Genetics

Question 1

general terms p1

Answer 1

Gene= sequence of DNA that codes for a polypeptide which occupies specific locus of a chromosome (fixed position on a chromosome where a particular gene or genetic marker is located) Allele- a variant nucleotide sequence for a particular gene at a given locus- codes for an altered phenotype- single gene may have 1 or more alleles- always occupy same locus- same versions of same gene Diploid- 2 full sets of chromosomes, haploid 1 full set homozygous- 2 of same allele heterozygous- 2 different alleles for same gene

Question 2

general terms p2

Answer 2

genotype= alleles individual contains Phenotype- can be described as appearance but more than that- includes characteristics that can’t be seen eg blood group- both genotype and environment control characteristics of an individual so phenotype is better thought as the expression of the genotype in a specific environment

Question 3

Monohybrid inheritance

Answer 3

Monohybrid inheritance= inheritance of 1 gene Gregor Mendel- investigated inheritance in garden pea plants- several varieties available that were true breeding eg offspring produced from self fertilising all have same characteristics -nobody knew why but useful for experiments as: easy to grow, flowers can self or cross-fertilise, make flowers and fruit in the same year, make large no. seeds from each cross so when phenotypes of next gen are counted - numbers make them statistically meaningful

Question 4

Monohybrid inheritance p2

Answer 4

To study inheritance Mendel chose pairs of contrasting characteristics (tall/ dwarf, yellow/ green seeds, round/wrinkled seeds) was fortunate in choice of characteristics as: controlled by single genes, controlled by genes on different chromosomes, clear cut and easy to tell apart Characteristics are examples of discontinuous variation- but most traits show continuous variation - have range of values/ controlled by various genes

Question 5

Genetic diagrams- what do they show

Answer 5

• The generations eg first gen F1 and second gen F2… Genotypes/ phenotypes of parents and offspring Alleles present in the gametes F1 = first filial generation-=offspring of parent’s cross F2= second filial generation= offspring of self fertilised F1 plant or between members of F1 generation- also grandchildren of og parent

Question 6

Writing cross diagram steps- diagram plssssssssss p199

Answer 6

1- suitable symbols for the alleles- single letter for each characteristic/ lower and upper case= differ in shape and size 2- Write parents genotypes w/ appropriate pairs of letters- label genotypes of parents and state their phenotypes 3- show the gametes produced by each parent- circle and label them gametes 4- use a matrix called a punnet square to show results of possible combinations from random crossing of gamete- label genotype of F1 - show the phenotype of each F1 genotype in punnet square 6- indicate ratio of phenotypes

Question 7

Test cross or back cross

Answer 7

Test cross or back cross= cross between individual with the phenotype of a dominant characteristic but unknown genotype w/w/ individual that’s homozygous recessive for the gene in question eg tall pea plant could be pure breeding TT but also could be Tt not possible to tell from appearance- to test its genotype the tall plant is crossed w/ a dwarf plant- dwarf phenotype has only 1 possible genotype- homozygous recessive- tt- If tall plant is TT all F1 would be tall but if Tt 50% of plants in F1 genotype would be short assuming enough F1 members were counted- too few and only TT offspring could be produced by chance

Question 8

When neither allele is dominant- codominance

Answer 8

Both alleles in heterozygote are expressed individually- as result heterozygote offspring has a combo of both homozygote’s parent’s characteristics- eg in human ABO blood group system the I gene has 3 alleles I^A, I^B, I^O - homozygous parent I^A I^A = group A and I^B I^B= group B- offspring= I^AI^B- both alleles expressed both have A and B antigens on red blood cells so are group AB- can also be shown in phenotype eg chicken with white feathers x chicken with black feathers= offspring w/ speckled feathers

Question 9

Incomplete dominance

Answer 9

For some genes the phenotype of the heterozygote is intermediate between 2 parent phenotypes rather than both being expressed eg red flowered carnations x white carnations have F1 w/ all pink flowers- allele for red or white flowers not completely represented so allele symbols not given upper or lower case instead symbols R or C^R for red or W or C^W for white- in cross between red/ white carnations- produce 2 different types of gamete so upon interbreeding 1: red: 2 pink: 1 white

Question 10

Independent assortment

Answer 10

Independent assortment-genes assorted independently of each other during meiosis-so if 1 gene ends up in 1 gamete= equally likely for gene to end up in another gamete as well- due to way- during meiosis 1- pairs of homologous chromosomes- two pieces of DNA within a diploid organism which carry the same genes, one from each parental source- However, each homologous chromosome can provide a different version of each gene. - homologous chromosomes replicate- 2 sister chromatids for each chromosome- homologous pairs line up along equator- different processes can happen during lining up- . It is random each time which side of the equator the paternal and maternal chromosome of each homologous pair align at the equator, and as a result, each gamete receives different combinations of the maternal and paternal chromosomes- many different combos of lining up of all chromomsomes.

Question 11

law of segregation vs independent assortment

Answer 11

Mendel stated that each individual has two alleles for each trait, one from each parent. Thus, he formed the “first rule”, the Law of Segregation, which states individuals possess two alleles and a parent passes only one allele to his/her offspring. One allele is given by the female parent and the other is given by the male parent. The two genes may or may not contain the same information. If the two alleles are identical, the individual is called homozygous for the trait. If the two alleles are different, the individual is called heterozygous. The presence of an allele does not promise that the trait will be expressed in the individual that possesses it. In heterozygous individuals, the only allele that is expressed is the dominant.

Question 12

Independent assortment- what does this mean

Answer 12

Mendel then crossed these dihybrids. If it is inevitable that round seeds must always be yellow and wrinkled seeds must be green, then he would have expected that this would produce a typical monohybrid cross: 75 percent round-yellow; 25 percent wrinkled-green. But he found 9:3:3:1 ratio for cross between traits- each trait inherited independently of the others, he formed his “second rule”, the Law of Independent Assortment, which states the inheritance of one pair of factors (genes) is independent of the inheritance of the other pair. Today we know that this rule holds only if the genes are on separate chromosomes- genes A and B behave independently- described as unlinked- A has no bearing on the allele it receives for gene B -

Question 13

Dihybrid inheritance

Answer 13

Dihybrid inheritance- simultaneous inheritance of 2 unlinked genes eg genes of different chromosomes- Mendel knew from his early experiments with monohybrid crosses that in pea seeds - yellow colour was dominant to green and found seed shape was dominant to wrinkled crossed contrasting characters R= round seeds, r= wrinkled seeds/ Y= yellow seeds, y= green seeds

Question 14

Dihybrid inheritance- continuation of example

Answer 14

Plant with phenotype RRYY= homozygous for 2 dominant characters - round and yellow seeds- crossed with rryy homozygous for 2 recessive characters- wrinkled and green- each parent only produced 1 type of gamete RY and ry so all F1 plants= RrYy- round and yellow seeds Each gamete of the F1 plants contained an allele for shape and colour R would combine with with Y or y with equal probability- same for r with Y or y- so RY, Ry, rY and ry produced in equal proportions

Question 15

Dihybrid inheritance- continuation of example for F2

Answer 15

when F1 self fertilises each gamete type in pollen would combine with all four gamete types in oospheres w/ equal probability- F2 = for different combos of alleles- look at p202- 9 round yellow: 3 wrinkled yellow: 3: round green: 1 wrinkled green If colour or texture gene was considered alone 12:4=3:1 would be for either round: wrinkled or yellow: green- monohybrid ratio obtained as only one gene considered To calculate ratio of progeny - total number is divided by no. homozygous recessive alleles eg 315 round yellow and 32 wrinkled green (homozygous recessive) 315/32= 9 for ratio no. for round yellow - p203

Question 16

Dihibrid ratio

Answer 16

Formulation of dihybrid ratio led to 2nd law of inheritance - attributed to Mendel- states: Either one of a pair of contrasted characters may combine with either of another pair- With our current understanding of genetics his statement can be rewritten as- each member of pair alleles may combine randomly w/ either of another pair on a different chromosome

Question 17

Dihybrid test cross

Answer 17

pea plant with round yellow seeds could be RrYY, RRYy or RrYy- dihybrid test crosses to see what type of genotype- cross it with individual homozygous for both genes- rryy- ratios of phenotypes in the progeny indicate the genotype of the parent- in each cross the parental phenotypes are round yellow seeds and wrinkled green seeds- look at bottom table p203

Question 18

Linkage

Answer 18

Linked- Description of genes that are on the same chromosome and therefore do not segregate independently by meiosis- can’t move to opposite poles- on same chromosome Some cells undergoing meiosis genetic crossing over can happen between genes such as Dd/ Ee- 4 gamete types- DE, De, dE, de- DE and de originally on chromosome together- parental genotypes- dE and De- recombinant genotypes-combo of characteristics is described as parental when DE/ de come together and recombinant when De or dE come together- look at graph doesn’t have to be capitals vs non capitals

Question 19

Linkage p2

Answer 19

Crossing between 2 given genes is a rare event and does not happen in most cells so majority of gametes would still be parental- so no.gametes with different genotypes is not equal- so Mendelian ratios are not produced in the next gen and majority of phenotypes in offspring would have parental phenotypes Further apart 2 genes are on a chromosome- more opportunity for crossing over - leads to more recombinant gametes - therefore more offspring w/ recombinant phenotypes

Question 20

Recognising linkage

Answer 20

If the no. of progeny with different combinations of characteristics do not respond with mendelian ratios- possible genes are linked - common explanation for rejecting null hypothesis in genetics exp eg cross 2 organisms- 1 homozygous for light, round other homozygous for skinty legend and dark and ratio is not 9:3:3:1 ratio- suggests genes for shape and colour is linked

Question 21

Probability

Answer 21

Presentation of genetic cross= prediction of a likely outcome- actual results though= unlikely to agree precisely w/ the prediction eg coin toss- predicted 50% heads and tails- but if this happened in first 100 throws= surprising- if coin landed 60H and 40T could be due to chance deviation or due to biassed coin- if 2 unbiased coins are tossed can get HH, HT, TH or TT

Question 22

Probability- rules

Answer 22

Rule of addition- when a coin is tossed must be either H or T - probability of 1 result or the other= sum of independent probabilities of getting H is 1/2 and getting T is 1/2 and - so probability of getting one or the other is 1/2+1/2=1 Rule of multiplication- Consider both coins- probability of throwing both H on both is found by multiplying the 2 independent probabilities- in this case the probability of getting H on first coin is 1/2 and probability of getting H on second coin is also 1/2 so probability of HH is 1/2s1/2=1/4

Question 23

Rules for probability- punnet squares

Answer 23

for dihybrid cross 3:1 ratio for both seed shape and seed colour genes in F2- can be used to calsulate probability when all 4 alleles are involved eg round (dominant)-3/4, wrinkled (recessive)-1/4, yellow (dominant)-3/4, green (recessive)-1/4 Probability of combinations of alleles examples round and yellow -3/4 x3/4= 9/16 round and green-3/4x1/4=3/16 not all combos but you get the gist

Question 24

chi squared test

Answer 24

Expected ratio of phenotypes for F2 of homozygous parents of a monohybrid cross is 3:1, of a dihybrid cross 9:3:3:3:1 and of a cross with incomplete or codominance= 1:2:1- ratios represent the probabilities of getting each of the phenotypes- would be surprising if we got exact ratios- chi squared test is used to see if numbers of the different phenotypes are close enough to predicted values to support genetic explanation of how they arose- if the numbers aren’t close enough- shows they have arisen for another reason

Question 25

Null hypotghesis

Answer 25

Statistical tests are designed to to test null hypothesis- which in genetics is a statement that there's no significant difference between the observed and expected results of a cross- in genetics a null hypothesis states that the observed results are due to Mendelian inheritance- deviation from expected ratio= due to chance Reason this is a null hypothesis= Mendelian inheritance is based on there being no difference in: no. of the different type of gamete, probability of each gamete type fusing with another type of gamete, the viability of embryos whatever their genotype and that genes aren't linked

Question 26

Carrying out Chi Squared/ how it's related to probability

Answer 26

look at mind map for method Biologists make predictions based on their model of how a phenomenon works- if predicted results happen 5% or more times the experiment is run- biological reasoning is correct- any deviation is not significant- due to chance If predicted results happen less frequently than 5% of time- deviation is significant- assumptions on which prediction was made= not correct- must be different explanation for phenomenon- 5% and above- null can be accepted

Question 27

Chi squared- constructing conclusion

Answer 27

Must include certain points calculated value of X^2 is less/ more than critical value... This is equivalent to a probability greater/ lower than 5% so the null hypothesis is accepted/ rejected at the 5% level of significance Inheritance is/ isn't Mendelian and any deviation from predicted ratio is/ isn't due to chance

Question 28

Sex determination

Answer 28

Most angiosperm species are hermaphroditic and their flowers make both pollen and ovules but 2 other main strategies- Monoecious plants have separate male and female flowers eg maize Dioecious plants have seperate male and female plants among animals there are hermaphrodites- but uncommon among vertebrates

Question 29

Sex determination examples

Answer 29

Temperature- lizard, crocodile and alligator eggs hatch as male when temp above 32C and female when below 32C- sea turtles hatch as females when in sun and as males when in shade Sequential hermaphrodism- certain mollusc species makes stacks of individuals - those at top are male- as more males join top of stack those below them become females Male sewadge sludge worm can become hermaphroditic and fertilise itself if females aren't available Ploidy level- bee eggs that aren't fertilised are haploid and develop as males - those that are fertilised are diploid= females Chromosome structure- In mammals, females have 2 X chromosomes- males have X and Y chromosome- reverse situation occurs in birds some have ZZ male and ZW female

Question 30

Human sex ddetermination

Answer 30

Human cells contain 46 chromosomes, 23 from each parent-chromosomes from 2 parents can be arranged in homologous pairs with each pair containing chromosomes of the same size and characteristics- the arrangement of homologous pairs in decreasing size order is called a karyotype Of 23 pairs in Karyotype- 22 pairs have homologous genes although may have different alleles- these chromosomes= autosomes- 23rd pair= gametes-females- comprise of 2 X chromosomes, males- XY- Y is much shorter- sex chromosomes= different sizes- called heterosomes 2 regions on X and Y chromosomes= homologous- can pair with each other at meiosis- czlled pseudoautosomal regions- PAR1 and PAR2- reflect evolutionary loss of genes from 1 sex chromosome- leaves larger X and smaller Y chromosome

Question 31

Maleness and the Y chromosome

Answer 31

X and Y chromosomes probably have a common ancestor in our distant evolutionary past- but the Y chromosome has lost so many of its genes that it shares few regions of DNA with the X - 1 gene that Y has that X doesn't- SRY gene - this is 'sex determining region' of the Y chromosome- role is to switch genes on other chromosomes eg genes on autosomes- are responsible for expression of male characteristics Commonest chromosomal abnormalities are those involving the number of sex chromosomes- a person can have any no. X chromosomes but only takes 1 X chromosome to make a male

Question 32

Chromosomal sex determination in humans

Answer 32

All females secondary oocytes contain an X chromosome so female is 'homogametic sex' eg gametes= identical regarding sex chromosomes In males at meiosis I an X chromosome passes into the other- results in 1/2 males' sperm contain X chromosome and other 1/2 contain Y chromosome- male is heterogametic sex- gametes are differnt types regarding sex cells At fertilisation the oocyte may be fertilised by either an X or Y carrying sperm w/ equal probability- equal chance of being male or female

Question 33

Sex linkage Homologous chromosomes are two pieces of DNA within a diploid organism which carry the same genes, one from each parental source. In simpler terms, both of your parents provide a complete genome. Each parent provides the same 23 chromosomes, which encode the same genes. So, our cells carry 46 total chromosomes, in two copies. However, each homologous chromosome can provide a different version of each gene.

Answer 33

Most of lengths of X and Y chromosomes aren't homologous - in those regions females have 2 copies of each gene as they have 2 X and males have only 1 copy- if female is heterozygous for 1 of those genes- dominant allele will be expressed as normal- whichever allele the male carries is expressed- even if he has recessive allele- no second allele to dominate it

Question 34

sex linkage- conditions

Answer 34

Definitions Carrier- a phenotypically normal individual (female in sex linked conditions) with 1 normal dominant allele and 1 mutant recessive allele Sex linkage- a gene is carried by a sex chromosome so a characteristic it encodes is seen predominantly in 1 sex Haemophelia- associated w/ blood disorder- can't produce 13 blood clotting proteins- blood clots slowly - slow persistent bleeding- gene on X chromosome=X linked gene- codes for blood clotting protein- Factor VIII- Allele coding for normal version- X^H but coding for mutant version= X^h-

Question 35

Haemophelia

Answer 35

X^HX^H= normal blood clotting X^hX^H- carrier X^hX^h- female has haemophelia X^HY- normal X^hY- haemophelia- male can't be carrier will have haemophelia or won't-condition far more common in males than females - condition is therefore described as sex linked - gene on X chromosome gives rise to sex linked condition= sex linked gene

Question 36

Inheritance of sex linked conditions

Answer 36

Male cannot pass alleles on his X chromosomes to his sons- must receive his Y chromosomes - his daughters however all recieve an X chromosome from their father DMD caused by an X linked recessive allele of the dystrophin gene- gene codes for protein dystrophin- component of a glycoprotein that stabilises the cell membranes of muscle fibres- Allele for normal protein has symbol X^D- allele for mutant protein has symbol X^d- normal male X^DY shows statistically carrier females have 50% chance of having affected sons - daughters would be phenoptypically but 50% would be carriers- Phenotypic ratio for sons=1:1 unaffected:carrier Affected males X^dY can't pass mutant gene to sons if mother is unaffected X^DX^D daughters will all be carriers- To be affected female must receive the X^d allele from both parents- mum can either be carrier or have DMD

Question 37

Pedigree diagrams

Answer 37

Inheritance of a medical condition marked on a family tree allows a geneticist to infer an explanation of how it is inherited- these pedigree diagrams can indicate whether a condition is sex linked and if it is the result of a dominant or recessive allele- diagram p211 (knee breakage if otherwise) - on page will say what you can infer from diagram

Question 38

Mutations

Answer 38

Mutation= change in amount, arrangement or structure in hereditary material of an organism- either DNA or for some viruses RNA- Mutations are: spontaneous- may happen without apparent cause Random- appear to happen w/ equal probability anywhere in genome of diploid organisms Mutations can occur in all cells but only mutations that occur in gametes can be inherited- most mutations= harmful eg UV light exposure can cause skin cancer- beneficial mutations= rare- give selective advantage

Question 39

Mutations p2

Answer 39

Mutations may contribute to variation between individuals- which is raw material for natural selection and so evolution- in haploid organisms any mutation is expressed eg can be seen in phenotype- unless lethal In diploid organisms dominant mutations are expressed but are rare- most mutations are recessive- they're likely to occur in a cell w/ a dominant allele and so are not expressed- mutation in diploid organisms, therefore has less impact on evolution than other sources of variation.

Question 40

Mutation Rate

Answer 40

Mutations can occur during DNA replication and during cell division- so generally organisms with short life cycles and frequent meiosis will show greater rate of mutation Other ways of increasing mutation rate Ionising radiation- gamma, X rays and UV light- UV light of wavelength 260 nm is particuarly mutagenic as it is wavelength that DNA absorbs most efficiently. Radiation joins adjacent pyrimidine bases in DNA strand so replication DNA polymerase may insert correct nucleotide.

Question 41

Mutation rate p2

Answer 41

Gene or point mutation- DNA is not copied accurately in S phase before cell division-errors involve 1 or a small numbers of bases Chromosome mutation- chromosomes may get damaged and break - broken chromosomes may repair themselves and the DNA and protein rejoin- but may not repair themselves correctly- alter their structure- potentially affecting a large number of genes -Aneuploidy- a whole chromosome or small number of chromosomes may be lost or added- in phenomenon called non-disjunction- when chromosomes fail to separate to poles of dividing cells at anaphase I or when chromosomes fail to separate at anaphase II Polyploidy- no. chromosomes may double if the cell fails to divide following the first nuclear division after fertilisation

Question 42

Gene (point) mutations

Answer 42

If DNA polymerase changes the base sequence, a point or gene mutation occurs- these are: Addition- a base is added- if this happens in 3 places an extra amino acid is added to the polypeptide chain at translation -Duplication- same base is incorporated twice -Subtraction- a base is deleted- if this happens in 3 places- the polypeptide has one fewer amino acids when translated -Substitution- a different base is incorporated Inversion- adjacent bases on the same DNA strand exchange position

Question 43

Point mutation p2

Answer 43

Point mutation causes change in allele in which it occurs and the bases of the messenger RNA codon- the effect on the polypeptide produced at translation and consequently the effect on the phenotype, depends on the nature of the mutation: - New codon may code for same amino acid so there's no change to the polypeptide = 'silent' mutation - If amino acid with a similar chemical nature is substituted the effect may be small eg if valine replaces alanine -If the mutation is at a significant site on the protein molecule- may make a significant difference to the activity of the protein- if protein were an enzyme- the structure of the active site could be destroyed -If 1 or 2 bases are added or deleted a frame shift mutation occurs and all subsequent amino acids incorporated will be altered

Question 44

Sickle cell disease

Answer 44

-A substitution point mutation in the gene producing the B polypeptide of haemoglobin results in sickle cell disease- a group of conditions affecting the haemoglobin -A group of conditions affecting the haemoglobin- A DNA triplet on the coding strand CTC codes for amino acid glutamate- substituting A for T gives CAC which codes for valine- side chain of glutamate is large and hydrophilic whereas valine is small and hydrophobic- when oxygen tension is low the affected haemoglobin within the red blood cell aggregates- the cell membrane collapses on the precipitated haemoglobin and red blood cell becomes sickle shaped- cells become fragile and might break in the capillaries

Question 45

Sickle cell disease- coding

Answer 45

Normal haemoglobin has symbol HbA produced by allele Hb^A- the mutant haemoglobin has the symbol HbS and is produced by allele Hb^S- Individuals who have genotype Hb^SHb^S have sickle cell anemia- may be severely affected with joint pain/ organ damage - the ability of their red blood cells to carry oxygen is reduced- resulting in anemia and possible death- alleles Hb^A and Hb^S are codominant- in heterozygotes HbA and HbS are produced have sickle cell trait - at least 50% of their haemoglobin is HbA- symptoms= less severe than people who have genotype Hb^SHb^S

Question 46

Chromosome mutations- changes in structure

Answer 46

Chromosome mutations= changes in structure or number of chromosomes in cells during prophase I of meiosis- homologous chromosomes pair and exchange material at chiasmata- mutation arises when a chromosome does not rejoin accurately at the corresponding condition on its homologous partner - the homologous chromosomes and so the gametes they're in end up with some different genes- each gamete may still fuse w/ another and produce a new individual but further meiosis will be impossible as mutant chromosomes will not be able to make homologous pairs at meiosis

Question 47

Mutations- changes in number of chromosomes

Answer 47

Changes in chromosome number are most likely to occur during meiosis Non disjunction- A faulty cell division in meiosis due to faulty spindle-following which one of the daughter cell receives 2 copies of a chromosome and the other receives none Down's syndrome- Chromosome 21 is affected- if non-disjunction happens during oogenesis- secondary oocyte has either no chromosome 21 or has 2 copies not 1 - those w/ no chromosome 21 cannot produce viable embryo 2 copies of 21 chromosome that fuses with a normal sperm produces viable embryo w/ cells containing 3 copies of chromosome 21 instead of 2 w/total of 47 chromosomes- called trisomy 21- produces individual w/ down's syndrome

Question 48

Translocation Down's

Answer 48

Fragment of one chromosome has attached to another- of people who have Down's syndrome 5% have 46 chromosomes - during meiosis forming a gamete that produced them, a fragment of chromosome 21 attached itself to chromosome 14- when that abnormal gamete fused with a normal one - it produced an embryo w/ 2 normal copies of chromosome 21 and the fragment of an additional one attached to chromosome 14

Question 49

Changes in numbers of sets of chromosomes

Answer 49

Cells with complete sets of chromosomes= euploid Small no. extra chromosomes or small no. too few may result from non-disjunction- aneuploid Polyploidy- Having more than 2 complete sets of chromosomes- may arise in different ways -defect in spindle at meiosis may result in all chromosomes at anaphase I or all chromosomes at anaphase II moving to same pole of cell- makes gametes w/ 2 of each chromosome- when diploid gamete is fertilised by 'normal' gamete- triploid zygote w/ 3 sets of chromosomes is made- may survive but won't be able to make homologous pairs at meiosis -unable to make gametes= infertile- most plants reproduce asexually- triploidy doesn't affect reproduction

Question 50

Changes in numbers of sets of chromosomes p2

Answer 50

Other ways of polyploidy -If 2 diploid gametes fuse a tetraploid (4n) is produced- -Endomitosis= replication of chromosomes that's not followed by cytokinesis- if this happens in early embryo 4 sets of chromosomes are incorporated into the new nuclear envelope and successive rounds of mitosis continue to produce tetraploid cells - in very rare situations triploids (3n) undergo endomitosis making hexaploids (6n)- hexaploids= fertile- can make homologous pairs in meiosis

Question 51

polyploidy- why more present in plants

Answer 51

polyploidy very common in plants and is associated with beneficial characteristics eg vigour/ disease resistance Much more common in plants than in animals, possibly because many plants can produce asexually are hermaphrodic and so do not use chromosomes to determine their sex

Question 52

Carcinogens and their genes

Answer 52

Carcinogen= agent that causes cancer- some mutants are carcinogenic Tumour suppressor genes- genes control cell division- division halted when enough cells have been produced for growth and repair- genes regulate mitosis- prevent cells dividing too quickly- mutation can cause 1 of these genes to lose its regulatory function- cell could then go through continual repeated mitosis- causes cancer- if cell escapes attack of immune system - produces collection cells called tumour- can be benign but sometimes tumours can spread throughout the body- invade other tissues- (secondary tumours/ metastases)= malignant

Question 53

TP53

Answer 53

Abnormalities in tumour suppressor gene TP53 which codes for P53 protein have been identified in more than 1/2 human cancers Normal p53 protein- activates repair of damaged DNA , prevents cell from entering S phase while damaged DNA is repaired, initiates apoptosis if damaged DNA can't be repaired Mutant p53 protein- No DNA repair, Cell with damaged DNA enters S phase and DNA is replicated, mutant cells survive and undergo mitosis

Question 54

Oncogenes

Answer 54

Oncogene= A proto- oncogene with a mutation that results in cancer Proto-oncogene codes for protein that contributes to cell division- mutation may switch on a gene permanently so excessive amounts of a protein are made= rapid uncontrolled, repeated mitosis- proto-oncogene now called oncogene if it causes cancer may happen if A mutation causes chromosomes to rearrange and places the proto-oncogene next to a DNA sequence that permanently activates it There is an extra copy of the proto-oncogene- too much of its product is made Tobacco smoke contains lots of chemicals eg tar, nicotine and carbon monoxide- over 40 are known to be carcinogenic and 400 others are toxic- tar collects in lungs as tobacco smoke cools- it's a mixture of lots of chemicals- some can enter nuclei of alveolar cells and slide between base pairs in DNA - cause mutation by preventing accurate replication- carcinogenic

Question 55

Control of gene expression- modification

Answer 55

Variation= differences between members of a species- traditionally related top differences in DNA nucleotide sequence eg different alleles/ physiological effects of the environment eg higher light intensity increasing plant growth Epigenetics=the control of gene expression by modifying DNA or histones but not affecting the DNA nucleotide sequence- evidence shows environment can alter expression of genes by affecting how they're transcribed without changing nucleotide sequence This underpins an embryo’s ability to differentiate its cells into specialised lineages for different organs and tissues in the adult: skin tissue, muscle tissue, nervous tissue, etc.

Question 56

Transcription- brief overview

Answer 56

Transcription – DNA is transcribed and an mRNA molecule is produced This stage of protein synthesis occurs in the nucleus of the cell Part of a DNA molecule unwinds (the hydrogen bonds between the complementary base pairs break) Catalysed by helicase, like in DNA replication This exposes the gene to be transcribed (the gene from which a particular polypeptide will be produced)

Question 57

Transcription brief overview p2

Answer 57

A complementary copy of the code from the gene is made by building a single-stranded nucleic acid molecule known as mRNA (messenger RNA) Free activated RNA nucleotides pair up (via hydrogen bonds) with their complementary (now exposed) bases on one strand (the template strand) of the ‘unzipped’ DNA molecule The sugar-phosphate groups of these RNA nucleotides are then bonded together by the enzyme RNA polymerase to form the sugar-phosphate backbone of the mRNA molecule When the gene has been transcribed (when the mRNA molecule is complete), the hydrogen bonds between the mRNA and DNA strands break and the double-stranded DNA molecule re-forms The mRNA molecule then leaves the nucleus via a pore in the nuclear envelope

Question 58

Translation overview- check

Answer 58

In the transcription stage of protein synthesis, the section of the DNA molecule where the gene is located (the gene coding for a particular polypeptide) unwinds – the hydrogen bonds between the complementary base pairs break, causing the two DNA strands to ‘unzip’ Free activated RNA nucleotides then pair up with the exposed bases on the DNA molecule but only with those bases on one strand of the DNA molecule This strand of the DNA molecule is called the template strand or the transcribed strand This is the strand that is transcribed to form the mRNA molecule (RNA polymerase binds the RNA nucleotides together to create the sugar-phosphate backbone of the mRNA molecule) This mRNA molecule will then be translated into an amino acid chain The strand of the DNA molecule that is not transcribed is called the non-template strand or the non-transcribed strand

Question 59

Epigeneics- background- histones

Answer 59

All of the chemical modifications to all histone proteins and DNA (except base changes) in an organism is called the epigenome In eukaryotic cells, the DNA in the nucleus is wrapped around proteins called histones Histone proteins can be chemically modified through the addition of acetyl DNA can also be chemically modified through the addition of methyl groups without changing the base sequence which also leads to the regulation of gene expression DNA is wrapped around histone proteins which form a nucleosome. Nucleosomes coil tightly around each other to form the chromosome structure.

Question 60

h

Answer 60

Transcription can be inhibited by specific means. A common way is increased DNA methylation. The methyl (CH3) group acts as a tag on the DNA at various locations and prevents transcription that might’ve occurred otherwise.

Question 61

More on what causes change to epigenome

Answer 61

The epigenome, like the genome, is heritable but can undergo change Identical twins become more distinguishable with age because despite having exactly the same DNA, their epigenomes change independently, leading to differences Changes to the epigenome are caused by changes in the environment Smoking, stress, exercise and diet can cause epigenetic changes Internal signalling from the body's own cells can also cause modifications to occur

Question 62

Histone modification p1

Answer 62

The chemical modification of histones and DNA controls how tightly the DNA is wound around them as the intermolecular bonding between the histones and DNA changes If the DNA is wound more tightly in a certain area, the genes on these section of DNA are 'switched off' as the gene and promoter regions are more hidden from transcription factors and RNA polymerase The modification of histones is reversible and therefore can be different in different cell types and can vary with age

Question 63

Histone modification p2-

Answer 63

Histone modification follows translation- examples: - an acetyl group (-CH3CO) to amino acid lysine -A methyl group (CH3) to lysine or arginine - a phosphate group (-PO4^2-) to serine or threonine chemical modification of histones and DNA controls how tightly the DNA is wound around them- If the DNA is wound more tightly in a certain area, the genes on these section of DNA are 'switched off' as the gene and promoter regions are more hidden from transcription factors and RNA polymerase The modification of histones is reversible and therefore can be different in different cell types and can vary with age

Question 64

Effects of epigenetics p1

Answer 64

Stem cells of the embryo progressively differentiate- switching off genes coding for enzymes that are not needed- changes mean that diferentiated cells only express genes needed for own activity - different epigenetic changes can happen to cells within same tissue or to cells in different tissues- allows for big difference in gene expression in different cells of an organism DNA damage happens thousands of times a day in cells of body- most repaired but epigenetic changes may remain- seen in monozygotic twins- identical nucleotide sequences epigenetic differences similar at start but increase w/ age

Question 65

Effects of epigenetics p2

Answer 65

Genomic printing can happen- if genes inactivated in gametes inactivation may be passed to next gen eg gene may be permanently switched off by DNA methylation on chromosome derived from 1 parent- if switching= damaged- medical condition may happen eg NOEY2 gene- only fathers copy is expressed- if like mother's it's not then offspring more at risk for breast/ ovarian cancer X inactivation- whole chromosome turned off- female mammals just have 1 X chromosome- other turns to densely staining chromatin called Barr body- look at cat- patchwork of fur shows random X inactivation- fur gene carried on these X chromosomes inactivated