Genetic development (6) Flashcards
what is importance of apoptosis in controlling development of the body form
Apoptosis = programmed cell death
- Models the body during development eg. digits into separate units. + metamorphosis eg. caterpillar to butterfly
- Destroy harmful self T lympthocytes during development of the immune system, prevent cells attacking own cells
- After 50 mitotic divisions, cells go into senescence (cell ageing)
- Cells experiencing cell stress eg. irreparable genetic damage, RNA decay
(not necrosis = unprogrammed/premature death of cells by autolysis eg. disease, injury, lack of blood)
what is the second cross in a DIHYBRID CROSS of Green (G) and yellow (g) peas, round (R) and wrinkled (r)
- parental phenotypes
- parental genotypes
GAMETES
F2 genotypes & phenotypes
PHENOTYPIC RATIO
GREEN + ROUND GgRr XGREEN + ROUND GgRr
gametes: (GR) (Gr) (gR) (gr) x (GR) (Gr) (gR) (gr)
punnet square shows cross of gametes: GGRR, GGRr, GgRR, GgRr, GGRr, GGrr, GgRr, Ggrr, GgRR, GgRr, ggRR, ggRr, GgRr, GGrr, ggRr, ggrr
9 GREEN + ROUND, 3 GREEN + wrinkled, 3 yellow + ROUND, 1 yellow + wrinkled
what are hox genes
In animals, the homeotic genes containing the homeobox sequence are HOX GENES
Main role in bilaterian animals (animal with bilateral symmetry) = specify anteroposterior identity i.e. front/head and tail/bottom
They also determine which body parts grow where on the body & regulate mitosis, apoptosis, cell cycles in all parts of the animal’s body. If there is mutation, the outcome in the phenotype is SUBSTANTIAL.
what is a point mutation & a silent mutation
one base is changed in the DNA sequence
(can be silent, missense, nonsense etc.)
silent : the phenotype is unaltered by this mutation
what is EPISTASIS
Two genes controlling a single character, but one of them can mask/suppress the effect of the other gene.
- EPISTATIC GENE* = one that masks the effect of the other
- HYPOSTATIC = gene whose effect is masked*
eg. Labrador colour; Gene B = pigment (B = black, b = brown) Gene E = placement of pigment (E = in both fur and skin, e = in skin only)
Gene at locus E is epistatic to gene at locus B (hypostatic) as it masks its expression.
what is a mutation
A random change in the DNA sequence of a genome.
- can occur during DNA replication
- can be caused by mutagens i.e. a physical (X-rays), chemical (cigarettes) or biological (viruses) agent, that can cause a mutation.
what is multiple allele condition
When a gene has 3 or >3 alleles, i.e. a single characteristi has 3 or more forms. The homologous pair of chromosomes still only carries two of the alleles at the locus for that gene.
eg. BLOOD TYPE
Gene = I and alleles are IA, IB and IO where A & B are codominant, whilst O is recessive to them.
Type O can be given universally because there are no antigens present on surface of RBC. If the wrong type given, agglutination will occur.
what is a transcription factor
+ examples
A protein or short non-coding piece of RNA that binds to the promoter region of a gene to inhibt/enable gene expression.
- Homeodomain can be a transcription factor.*
- STEROID hormones, tumour suppressor gene, proto-oncogenes*
for what reasons may a dihybrid phenotypic ratio not be 9 : 3 : 3 : 1
EPISTASIS
AUTOSOMAL LINKAGE because both genes occur on the same chromosome/autosome/chromatid or no independent assortment, so alleles are inherited together/in same gamete. Unless crossing over occurs/chiasma forms between gene loci
GENETIC DRIFT if number of individuals is small
what are deleterious mutations + examples
Mutations that change the phenotype of an organism adversely.
eg. sickle cell anaemia & cystic fibrosis
how can histones be modified to affect gene expression
ACETYLATION- addition of an acetyl group
= histone is less positively charged, DNA coils less tightly, gene expression is increased/enabled.
METHYLATION- addition of a methyl group -CH3
makes histone+DNA more hydrophobic so they pack more tightly, so gene expression decreasing/inhibiting.
what happens in gel electrophoresis (3 marks)
Fragments of DNA from restriction enzyme cleavage are separated from each other by migrating through a support called agarose gel.
how is DNA cut into fragments
RESTRICTION ENDONUCLEASES are used to cut/cleave within the short tandem repeats/microsatellites - highly variable short repeating lengths of DNA - variability is unique for individuals. 2-5 nucleotides repeated 10-30x due to inaccuracy of DNA polymerase copying these regions vs. genes that are very similar/identical in sequence between individuals
THIS GIVES A MIXTURE of DIFFERENT-SIZED FRAGMENTS which can be separated by gel electrophoresis.
what is DNA profiling/ what is its role
what are the uses of it
DNA profiling = genetic fingerprinting (when for people)
Individuals within a species have a unique DNA sequence/genome that gives each an unique DNA profile. except clones/identical twins
A technique used to identify individuals.
- Forensic science, historical identification- crime, lost familes
- Maternity/paternity disputes
- Analysis of disease- eg. Huntingdon’s
what are silent mutations
They do not affect structure and tf function of a protein because:
- They occur in the non-coding regions of DNA: SATELLITE DNA between genes or INTRONS which are non-coding sections within a gene which are spliced out before mRNA leaves the nucleus.
- They occur in the coding region of a gene but do not change the amino acid coded for bc of the degenerate nature of the genetic code.
- They occur in the coding region of the gene and change an amino acid that doesn’t take part in the folding of the protein, so tertiary structure is not affected- function is not affected. (i.e. only primary structure is affected)
how are the rates of mitosis/apoptosis balanced/regulated
& when they aren’t?
CELL SIGNALLING by external factors eg. hormones, growth factors, cytokines (immune s) + nitric oxide.
external factors eg. virus, bacteria, harmful pollutants/UV light may upset the balance
TOO MUCH MITOSIS/too little apoptosis = tumours
TOO LITTLE MITOSIS/too much apoptosis = degeneration/tissue loss
Cells undergoing apoptosis release chemical signals stimulating mitosis/cell profileration to model tissues.
what is the Lac operon
The LC is a stretch of DNA with a regulatory site containing an operator and promoter, followed by the structural genes which code for the enzymes required for lactose metabolism/utilisation for respiration.
what are frameshift mutations
Those caused by deletion/insertion of a base. Entire gene following this mutation is read incorrectly.
what is an advantageous mutation + examples
(very occasionally) a mutation that changes the phenotype of an organism and is advantageous, so will be selected for.
eg. lactose tolerance + HIV resistance
describe the steps in the lac operon when lactose is absent
Lactose absent, repressor _active_, operon off.
Repressor binds to operator, blocking the binding of RNA polymerase to the promoter region.
The RNA cannot synthesise the necessary enzymes from genes lacZ, lacY, lacA.
For an autosomal dihybrid cross, where parental phenotype is
TALL + PURPLE flower AABB X dwarf + white flower aabb
gives the gametes, F1 genotypes and phenotypes
and the resulting cross/ratio
(AB) X (ab) on the same chromosome
GENOTYPE: AaBb = tall + purple
PARENTAL PHENOTYPE: tall + purple X tall + purple
AaBb X AaBb
GAMETES: (AB) (ab) X (AB) (ab)
genotypes: AABB AaBb AaBb aabb
tall + purple 3 : 1 dwarf + white
what is a DNA probe
A labelled short DNA sequence, usually radioactively and used to detect complementary DNA sequences to it by DNA hybridisation.
It’s made either in vivo/in vitro from:
- same gene as DNA of interest but from another organism
- a short segment of the gene synthesised from known aa sequence of the protein which the gene codes for
what is the process of apoptosis
- Hydrolytic enzymes break down the proteins of the cell cytoskeleton
- Cell starts to shrink and cytoplasm becomes dense with tightly packed organelles
- CSM changes shape, blebs form (small protrusions)
- Nuclear envelope breaks down, chromatin condenses & DNA broken into fragments
- Cell breaks into membrane-bound vesicles = apoptotic bodies. Ingested by phagocytes. No cell debris and surrounding cells not damaged.
what are the 4 outcomes of mutations on the organism
- NONE/ no change in the phenotype- SILENT mutations (most mutations)
- DELETERIOUS
- ADVANTAGEOUS
- NEUTRAL
describe post-translational level gene expression regulation
ACTIVATION of PROTEINS by cyclic AMP
regulates the function of proteins by;
- activating a cascade of enzymes i.e. in glycogen metabolism
- activating transcription factors (transcriptional level)
+
post-transitional MODIFICATION of PROTEINS eg. addition of carbohydrate, lipids, metal ions to form a conjugated protein. - Determines the function of the proteins.
what is the homeodomain comprised of
2 alpha helices connected by a turn. It attaches to a TAAT sequence of the enhancer region of a gene
outline the process of DNA profiling
- AMPLIFY the quantity of DNA if there is too little by PCR
- CUT the DNA into different sized fragments using restriction endonuclease enzymes
- SEPARATE the fragments using GEL ELECTROPHORESIS
- HYBRIDISE radioactive probes from STRs to the separated fragments to create the pattern of DNA bands on a NYLON TEMPLATE and develop on a RADIOGRAPH to make the genetic fingerprint
what are the first stages in genetic engineering where gene & plasmid obtained
- Obtaining gene of interest as mRNA - DNA probes to identify position on southern blots of gel electrophoresis + spliced with restriction enzyme to isolate from respective mRNA + amplification by PCR
- Reverse transcription (from retro-viruses incubated) with extracted mRNA makes copyDNA/cDNA
- Isolating the plasmid by lysing bacterial cells with detergent/centrifugation. Plasmid DNA is in supernatant
explain the action of housekeeping genes
TRANSCIPTIONAL LEVEL REGULATION OF GENE EXPRESSION
All cells of an organism contain the same genome (except gametes). certain proteins are needed at all times eg. ATP synthase + Cytochrome C (except erythrocytes), and are expressed by housekeeping genes.
Proteins that are needed at certain times by certain tissues are expressed by tissue-specific genes. eg. trypsin produced by exocrine pancreatic tissue, haemoglobin in bone marrow, insulin in ß-cells in islets of Langerhans.
These genes can be switched on & off when necessary.
what are missense and nonsense mutations
MISsense = one amino acid is incorrectly coded for
NONsense = the codon is mutated into a STOP codon and synthesis of polypeptide chain terminates
BOTH ARE BASE SUBSTITUTION mutations
WHAT IS A PLASMID
how does a virus differ as a vector
A v small segment of circular DNA
with an origin of replication naturally present in bacteria
possible to have many copies of plasmid in one bacterial cell
virus vectors are viruses that infect eukaryotic cells
what happens after gene + plasmid obtained
- Splicing** the DNA of interest **into** the **plasmid
- Reintroduction of hybrid plasmid into host cell
For a sex-linked monohybrid cross: haemophilia is a recessive sex-linked condition preventing production of factor VIII in the clotting cascade. The gene is present on the non-homologous portion of the X chromosome. H = normal dominant allele for factor VIII. h = recessive haemophilia.
Give the PARENTAL PHENOTYPE & GENOTYPE
GAMETES
F1 genotypes & phenotypes
FEMALE CARRIER X NORMAL MALE
XH Xh x XH Y
(XH) (Xh) x (XH) (Y)
3 no haemophilia: female XH XH male XH Y female Xh XH 1 HAEMOPHILIA: male Xh Y
what are the 4 stages of PCR and at what temperatures
- DENATURATION of sample DNA at 94’C, H bonds between bases break.
- ANNEALING of primers at 55’C to complementary regions of DNA adjacent to target gene.
- EXTENSION of primers by Taq polymerase at 72’C by bonding free nucleotides to build two identical DNA strands (copies of template DNA).
what can affect the ratio of a dihybrid cross with autosomal linkage
& a normal dihybrid cross
CROSSING OVER in meiosis
- closer the loci of genes to the centromere, the less affected the linkage is by crossing over*
9: 3:3:1 is a theoretical ratio, actual/practical values may differ due to random fertilisation.
what is genetic engineering
The addition of an extra segment of DNA to the genetic material of the cell.
Needs to be done in such a way that:
- Replication of the new DNA can occur
- It will consequentially be transmitted to daughter cells on cell division (mainly for cloning reasons)
- If the segment is a gene to allow expression of gene
what are the differences between a monohybrid and dihybrid cross
Mono = one set of alleles inherited on one pair of homologous chromosomes
Di = two sets of alleles inherited on two pairs of homologous chromosomes (4 X-somes in total, each gamete inherits 2 X-somes, opposed to 1 in monohybrid cross)
what is post-transcriptional gene regulation
ONLY IN EUKARYOTES
Initial transcription = heteronuclear RNA/pre-mRNA containing exons & introns (coding & non-coding)
Then, introns are cut out & exons spliced together to form mRNA- (leaves the nucleus)- by spliceosomes hydrolysing the sugar-phosphate backbone at intron-exon junctions and condensation reactions at exon-exon junctions.
The proteome i.e. entire set of proteins expressed by an organism is much larger than its genome because introns can be spliced differently.
how is DNA amplified + what is needed
PCR- polymerase chain reaction
- DNA sample that needs to be amplified
- Primer - short piece of DNA which will be complementary to the DNA sample. Needs to be DOUBLE-STRANDED DNA for DNA polymerase to initiate replication.
- Free nucleotides
- Taq polymerase (THERMOSTABLE DNA polymerase)
what is recessive epistasis
When the presence of 2 recessive alleles at one gene locus masks expression of another gene. eg. if gene A produces an enzyme that converts white precursor to red pigment, aa = masks the gene and it stays white.
DIHYBRID CROSS PURE-BREED PARENTAL STOCK
Green + round GGRR X yellow + wrinkled ggrr
what are the gametes, F1 genotypes & phenotypes
gametes of green+round are (GR) whilst gametes of yellow+wrinkled are (gr)
genotype = GgRr
phenotype = green + round
after a monohybrid cross resulting in F1 genotypes of Rr and phenotypes of all round (none wrinkled), the F1 are self-fertilised
name the Parental phenotype + genotype
- Gametes*
- F2 genotypes + phenotypes*
and RATIO of phenotypes
ROUND PEAS X ROUND PEAS
Rr x Rr
(R) (r) (R) (r)
GENOTYPES: RR Rr rR rr
3 round peas : 1 wrinkled
what is sex-linkage (example)
& difference between sex and all other chromosomes
All genes carried on sex chromsomes are sex-linked.
i.e. sex chromosomes are heterosomes, compared to autosomes.
X and Y share a homologous portion i.e. same alleles, but the Y heterosome is shorter, no the X heterosome contains a non-homologous portion.
Any characteristic on the non-homologous of the X chromosome will always be expressed in the male, even if recessive.
Eg. HAEMOPHILIA
give the ratios for the different crosses we need to know
monohybrid cross: hetero x hetero: 3 : 1
dihybrid cross: hetero x hetero: 9 : 3 : 3 : 1
dihybrid cross w/ autosomal linkage: 3 : 1
sex-linked monohybrid cross 3 : 1 or 1 : 2 in male offspring
polygenic inheritance: hetero x hetero: 9 : 3 : 3 : 1
are the genes in prokaryotes housekeeping or tissue-specific
No tissue-specific genes because they are unicellular.
Some genes are housekeeping genes.
Other genes are expressed when the need arises eg. beta galactisidase for metabolising lactose.
describe the stages when lactose is present**/glucose is absent for the Lac operon
(positive control)
Lactose present/glucose absent, repressor inactive, operon on.
Lactose acts as an inducer, binding to the repressor protein’s allosteric site, changing its tertiary stucture so it cannot bind to the operator on the promoter region. Therefore the operon is ‘on’. I.e. lactose depresses the operon, so enzymes for lactose utilisation are induced.
This allows RNA polymerase to bind to the regulatory gene and transcribe a strand of mRNA, producing ß-Galactosidase (hydrolytic enzyme i.e. lactose to ß-galactose + a-glucose) Permease (makes cell membrane permeable) & Transacetylase.
In genetic engineering, how is the DNA to be cloned given to the host cell
CLONED DNA
> can be from same species
> from different species (eg. human insulin in bacteria)
> artificially synthesised in a lab
The DNA to be cloned must be spliced to another segment of DNA i.e. a vector, which must be recognised + replicated by the host cell. (i.e. three necessary components: DNA of interest, vector & host)
Vector = plasmids or viruses
what are neutral mutations & example?
changes the phenotype but give no advantage or disadvantage to the organism eg. earlobes
how can one ascertain whether an individual is heterozygous (Bb) or homozygous dominant (BB) for a single characteristic eg. coat colour B=black, b=brown
CROSS THE INDIVIDUAL WITH A HOMOZYGOUS RECESSIVE (bb) parent.
If HOM. DOM. all offspring will be Bb/ black
If HETEROZYGOUS 50% offspring bb/brown
50% offspring Bb/black
why do the molecules of DNA separate in gel electrophoresis
They separate based on size when an electric current (max. 45 volts) is applied to an agarose gel because DNA is negatively charged because of phosphates in s-p backbone, and will move through the gel to the positive electrode/anode.
The SMALLEST DNA fragments move FURTHEST and the LARGEST fragments move SLOWEST.
what happens to DNA after gel electrophoresis
HYBRIDISATION of RADIOACTIVE DNA PROBE
(southern blotting)
A DNA probe is radioactively labelled to identify within which specific bands on the gel the DNA of interest is contained.
how are genes’ expression turned ‘on/off’ in eukaryotic cells
By the degree to which they are coiled during Interphase.
(DNA is negatively charged, due to phosphates, and wound around positively charged proteins = histones to form chromatin)
There are two forms of chromatin: heterochromatin is tightly wound DNA, inhibiting gene expression.
euchromatin is loosely wound DNA, enabling gene expression.
what is dominant epistasis
If the presence of a dominant allele at a gene locus results in that gene having an effect on another gene.
eg. interrupts a sequence
what is the importance of mitosis in controlling development of body form
M increases the # of cells = GROWTH.
Tissue replacement/repair. Every daughter cell is genetically identical to parent.
(regulated by homeobox/hox genes)
how can cAMP be involved in gene expression
- (non-steroid hormones use cAMP as a secondary messenger by activating a G protein that activates adenylate cyclase to convert ATP to cAMP)*
- cAMP activation of protein kinase results in a cascade, and a catalytic subunit may cross the nuclear membrane & bind to a protein transcription factor leading to expression of a gene*
MONOHYBRID inheritance
Give the gametes, F1 genotypes & F1 phenotypes of wrinkled peas (rr) and round peas (RR)
Gametes of round peas then wrinkled peas (in circles) (R) (R) (r) (r)
Offspring genotypes will be: All Rr and phenotypes: all round
what is an endonuclease
An enzyme that cuts in the middle of a DNA strand.
Restriction endonucleases recognise a specific sequence within the DNA.
why can introns be spliced differently
During transcription in an EU cell, the entire base sequence of a gene is copied into a base sequence of pre-mRNA. It contains non-coding + coding regions of DNA. Non-coding regions are edited out.
what is the function of Lac operon in E.coli bacteria
Allows the production of the enzyme to break down lactose when lactose is present/ when glucose is absent.
Glucose is the preferential substrate for respiration.
what is polygenic inheritance
When 2 or more genes at different loci can combine to affect one characteristic.
9 : 3 : 3 : 1 but with one characteristic and a more diverse phenotype that monohybrid cross
- eg. Siamese cats: BBDD, BBDd, BbDD, BbDd = BLACK DENSE*
- BBdd, Bbdd = BLACK DILUTE*
- bbDD, bbDd = BROWN DENSE*
- bbdd = BROWN DILUTE*
what is the homeobox gene
Homeotic genes contain a very conserved region of DNA, 180 bases long = HOMEOBOX
Very conserved across all eukaryotic/multicellular kingdoms.
The 180-bp homeobox codes for 60 amino acids, resulting polypeptide = homeobox domain/homeodomain. It is used to turn genes on/off.
what is autosomal linkage of a dihybrid cross
When 2 pairs of alleles are on the same pair of homologous chromosomes
eg. Aa and Bb on one pair, AB on one chromosome, ab on the other
what are the ways of regulating gene expression in a cell
TRANSCRIPTIONAL LEVEL
- housekeeping genes
- lac operon (only in bacteria)
- transcription factors
POST-TRANSCRIPTIONAL LEVEL
-editing of primary mRNA & removal of introns to produce mature RNA
POST-TRANSLATIONAL LEVEL
- activation of proteins by cyclicAMP
- modification of proteins
which factors can cause variation in a plant phenotype
Inheritable variation; Genetic:
- Polygenic genes, dominant + recessive alleles
- Mutations i.e. mistakes in DNA replication/nuclear division
- Crossing over, independent assortment in meiosis (Homologous chromosomes in M1, non-identical sister chromatids in M2)
- Gene interactions
- Random mating + fertilisation
Non-inheritable; Environmentally induced:
- Climate and altitude
- Water availability, diet
- Acidity (pH), soil type
- Light intensity
- Predation, competition
- (Mutations caused by environment)
what are the characteristics of discontinuous variation
- Discontinuous variation = discrete = qualitative variation
- DISTINCT CATEGORIES without intermediates
- Monogenic
- Environment has little effect
- Represented by bar graph
- Limited number of phenotypes
eg. blood groups
what are the types of ADAPTATION
- STRUCTURAL:internal structure = anatomy, structural appearance = morphology
- Eucalyptus trees in Australia have vertically hanging leaves to reduce SA exposed to light, reducing transpiration. Thick bark prevents fire.*
- PHYSIOLOGICAL: functions of body systems, biochemistry of cells
- Eucalyptus trees in Aus release seeds after fire, when there is less competition from others that don’t survive the fire.*
- BEHAVOURIAL: eg. mating rituals, thermoregulation, hiding from predators etc.
- E trees produce toxic compounds in their leaves to deter grazing animals.*
why are adaptations necessary to an organism
For it to be successful in its environment, it must possess features that will help it to survive.
An adaptation is a modification to an organism’s structure, function or behaviour to help it survive.
Typically an organism living in an extreme environment will have many adaptations.
what are the characteristics of continuous variation
Continuous = quantitative variation = characteristic can be measured.
- POLYGENIC (determined by many genes) (+environment)
- Environment has a large effect
- Represented by a histogram (no distinct categories)
- No limit on values
Usually standard distribution, i.e. range of phenotypes between two extremes such as HEIGHT
label the type of adaptation for the following thermophilic extremophiles:
ANIMAL:
- Fennec fox has large ears and eyes to give it good hearing/vision for predation. It loses heat from the ears during the day and thick fur retains heat while hunting in the cold nights.
- Its kidneys reabsorb most water in its urine (concentrated) to avoid dehydration.
- The fox is nocturnal, remaining in burrows to avoid heat/being predated by eagles.
MICROORGANISM:
- In a thermophilic archaean, The formation of matrix that sticks the bacteria into biofilms to withstand high temperatures.
- Cell membrane lipids have strong ether linkages. Has an enzyme that supercoils DNA to make it compact to withstand heat near boiling. Tertiary structure stabilised by large number of polar amino acids to form H bonds and ionic - reduces chances of denaturation.
- Bacteria produces heat shock proteins to protect cell against heat damage, and a heat-resistant DNA polymerase allows replication at 100’C.
- Structural
- Physiological
- Behavioural
- Behavioural
- Structural
- Physiological
what is genetic drift
The random change in allele frequency that occurs without natural selection.
It has large impact in a small population eg. if frequency of the presence is 1% in 1,000,000 is 10,000 so the allele won’t be lost. In an isolated deme (small population) eg. 100 individuals. The presence is 1 individual. HIGH PROBABILITY of losing allele/frequency increasing in whole population is large.
If the allele has a selective advantage, could lead to EXTINCTION if lost, EMERGENCE OF A NEW SPECIES if its frequency increases.
what are the two types of Genetic Drift
GENETIC BOTTLENECK
large reduction in population due to an _epidemic/natural disaster_eg. Cheetah, Northern Elephant Seals
FOUNDER EFFECT
small population colonises a new area. only the alleles present in colonisers are effect until mutation- rare/recessive are expressed/have a large impact/high frequency eg. Amish in Pennsylvania
what is the first ISOLATING MECHANISM (G)
GEOGRAPHICAL Isolation i.e. PHYSICAL separation of 2 or more populations
