Questions on meiosis and the sources of genetic variation

Question 1

Describe how crossing over happens during prophase I of meiosis, and describe its effect on the chromatids involved.

Accepted Answer

B1 each chromosome finds its homologue and the two lie alongside each other along their whole length, forming a bivalent of four chromatids B1 non-sister chromatids, one maternal and one paternal, break at the same point and each joins to the other's cut end B1 the visible point at which they cross is a chiasma, and equivalent sections of DNA have been exchanged A1 the two chromatids that took part leave carrying a combination of alleles that neither parental chromosome had, so sister chromatids are no longer genetically identical; the other two leave unchanged

Question 2

Explain how meiosis produces gametes that are genetically different from one another, naming the stage at which each process happens.

Accepted Answer

B1 crossing over in prophase I exchanges sections of DNA between non-sister chromatids of a homologous pair B1 this makes new combinations of alleles along a single chromosome, combinations that were on neither chromosome the cell started with B1 independent assortment at metaphase I decides which member of each bivalent faces which pole, and the decision is made separately for every bivalent A1 this makes new combinations of whole chromosomes, giving 2 to the power n kinds of gamete where n is the number of homologous pairs

Question 3

A species of plant has a diploid number of 14. Calculate the number of different chromosome combinations its gametes could carry from independent assortment alone, and calculate the number of different chromosome combinations possible in a zygote formed by two such plants.

Accepted Answer

M1 halve the diploid number to get the number of homologous pairs, so n = 7, because one decision is made per pair A1 2 to the power 7 is 128 chromosome combinations in a gamete M1 a zygote takes one gamete from each parent and the two are independent, so multiply 128 by 128 A1 16 384 combinations, and crossing over would make the true figure larger still

Question 4

Compare mitosis with meiosis, referring to the number of divisions, the chromosome number of the daughter cells, the behaviour of homologous chromosomes and the genetic result.

Accepted Answer

B1 mitosis is one division producing two daughter cells, whereas meiosis is two divisions producing four B1 mitosis leaves the chromosome number unchanged, diploid to diploid, whereas meiosis halves it, diploid to haploid B1 homologous chromosomes do not pair in mitosis, whereas in meiosis they pair as bivalents in prophase I and cross over B1 the daughters of mitosis are genetically identical to the parent cell, whereas the four products of meiosis all differ from one another

Question 5

Describe what happens to the spindle and to the nuclear envelope during prophase I and during telophase II of meiosis.

Accepted Answer

B1 in prophase I the nuclear envelope breaks up into small vesicles and the nucleolus disappears B1 a spindle of microtubules is built between the two poles of the cell, growing from a pair of centrioles at each pole in an animal cell and from a region without centrioles in most plant cells B1 in telophase II the spindles break down A1 a nuclear envelope re-forms around each of the four groups of chromosomes, the chromosomes uncoil and the nucleoli reappear

Question 6

A cell of a species with a diploid number of 20 is about to begin meiosis, immediately after DNA replication. Calculate the number of chromosomes and the number of chromatids present at the start of meiosis, at the end of meiosis I, and at the end of meiosis II.

Accepted Answer

M1 a diploid number of 20 means the cell, after replication, has 20 chromosomes and 40 chromatids at the start of meiosis (each of the 20 chromosomes now two chromatids) A1 at the end of meiosis I: 10 chromosomes, 20 chromatids, since homologues have separated but the centromeres have not divided M1 chromosome number does not change again at the second division: the centromeres divide so each chromatid becomes a chromosome, and the cell then divides, so the chromatid count per cell halves A1 at the end of meiosis II: 10 chromosomes, 10 chromatids in each of the four cells

Question 7

In a species with a diploid number of 16, non-disjunction of one homologous pair occurs at anaphase I of meiosis in one cell, and the rest of the division is normal. Describe the chromosome number you would expect to find in each of the four cells that result from this meiosis.

Accepted Answer

B1 the normal haploid number for this species is 8 B1 because non-disjunction happened at anaphase I, both homologues of the affected pair went to the same pole, so one of the two cells produced by meiosis I has one extra chromosome, 9, and the other has one too few, 7 B1 meiosis II proceeds normally on each of these two cells, simply separating the sister chromatids of each without changing the chromosome number further B1 the four final cells therefore have chromosome numbers 9, 9, 7 and 7, because non-disjunction at anaphase I affects all four products, unlike non-disjunction at anaphase II, which would affect only two of the four

Question 8

A person with Turner's syndrome has 45 chromosomes, with a single X chromosome and no other sex chromosome. Explain how non-disjunction during meiosis in the father can produce a child with Turner's syndrome, and explain why the child is female.

Accepted Answer

B1 the father's X and Y chromosomes fail to separate at anaphase I, or the sister chromatids of a sex chromosome fail to separate at anaphase II B1 so a sperm is produced with no sex chromosome, carrying 22 chromosomes instead of 23 B1 it fertilises a normal egg carrying 22 autosomes and an X, giving a zygote with 45 chromosomes and one X: a monosomy B1 the zygote has no Y chromosome, so it lacks the gene on the Y that switches development onto the male route, and develops as a female

Question 9

State what is meant by polysomy and by monosomy, and state the chromosome change found in Down's syndrome and in Turner's syndrome.

Accepted Answer

B1 polysomy is the presence of one or more extra copies of a chromosome in a cell B1 Down's syndrome is a polysomy: three copies of chromosome 21 (trisomy 21), 47 chromosomes in all B1 monosomy is the presence of only one copy of a chromosome where there should be a pair B1 Turner's syndrome is a monosomy: a single X chromosome and no second sex chromosome, 45,X

Question 10

A chemical prevents chiasmata from forming in prophase I but allows the rest of meiosis to run normally. Suggest what this does to the genetic variation of the gametes produced, and suggest why they would still not all be the same.

Accepted Answer

B1 no crossing over would occur, so no new combinations of alleles would be made along a chromosome and each chromatid would carry the alleles of the parental chromosome it came from B1 sister chromatids would therefore stay genetically identical, so the two cells produced by each meiosis II would be identical to each other A1 independent assortment at metaphase I would still combine whole chromosomes in new ways, giving 2 to the power n kinds of gamete, so variation would be reduced rather than abolished

Question 11

Compare the way the cytoplasm divides at the end of meiosis in an animal cell with the way it divides in a plant cell.

Accepted Answer

B1 in an animal cell a ring of protein filaments just inside the cell surface membrane contracts and draws the membrane inwards as a cleavage furrow, which deepens until the cell is pinched in two B1 in a plant cell vesicles from the Golgi apparatus line up along the old equator and fuse into a cell plate, which becomes the new cell surface membranes and the new cell wall between the daughter cells A1 the difference exists because the rigid cell wall of a plant cell cannot be pinched inwards

Question 12

The drawing shows a single cell in which the chromosomes lie across the middle of the cell in homologous pairs, with no nuclear envelope and with spindle fibres running to each end. Identify the stage shown, and give two features of the drawing that rule out the other stages.

Accepted Answer

B1 the stage is metaphase I B1 the chromosomes are on the equator in pairs, two chromosomes deep, which happens only in the first division; at metaphase II they would lie in a single line A1 there is a single cell rather than two, and the chromosomes are on the equator rather than strung out towards the poles, which rules out the second division and rules out anaphase

Question 13

Before crossing over takes place, the two sister chromatids of one chromosome are genetically identical. Suggest why, once crossing over has taken place in a bivalent, this is no longer necessarily true, and suggest what would be different about the cells produced by meiosis II from a bivalent in which no crossing over occurred at all.

Accepted Answer

B1 crossing over exchanges a section of DNA between one chromatid of the maternal chromosome and one non-sister chromatid of the paternal chromosome, so those two participating chromatids each carry a new combination of maternal and paternal alleles and are no longer identical to their own sister chromatid B1 where no crossing over takes place in a particular bivalent, each chromatid remains identical to its sister, carrying only the alleles of the parental chromosome it came from B1 the two cells produced by meiosis II from that bivalent would therefore be genetically identical to each other at those loci, whereas if crossing over had occurred the two meiosis II products from that cell could differ from each other at those loci

Question 14

Haemophilia A is caused by a recessive allele on the X chromosome. A girl has a single X chromosome and no other sex chromosome, and she does not have haemophilia. Her father has haemophilia and her mother is a carrier. Explain in which parent non-disjunction occurred.

Accepted Answer

B1 the girl is unaffected with only one X, so her X carries the dominant allele, X^H B1 her father is X^hY and has no X^H to give, so her X came from her mother's egg B1 the sperm therefore carried no sex chromosome, so non-disjunction occurred in the father, his X and Y failing to separate at anaphase I or a pair of sister chromatids failing to separate at anaphase II

Question 15

State the stage of meiosis at which homologous chromosomes are separated, and state the stage at which sister chromatids are separated.

Accepted Answer

B1 homologous chromosomes are separated at anaphase I B1 sister chromatids are separated at anaphase II, when the centromeres divide

Question 16

State what is meant by a bivalent, and state how many chromatids it contains.

Accepted Answer

B1 a bivalent is the structure formed when a homologous pair of chromosomes pairs up (synapsis) in prophase I of meiosis B1 a bivalent contains four chromatids, two from each of the two homologous chromosomes, because both chromosomes were already replicated before pairing

Question 17

In a dividing cell, two separate daughter cells each contain single chromatids being pulled to opposite poles, with the number moving to each pole equal to the haploid number of the species. Identify the stage described in each cell, and identify one feature of that description which shows this is the second division rather than the first.

Accepted Answer

B1 the stage is anaphase II B1 there are two separate cells, each already a product of meiosis I, rather than one, and what is moving to each pole is a single chromatid becoming its own chromosome rather than a whole chromosome of two chromatids, both of which occur only in the second division

Question 18

State when DNA replication occurs relative to the two divisions of meiosis, and state the chromosome number and chromatid number of a cell immediately after that replication in a species with a diploid number of 24.

Accepted Answer

B1 DNA replication occurs once, during the interphase before meiosis begins, and does not happen again between meiosis I and meiosis II B1 immediately after that replication such a cell has 24 chromosomes and 48 chromatids, since each of the 24 chromosomes now consists of two sister chromatids