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A Level Biology > Topic 2.4- Meiosis and sexual reproduction > Flashcards

Topic 2.4- Meiosis and sexual reproduction Flashcards

1
Q

What is meiosis?

A

A form of cell division that produces 4 genetically different haploid cells, known as gametes

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2
Q

What happens during meiosis I?

A
  1. Homologous chromosomes pair to form bivalents
  2. Crossing-over (exchange of sections of genetic material) occurs at chiasmata
  3. Cell divides into two. Homologous chromosomes separate randomly. Each cell contains either maternal or paternal copy
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3
Q

What happens during meiosis II?

A
  1. Independent segregation of sister chromatids

2. Each cell divides again, producing 4 haploid cells

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4
Q

How does meiosis produce genetic variation?

A

-crossing over during meiosis I
-independent assortment (random segregation) of homologous chromosomes + sister chromatids
result in new combinations of alleles

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5
Q

What is a chromosome mutation?

A 
involves sections of a chromosome rather than a base
types include:
-deletion
-inversion
-duplication
-translocation
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6
Q

What is a translocation mutation?

A

a mutation where one part of a chromosome breaks off and rejoins to another completely different chromosome, may be balanced or unbalanced

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7
Q

What is a non-disjunction mutation?

A
  • chromosomes fail to separate correctly in meiosis

- gametes have at least one more (polysomy) or one less (monosomy) chromosomes than normal

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8
Q

Give examples of conditions caused by non-disjunction

A

trisomy-21= Down’s syndrome

X-monosomy of sex chromosomes= Turner’s syndrome

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9
Q

Name the 2 types of gametogenesis

A
  1. spermatogenesis to produce spermatozoa

2. oogenesis to produce ova

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10
Q

What are the primordial germline cells?

A

diploid precursors to gametes

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11
Q

Outline the stages of spermatogenesis

A

primordial germline cell —> spermatogonia —> primary spermatocytes —> secondary spermatocytes —> spermatids —> 4 spermatozoa

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12
Q

How do primary spermatocytes form?

A

Primordial germline cells of seminiferous tubules of testes divide by mitosis. forming spermatogonia
Growth period of spermatogonia without further division forms primary spermatocytes

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13
Q

How do spermatids form?

A

Primary spermatocytes undergo meiosis to form 4 undifferentiated haploid cells
Meiosis I: 2 haploid secondary spermatocytes
Meiosis II: haploid spermatids

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14
Q

How do spermatozoa form?

A

Spermatids differentiate to gain flagellum, acrosome and many mitochondria

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15
Q

Outline the stages of oogenesis

A

Primordial germ cell —> oogonia —> primary oocyte —> secondary oocyte + polar body —> ootid + polar bodies —> 1 ovum

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16
Q

How do primary oocyte form?

A

Primordial germline cells in the ovaries and oviduct divide by mitosis, forming oogonia
Growth period of oogonia without further division forms primary oocytes, which remain in prophase I in follicles until puberty

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17
Q

What happens during the first meiotic division of oogenesis?

A

one secondary oocyte with all the cytoplasm forms

one polar body buds off and sticks to the oocyte

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18
Q

What happens during the second meiotic division of oogenesis?

A

Secondary oocyte divides into haploid ootid and polar body
polar body divides in 2
overall 3 polar bodies form. they degenerate as ootid develops

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19
Q

How does an ovum form?

A

Meiotic division in utero halts at prophase. Fertilisation triggers influx of Ca2+ This stimulates meiosis II to form ovum

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20
Q

Describe the structure of an ovum

A
  • outer layer of follicle cells
  • zona pellucida; coating above cytoplasm prevents polyspermy + hardens when cortical granules release chemicals
  • haploid nucleus; fertilisation restores diploid chromosome number
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21
Q

What is a mutagen?

A

chemicals known to increase the rate of mutation

22
Q

Define chiasmata

A

the points where chromosomes break during recombination (crossing-over)

23
Q

What is monosomy?

A

when only one member of a pair of chromosomes is present in a cell

24
Q

What is polysomy?

A

when a cell contains 3 or more rather than 2 chromosomes of a particular type

25
Q

Define aneoploidy

A

when a cell contains too few or too many chromsomes

26
Q

Give an example of a polysomy mutation

A

down’s syndrome

27
Q

Give an example of a monosomy mutation

A

Turner’s syndrome

28
Q

What is the process of spermatogenesis?

A

1) the diploid primordial germ cell divides several times by mitosis to form diploid spermatogonia
2) the spermatogonia then grow without further division until they’re big enough to be a primary spermatocyte
3) the spermatocytes undergo meiosis. the first meiotic division results in 2 haploid cells called secondary spermatocytes
4) the second meiotic division results in 4 haploid cells called spermatids
5) the spermatids differentiate in the tubules of the testes to form spermatozoa

29
Q

How does a pollen grain form in the anther?

A
  1. meiosis of diploid microspore cells in the anther forms 4 haploid microspores
  2. haploid microspores mature into pollen grains via mitosis
30
Q

Describe the structure of pollen grains

A
  • generative cell with haploid nucleus produces 2 male gametes via mitosis
  • pollen tube cell elongates to penetrate the ovule
  • outer protective coating
31
Q

How does the embryo sac form in the ovule?

A
  1. mitosis of diploid megaspore cells in ovule from 1 ovule and 3 polar bodies. polar bodies degenerate then are reabsorbed
  2. after 3 mitotic divisions, ootid forms embryo sac
32
Q

Describe the structure of the embryo sac

A
  • 2 polar nuclei form endosperm
  • ovum forms zygote
  • 2 synergids help generative nucleus of pollen grain to reach ovum
  • 3 antipodal cells
  • outer protective coating
33
Q

How do male nuclei reach the embryo sac?

A
  1. pollen grain germinates on the stigma
  2. enzymes digest surrounding tissue, providing nutrient source. enables pollen tube to grow down the style and through the micropyle
  3. mitosis of pollen grain generative cell produces 2 male gametes which enter embryo sac
34
Q

Where does double fertilisation occur?

A

embryo sac of ovule

35
Q

What happens during double fertilisation?

A
  • diploid zygote forms when 1 male gamete and ovum nucleases fuse
  • triploid endosperm forms when 1 male gamete and 2 polar nuclei fuse
36
Q

What is the purpose of a triploid endosperm?

A

nutrient source for embryo

37
Q

At what sucrose concentration do pollen tubes grow best?

A

near isotonic as it prevents osmotic damage

38
Q

Name the 2 types of gametogenesis

A
  • spermatogenesis to produce spermatozoa

- oogenesis to produce ova

39
Q

What are primordial germ cells?

A

diploid precursors to the gametes

40
Q

Outline the stages of spermatogenesis

A
  • primordial germ cell
  • spermatogonia
  • primary spermatocytes
  • secondary spermatocytes
  • spermatids
  • 4 spermatozoa
41
Q

Outline the stages of oogenesis

A
  • primordial germ cell
  • oogonia
  • primary oocyte
  • secondary oocyte and polar body
  • ootid and polar bodies
  • 1 ovum
42
Q

How do primary spermatocytes form?

A
  • primordial germ cells in tubules of testes divide by mitosis, forming spermatogonia
  • growth period of spermatogonia without further division forms primary spermatocytes
43
Q

How do spermatids grow?

A
  • primary spermatocytes undergo meiosis to form 4 undifferentiated haploid cells
  • meiosis 1: diploid secondary spermatocytes
  • meiosis 2: haploid spermatids
44
Q

How do spermatozoa form?

A

spermatids differentiate to gain flagellum, acrosome and many mitochondria

45
Q

How do primary oocytes form?

A
  • primordial germ cells in the ovaries and oviduct divide by mitosis, forming oogonia
  • growth period of oogonia without further division forms primary oocytes, which remain in prophase I in follicles until puberty
46
Q

What happens during the first meiotic division in oogenesis?

A
  1. secondary oocyte with all the cytoplasm forms

2. polar body buds off and sticks to the oocyte

47
Q

What happens during the second meiotic division of oogenesis?

A
  • secondary oocyte divides into haploid ootid and polar body
  • polar body divides in 2
  • overall 3 polar bodies made and they degenerate as the ootid develops
48
Q

How does an ovum form?

A

meiotic division in uterus halts at prophase. fertilisation triggers an influx of Ca2+. this stimulates meiosis II to form ovum

49
Q

Describe the structure of the ovum

A
  • outer layer of follicle cells
  • zona pellucida- coating above cytoplasm prevents polyspermy + hardens when cortical granules release chemicals
  • haploid nucleus- fertilisation restores diploid chromosome number
50
Q

What happens during fertilisation of an egg/ovum?

A
  1. enzymes in acrosome of sperm head digest zona pellucida
  2. sperm head fuses with ovum cell membrane, allowing sperm nucleus to enter
  3. cortical reaction causes zona pellucida to harden, preventing polyspermy
  4. nuclei fuse, forming a diploid zygote
51
Q

Outline the stages of early embryo development

A

zygote –> morula –> blastocyst –> implantation

52
Q

How does a blastocyst form?

A
  • several mitotic divisions of zygote produce morula (solid ball of cells)
  • morula divides further and then undergoes differentiation and cavity formation

A Level Biology (32 decks)

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