2.4-meiosis and sexual reproduction

Question 1

meiosis- interphase 1

Answer 1

-chromatin not visible
-DNA and proteins are replicated

Question 2

meiosis- prophase 1

Answer 2

-chromosomes visible
-homologous chromosomes join to form bivalent, chromatids cross over (chiasmata)

Question 3

meiosis- metaphase 1

Answer 3

bivalents line up along equator

Question 4

meiosis- anaphase 1

Answer 4

-chromosomes separate
centromere doesn’t split

Question 5

meiosis- telophase 1

Answer 5

-nuclei form, cell divides, cells have 2 chromosomes, 4 chromatids

Question 6

aims of meiosis 1

Answer 6

1.to create genetic variation
2.to halve the number of chromosomes

Question 7

products of meiosis 1

Answer 7

two haploid cells

Question 8

meiosis- interphase 2

Answer 8

-short
-no DNA replication
-chromosomes remain visible

Question 9

meiosis- prophase 2

Answer 9

-centrioles replicate and move to new poles

Question 10

meiosis- metaphase 2

Answer 10

-chromosomes line up along equator

Question 11

meiosis- anaphase 2

Answer 11

-centromeres split, chromatids separate

Question 12

meiosis- telophase 2

Answer 12

-4 haploid cells, each with 2 chromatids

Question 13

products of meiosis 2

Answer 13

4 haploid daughter cells , genetically different

Question 14

importance of meiosis- crossing over

Answer 14

-takes place when large multi enzyme complexes ‘cut and join’ bits of maternal and paternal chromatids together
-points where chromatids break are called chiasmata, they are important in 2 ways;
1.exchange of genetic material, leading to variation
2.errors in process lead to mutation, introducing new combinations into genetic makeup

Question 15

importance of meiosis- independent assortment

Answer 15

-chromosomes that came from parents are distributed into gametes and therefore offspring, randomly
-e.g. each gamete receives 23 chromosomes, in each gamete any number from 0-23 could come from maternal or paternal
-this alone guarantees great variety

Question 16

independent assortment and crossing over diagrams

Answer 16

look at flashacrds

Question 17

what is a mutation?

Answer 17

• a change in the DNA base sequence which results in a new and different allele

Question 18

gene mutations- types

Answer 18

1. addition or deletion of a base- causes a frame shift
2. substitution- causes a change in sequence but only of one area, no frame shift

Question 19

chromosome mutations- translocations

Answer 19

-take place during meiosis
-some change the phenotype completely or make it incompatible with life
-translocations between certain chromosomes can make you predisposed to cancer e.g. burkitts lymphoma which is a cancer of white blood cells in young people

Question 20

what happens in translocations

Answer 20

-chromosome from one pair becomes attached to a chromosome from another pair
OR
-parts of 2 chromosomes break off and attach to different chromosomes

Question 21

chromosome mutations- non-disjunction

Answer 21

-affects whole chromosomes
-homologous pairs fail to separate, so gamete ends up with 2 copies of that chromosome or non at all
-if one of these abnormal gametes join with a normal one, results in monosomy or polysomy

Question 22

2 types of aneuploidy

Answer 22

1.monosomy- sex cells missing one chromosome e.g. turners syndrome
2.polysomy- more chromosomes than required e.g. down syndrome

Question 23

early stages of the development of the embryo (6)

Answer 23

1.zygote is totipotent(can form 216 cell types)
2.cleavege-type of mitosis, cells divide rapidly with no interphase- happens as egg travels down oviduct
3.blastocyst (mass of undifferentiated cells) as the zygote moves from oviduct to uterus (5-6days)
4.cells are pluripotent embryonic stem cells (can form most, not placenta)
5.outer layer of blastocyst cells form placenta
6.blastocyst breaks free of outer layer and implants in uterus

Question 24

5 stages of spermatogenesis

Answer 24

1.diploid(46) germ cell divides by mitosis to give spermatogonia
2.spermatogonia then grow until considered primary spermatocytes
3.spermatocytes divide by meiosis to give 2 haploid (23)secondary spermatocytes
4.secondary spermatocytes undergo meiosis again to form 4 haploid spermatids
5.spermatids differentiate into spermatozoa capable of fertilisation

mini, many, motile

Question 25

products of spermatogenesis

Answer 25

-4 haploid spermatozoa, capable of fertilisation

Question 26

7 stages of oogenesis

Answer 26

1.diploid germ cell divides by mitosis to form diploid oogonia
2.oogonium continues to grow to form diploid primary oocyte
3.the primary oocyte undergoes meiosis to form a haploid secondary oocyte (ova) and much smaller cell (first polar body)
4.ovulation occurs
5.second meiotic division does not occur until fertilisation has happened
6.haploid secondary oocyte divides by meiosis again to form ovum and another polar body
7. polar body disintergrates

few, fat, fixed

Question 27

product of oogenesis

Answer 27

haploid ovum and polar body

Question 28

fertilisation in humans (mammals)- summary

Answer 28

1.sperm meet oocyte in oviduct, acrosomes have matured since sperm left male reproductive tract
2.sperm touch zona pellucida, triggering acrosome reaction
3.digestive enzymes pour out acrosome and digest zona pellucida, projections of sperm surface shoot forward due to actin like proteins
4.fertillisation occur when sperm touch oocyte and membranes fuse
5.sperm nucleus decondenses, releases chromosomes into oocyte, meiosis completed in oocyte forming ovum
6.m+f chromosomes roup together, corticle granules from oocyte combine with zona pellucida to form fertilisation membrane

Question 29

formation of pollen grain

Answer 29

-formed in anther
-meiosis of diploid microspore cells in anther form 4 haploid microspores
-haploid microspores mature, into pollen grains via mitosis

Question 30

formation of embryo sac in plants

Answer 30

-mitosis of diploid megaspore cells in ovule form one ootid and 3 polar bodies
-polar bodies degenerate and reabsorbed
-after 3 mitotic divisions, ootid forms embryo sac

Question 31

fertilisation in plants

Answer 31

-generative nucleus divides to form two male gametes by mitosis
-pollen tubes fuse with embryo sac
-double fertilisation occurs
-one male nucleus fuses with egg cell nucleus to produce diploid zygote
-other male nucleus fuses with 2 polar nuclei to produce triploid endosperm