Topic 2.4 Meiosis and Sexual Reproduction

Question 1

Diploid

Answer 1

-A cell which contains two sets of chromosomes
-(2n) eg. humans 2n=46

Question 2

Haploid

Answer 2

-A cell which contains a single set of chromosomes
-(n) eg. n=23

Question 3

The two divisions in meiosis

Answer 3

-Meiosis 1:
Homologous pairs of chromosomes are separated
-Meiosis 2:
The pairs of chromatids are separated

Question 4

Meiosis 1 summary (9 steps)

Answer 4

-The DNA replicates
-Chromosome condense and consist of 2 sister chromatids joined by a centromere
-The homologous chromosomes pair up
-Crossing over occurs at ‘chiasmata’ this introduces variation
-Spindle fibres line the homologous chromosomes up along the equator
-The homologous chromosome can line up in any order this is known as ‘independent assortment’ and also introduces variation
-The spindle fibres contract and pull the homologous chromosomes to opposite poles
-A nuclear membrane forms around the chromosomes
-The cytoplasm divides and 2 daughter cells are formed

Question 5

Meiosis 2 summary

Answer 5

-Half the amount of DNA
-Independent segregation of sister chromatids
-Finally create 4 genetically unique cells that have half the percent of chromosomes and DNA as the parent cell

Question 6

Importance of meiosis

Answer 6

-Reduces the number of chromosomes
-Diploid—>Haploid
-Introduces genetic variation

Question 7

Crossing over (recombination)

Answer 7

-Blocks of genes are swapped between homologous pairs (maternal and paternal)
-New and unique combination of alleles (variation)

Question 8

Independent assortment

Answer 8

-The haploid gamete contains one member of the homologous pair
-Which it receives is completely random
-23 pairs = 2^23 different combinations (over 8 million different combinations)

Question 9

Random fertilisation

Answer 9

Any one sperm can fertilise any one egg

Question 10

Gene deletion

Answer 10

Whole genes are missing from the codes which results in missing amino acids

Question 11

Positions of genes changing: Duplication

Answer 11

Whole genes are replicated
- 1 2 3 3 4 5 6

Question 12

Positions of genes changing: Inversion

Answer 12

The position of genes are swapped
- A B C D E G F

Question 13

Positions of genes changing: Translocation

Answer 13

Parts of two chromosomes break off and swap places with each other
(A base sequence from one chromosome fuses with a non-homologous chromosome).

Question 14

Translocation (balanced or unbalanced)

Answer 14

-Some translocations are balanced; a piece is swapped between two different chromosomes
-Some translocations are unbalanced; one chromosome loses a piece and another chromosome gains it

Question 15

Translocation effects

Answer 15

Translocations can have severe affects:
-eg. translocation between chromosome 8 and 21 can cause myeloid leukaemia (blood cancer)
-eg. translocation between chromosome 8 and 14 can cause Burkitt’s lymphoma (white blood cell cancer commonly found in children and young adults)

Question 16

Non-disjunctions of the chromosomes

Answer 16

-Some mutations affect whole chromosomes
-In humans, each gamete should contain 23 chromosomes, including one sex chromosome. An error in this is non-disjunction.

Question 17

How does non-disjunction occur

Answer 17

-During a reduction division of meiosis, one of the homologous pairs of chromosomes fail to separate during anaphase 2
-As a result one of the gametes has two copies off that chromosome, and another has no copies

Question 18

Monosomy in non-disjunction

Answer 18

-If one of these abnormal gametes fertilises with a normal gamete, the individual can have monosomy with only one member of the homologous pair present from the normal gamete

Question 19

Polysomy in non-disjunction

Answer 19

-If one of these abnormal gametes fertilises with a normal gamete, the individual can have polysomy with three or more rather than two chromosomes of a particular type

Question 20

Gametogenesis

Answer 20

-Gametes are formed
-In mammals male gamete are called spermatozoa and are produced in the testes
-Female gametes are called ova and are produced in the ovaries

Question 21

Primordial germ cells

Answer 21

Special cells called primordial germ cells in gonads divide and grow, then differentiate into gametes
(Mitosis produces the primordial germ cell and meiosis produces a gamete)

Question 22

Spermatogenesis (5)

Answer 22

-Formation of spermatozoa (sperm) in the testes
1. Diploid (46) germ cell divides by mitosis to give spermatogonia
2. Spermatogonia then grow until they have considered primary spermatocytes
3. Spermatocytes divide by meiosis to give 2 haploid (23) secondary spermatocytes
4. Secondary spermatocytes then undergo meiosis again to form 4 haploid spermatids
5. Spermatids then differentiate into spermatozoa which are capable of fertilisation
(picture)

Question 23

Oogenesis (6)

Answer 23

-Formation of ova (eggs)
1. Primordial germ cell (46) divides by mitosis several times to produce diploid oogonia
2. Only one oogonia grows bigger and develops into the primary oocyte. The other three are broken down (degenerate)
3. Primary oocyte grows and contains lots of storage material
4. The primary oocyte undergoes meiosis to produce a secondary oocyte and the first polar body
5. The first polar body sticks to the secondary oocyte
6. After fertilisation the second meiosis division occurs producing the ovum and three polar bodies which degenerate and die
(picture)

Question 24

Spermatozoa

Answer 24

Spermatozoa must remain in suspension in the semen so they can be transported through the female reproductive tract, and they must be able to penetrate the protective barrier around the ovum and deliver the male haploid genome safely inside.
(picture)

Question 25

Ova

Answer 25

-The human ovum is about 0.1mm across.
-Eggs don’t move of their own, so they don’t need contractile proteins
-They usually contain food for the developing embryo
-The main difference between eggs of various species is the quantity of stored food they contain
-In mammals, once the developing foetus has implanted in the uterus it is supplied with nutrients from the blood supply of the mother and so large food stores in the egg are necessary.

Question 26

Pollen formation (Meiosis stage)

Answer 26

Part 1:
-Meiosis occurs here, resulting vast numbers of the pollen grains that carry the male gametes
-Each anther contains pollen sacs where the pollen grains develop.
-In each pollen sac there are large numbers of diploid microspore mother cells
-They divide by meiosis to form haploid microspores.

Question 27

Egg cell formation in plants

Answer 27

-Meiosis results in the formation of a relatively small number of ova contained within ovules inside the ovary
-The ovule is attached to the wall of the ovary by a pad of special tissue called the placenta
-A complex structure if integuments (coverings) forms arounds tissue known as the nucellus
-In the centre the embryo sac forms the gametophyte generation.

Question 28

Analogous

Answer 28

(of organs) performing a similar function but having a different evolutionary origin, such as the wings of insects and birds.

Question 29

Pollen formation (Mitosis stage)

Answer 29

Part 2:
-The gametes themselves are formed from the spores by mitosis
-They contain two haploid nuclei; the tube nucleus and the generative nucleus.

Question 30

Pollen formation (fusing)

Answer 30

Part 3:
-The tube nucleus has the function of producing pollen that penetrates through stigma, style and ovary and into the ovule
-The generative nucleus then fuses with the nucleus of the ovule to form a new individual.

Question 31

Fertilisation definition

Answer 31

Fertilisation is the meeting a fusing of gametes.

Question 32

Internal fertilisation

Answer 32

Male gametes are transferred directly into the female. Fertilisation is much more likely.

Question 33

External fertilisation

Answer 33

-Occurs outside the body
-Gametes released directly into the environment
-Common in aquatic animals
-Fertilisation occurs by chance

Question 34

Fertilisation in mammals: sperm (pt 1)

Answer 34

-Around 100 million sperm will be released into cervix and a few thousand will reach the oviduct
-As they move the acrosome region matures so it can release digestive enzymes
-The first sperm to break through touches the surface membrane of oocyte.

Question 35

Fertilisation in mammals: egg (pt 2)

Answer 35

-This causes the second mitotic division in the egg creating a haploid nucleus
-Entry of other sperm are blocked as the inside of the egg changes from negative to positive
-The head detaches from the tail and absorbs water
-Chromosomes are released to fuse with the ovum
-This forms a diploid zygote.

Question 36

Location of gametes in plants

Answer 36

-The male gamete is contained within the pollen grain
-The female gamete is embedded deep in thee tissue of the ovary.

Question 37

Fertilisation in plants: Pollination (pt 1)

Answer 37

-The pollen grain lands on the surface of the stigma of the flower during pollination
-The molecules on the surface of the pollen grain and the stigma interact .

Question 38

Fertilisation in plants: Germination (pt 2)

Answer 38

-If they ‘recognise’ each other as being from the same species the pollen grain begins to grow or germinate
-Often the pollen grain will only germinate if it is from the same species but a different plant
-This helps prevent self-fertilisation which would reduce variety.

Question 39

Fertilisation in plants: Pollen tube growth (pt 3)

Answer 39

-A pollen tube begins to grow out from the tube cell of the pollen grain through the stigma into the style, so the pollen tube can make its way down between the cell
-The digested tissue acts as a nutrient source for the pollen tube
-As the pollen tube grows through the ovary, the generative cel containing the generative nucleus travels down it.

Question 40

Fertilisation in plants: Fertilisation stage (pt 4)

Answer 40

-The nucleus of this cell divides by mitosis as it moves down the tube to form two male nuclei
-Eventually the pollen tubes grow through the ovary and the tip passes through the micropyle of the ovule
-The growth is due to rapid elongation of the cell
-The two male nuclei then pass into the ovule so fertilisation can occur.

Question 41

Double fertilisation

Answer 41

Flowering plants undergo double fertilisation
-One male nucleus fuses with the two polar nuclei to form an endosperm nucleus which is triploid (extraset of chromosomes)
-The endosperm is involved in supplying the embryo plant with food when it begins to germinate
-The other male nucleus fuses with the egg cell to form the diploid zygote
-At this point fertilisation is complete and the developments of the seed and the embryo within can begin.

Question 42

2 examples of conditions caused by non-disjunction

Answer 42

-trisomy-21 = Down’s syndrome
-X-monosomy of sex chromosomes = Turner’s syndrome

Question 43

Outline of spermatogenesis

Answer 43

Primordial germ cell –> spermatogonia –> primary spermatocytes –> secondary spermatocytes –> spermatids –> 4 spermatozoa.

Question 44

Outline of oogenisis

Answer 44

Primordial germ cell –> oogonia –> primary oocyte –> secondary oocyte & polar body –> ootid & polar bodies –> 1 ovum.

Question 45

Structure of ovum

Answer 45

-Outer layer of follicle cells.
-Zona pellucida: coating above cytoplasm prevents polyspermy and hardens when cortical granules release chemicals.
-Haploid nucleus: fertilisation restores diploid chromosome number.

Question 46

Outline of stages of early embryo development

Answer 46

zygote –> morula –> blasocyst –> implantation

Question 47

How does a blastocyst form?

Answer 47

Several mitotic divisions of zygote produce morula (solid ball of cells).
Morula divides further then undergoes differentiation and cavity formation.

Question 48

Structure of a blastocyst

Answer 48

Inner cell mass develops into embryo.
Outer layer (trophoblast) develops into placenta.
Fluid filled cavity (blastocoele).

Question 49

What happens during implantation?

Answer 49

1) Blastocyst breaches surrounding jelly coat, so it can attach to endometrium.
2) Release of autocrine hormones and digestive enzymes triggers implantation into uterine wall.
3) Growing embryo gains nutrients and oxygen from endometrial tissue fluid.

Question 50

Prophase I

Answer 50

Each chromosome appears in the condensed form with two chromatids.
Homologous pairs of chromosomes associate with each other.
Crossing over occurs.

Question 51

Metaphase I

Answer 51

The spindle forms and the pairs of chromosomes line up on the metaphase plate.

Question 52

Anaphase I

Answer 52

The centromeres do not divide.
One chromosome from each homologous pair moves to each end of the cell.
As a result the chromosome number in each cell is half that of the original.

Question 53

Telophase I

Answer 53

The nuclear membrane re-forms and the cells begin to divide.
In some cells this continues to full cytokinesis and there may be a period of brief or prolonged interphase. During this interphase there is no further replication of the DNA.

Question 54

Prophase II

Answer 54

New spindles are formed.

Question 55

Metaphase II

Answer 55

The chromosomes line up on the metaphase plate.

Question 56

Anaphase II

Answer 56

The centromeres now divide and the chromatids move to the opposite ends of the cell.

Question 57

Telophase II

Answer 57

Nuclear envelopes re-form.
The chromosomes return to their interphase state and cytokinesis occurs, giving four daughter cells each with half the chromosome number of the original diploid cell.