation is random
majority of DNA is non-coding; mutations more likely to occur in non-coding regions
mutations in non-coding regions do not affect phenotype
Discuss why beneficial mutations are rare and suggest a process that beneficial mutations underpin
Majority of mutations are silent
Idea that random change to protein structure is more likely to reduce function
idea that beneficial mutations increase chances of survival if environment changes
surviving organisms reproduce and pass new alleles to offspring
leading to evolution
What are housekeeping genes?
Genes that code for enzymes necessary for metabolic reaction pathways eg. respiration
When are protein-based hormones only required for?
Needed by Certain cells at certain times to carry out short lived responses
Coded for by tissue-specific genes
What is morphogenesis
The regulation of the pattern of anatomical development
What is the usual choice in species for genetic studies and why
Fruit Flies
Small, easy to keep and have a short life cycle
What are homeobox genes
Group of genes which all contain a homeobox
What is a homeobox
A section of DNA 180 base pairs long coding for a part of the protein 60 amino acids long that is highly conserved (very similar) in plants, animals and fungi
What is a homeodomain?
Part of the protein that binds to DNA and switches other genes on or off
What are homeobox genes an example of
Regulatory genes
What happens when the Pax6 gene (a homeobox gene) mutates
Causes a form of blindness due to an underdevelopment of the retina in humans, causes blindness in mice and fruit flies who also have the gene
Pax6 involved in development of eyes in all 3 species - highly conserved
What are hox genes?
A group of Homeobox genes that are only present in animals
What are hox genes responsible for
Correct positioning of body parts
How are hox genes found?
Gene clusters
eg. mammals have 4 clusters on different chromosomes
How many hox genes do humans have and how has it arisen?
39, believed to have arisen from one ancient homeobox gene by duplication and accumulated mutations over time
How is the order in which genes along the chromosome are organised
In order in which their effects are expressed in the organism
How are body plans usually represented
As cross-sections through the organism showing fundamental arrangement of tissue layers
What are Diploblastic animals
Animals which have 2 primary tissue layers
What are Tripoblastic animals
Animals which have 3 primary tissue layers
What is a common feature of animals
Segmentation
eg. Head, thorax, abdomen of an insect
Backbone of vertebrates
Rings of a worm
What do segments in an organism do
Segments have multiplied over time and specialised to form different functions
What do hox genes in the head control
Development of mouthparts
What do hox genes in the thorax control
Development of wings, limbs or ribs
What are Somites and what have developed from them
Somites are segments in the embryo
From which individual vertebrae and associated structures have all developed
What are somites directed by
Hox genes to develop in a particular way depending on their position in the sequence
What is radial symmetry
Seen in diploblastic animals like jelly fish
No Left or right sides, only top and bottom
What is bilateral symmetry
Seen in Most mammals
The organisms have both left and right sides and a head and tail rather than just a top and bottom
What is Asymmetry
Seen in Sponges
No lines of symmetry
What is the role of mitosis
Increase the number of cells leading to growth (growth and repair)
What is apoptosis
Controlled cell death
(pg 516)
What are both mitosis and apoptosis controlled by
Hox genes
What is the role of apoptosis
Shaping different body parts by removing unwanted cells and tissue
What is cell proliferation
Rapid increase in the number of cells
What factors can affect the expression of regulatory genes
External - Change in temperature or intensity of light
Internal - Psychological stress or hormone release, Drugs (eg. thalidomide)
What affect did thalidomide have and how is it being used now?
Used to treat morning sickness in pregnant women
Prevented normal expression of particular hox gene
Resulted in the birth of babies with shortened or no limbs
Now being used as a treatment for cancer, to stop its growth
What is the theory of recapitulation
As organisms develop from a fertilised egg to embryo, they repeat the evolutionary process they have been through
How can the theory of recapitulation be summarised
Ontology (development of an organism) mimics phylogeny (evolutionary history of an organism)
Explain with reference to the body shape, while human beings are referred to as bilaterally, symmetrical, but jellyfish are radially symmetrical
Bilateral symmetry is along (single plane through) central axis
e.g., two arms, two legs
radial symmetry is along a plane at any angle through central axis
e.g., tentacles around central axis
The hox gene Pax6 is necessary for the normal development of the retina in humans. Mutation in this gene can lead to blindness. Pax6 mutations can also cause blindness in mice and fruit flies. Describe how scientist could have tested the idea that Pax6 plays a role in our development in all three species.
Isolate (Pax6) gene from one species
detail e.g., PCR, restriction enzymes
test in different, tissue/species
example of positive result e.g., eyes develop on legs
switch gene off early in development and eyes will not develop
DNA sequencing and compare genes from different species
Consider the statement:
“All hox genes are homeobox genes but not all homeobox genes are hox genes”
Discuss the validity of this statement
Statement is valid
Hox genes are one form of homeobox gene
present in vertebrates
in Hox clusters
other forms of homeobox gene present in other clusters
Where is the entire genome of an organism present
in every prokaryotic cell or eukaryotic cell that contains a nucleus
How do bacteria respond to changes in the environment
Gene regulation
Expressing genes only when the products are needed also prevents vital resources being wasted
How are genes regulated and categorised (by the level at which they operate)
Transcriptional - genes can be turned on or off
Post-transcriptional - mRNA can be modified which regulates translation and they types of proteins produced
Translational - Translation can be stopped or started
Post-translational - Proteins can be modified after synthesis which changes their function
What type of gene regulation is chromatin remodelling
Transcriptional control
What is chromatin
DNA/Protein complex when DNA is wound around histones in order to be packed into the nucleus
What is Heterochromatin
Tightly wound DNA causing chromosomes to be visible during cell division
What is Euchromatin
Loosely wound DNA present during interphase
Why is transcription of genes not possible when DNA is tightly wound (heterochromatin)
RNA polymerase cannot access the genes
Why is transcription of genes possible when DNA is loosely wound (Euchromatin)
RNA polymerase can access the gene
Why doesnt protein synthesis occur during cell divisions but does during interphase
Simple form of regulation, Ensures proteins are synthesised in time,
Prevents the complex and energy-consuming process of protein synthesis occuring when cells are actually dividing
What is chromatin remodelling (brief summary)
Tightness at which DNA binds to histones
What is Histone modification (brief summary)
Adding of groups to either increase or decrease packing and coil tightness of DNA/histone complex
What charge is on DNA coils
Negative
What charge is on histones
Positive
What is condensation of DNA
Increasing the degree of packing - tightness of DNA/histone complex
What is Acetylation and phosphorylation upon DNA packing and Chromatin complex (DNA/Histone)
Reduces positive charge on histones (makes them more negative)
Repels DNA more/ Makes bonds weaker/ less EFOA so they coil less tightly
So genes can be translated
What affect does methylation have upon DNA/Histone complex
Makes histones more hydrohpobic
Binds tighter to DNA, preventing transcription of genes
What are epigenetics
The control of gene expression by the modification of DNA, or all the different ways in which gene expression is regulated
What is an operon
A group of genes that are under the control of the same regulatory mechanism and are expressed at the same time`
Why are operons efficient at saving resources
If a certain gene product arent needed, then all of the genes involved in their production can be switched off
What are the 3 genes in the lac operon
LacZ, LacY, LacA
What are the 3 enzymes the Lac Operon
Beta-galactosidase, Lactose permease and transacetylase
Describe the events in the Lac Operon activation
Repressor protein binds and blocks to the promotor and operator, preventing RNA polymerase from copying the necessary DNA
When lactose is present, it binds and changes the shape of the repressor protein, meaning it is no longer fits the binding region
RNA polymerase can now code for the new proteins to code for necessary enzymes
(pg 511)
What are the three components of the lac operon
Regulatory gene, promotor, operator
What is the role of cAMP
cAMP acts as a secondary messenger, binds to CRP (cAMP receptor protein)
Increases or up-regulates production of enzyme to metabolise lactose
What are RNA processing and RNA editing an example of
Post-transcriptional/pre-translational control
Describe the events in RNA processing
pre-mRNA is the product of transcription
Modified forming mature mRNA before it can bind to a ribosome and code for the synthesis of required protein
A cap (modified nucleotide) is added to 5’ end and a tail (long chain of Adenine nucleotides) is added to the 3’ end
Both help stabilise mRNA and delay degradation in the cytoplasm
cap aids binding of mRNA to ribosomes. Splicing also occurs where the RNA is cut at specific points introns (non-coding DNA) are removed and the exons (coding DNA) are joined together
Process occurs within the nucleus
What is a cap in RNA processing
A modified nucleotide that is added to the 5’ end
What is a tail in RNA processing
A long chain of adenine nucleotides added to the 3’ end
What is splicing in RNA
Introns are removed and exons are joined together in the nucleus
What are introns
non-coding regions of DNA
What are exons
coding regions of DNA
What is RNA editing
Changing of some mRNA molecules through base addition, deletion or substitution.
Same affect as point mutations and result in the synthesis of different proteins which may have different functions
Increases the range of proteins that can be produced from a single mRNA molecule or gene
What are some examples of translational control
Degradation of mRNA - the more resistant the molecule, the longer it will last in the molecule, so more of the protein is synthesised
Binding of inhibitory proteins - to mRNA prevents it binding to ribosomes and the synthesis of proteins
Activation of initiation factors which aid the binding of mRNA to ribosomes - the eggs of many organisms produce large quantities of mRNA which isn’t required until after fertilisation
What do protein kinases do
Enzymes that catalyse the addition of phosphate groups to proteins.
Phosphate groups changes the tertiary structure and so the function of a protein
Many enzymes are activated by phosphorylation, so protein kinases are important regulators of cell activity, and activated sometimes by cAMP
What are some examples of post-translational control
Addition of non-protein groups eg. Carbohydrate chains, lipids or phosphates
Modifying amino acids and the formation of bonds such as disulfide bridges
Folding or shortening of proteins
Modification by cAMP - eg. Lac Operon cAMP binds to CRP increasing rate of transcription
The lac operon is often referred to as being ‘leaky’ meaning that it is still transcribed to a limited extent even if even in the absence of lactose.
1) Using your knowledge of how the lac operon works, explain why this is necessary
2) Suggest the functions of beta-galactosidase and lactose permease synthesised by the lac operon
1) enzyme coded for by lac operon enables lactose to enter bacteria
lactose binds to repressor
protein; (repressor) protein changes shape
transcription no longer blocked
enzymes needed
to metabolise lactose are synthesised
2) β-galactosidase catalyses the hydrolysis of lactose
to galactose and lactose
lactose
permease enables the entry of lactose into cells
Another example of gene regulation in prokaryotes in the trp operon. This operon codes for the production of tryptophan, an essential amino acid for the bacterium E.coli. When tryptophan is available in the environment the structural genes in the trp operon are not expressed.
Suggest a mechanism for the genetic regulation of this operon
Tryptophan binds to repressor protein
shape of repressor protein changes
repressor protein
binds to promoter
blocks RNA polymerase from binding
transcription prevented
of genes
coding for enzymes responsible for tryptophan synthesis
Using your knowledge of enzymes, explain how enzyme cofactors could play a role in gene regulation
Cofactors bind to proteins that regulate transcription
changes binding of proteins to control
elements
rate of transcription changed
RNA polymerase activated
