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Biology A-Level > Meiosis and Gametogenesis (Chapter 16) > Flashcards

Meiosis and Gametogenesis (Chapter 16) Flashcards

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1
Q

What are homologous chromosomes?

A

A pair of chromosomes in a diploid cell that have the same structure as each other, with the same genes (but not necessarily the same alleles of those genes) at the same loci and that pair together form a bivalent during the first division of meiosis

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2
Q

Where are the chromosomes from in the original zygote?

A

One chromosome of each pair came from the mother and one from the father

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3
Q

How many sets of how many chromosomes are there in a human?

A

Two sets of 23 chromosomes - one set of 23 from the mother and one set of 23 from the father

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4
Q

What are the sex chromosomes?

A

The non-matching x and y chromosomes, which determine the sex of the individual

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5
Q

What are the sex chromosomes in female humans?

A

2 x chromosomes

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6
Q

What are the sex chromosomes in male humans?

A

1 x chromosome and 1 y chromosome

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7
Q

What are autosomes?

A

All chromosomes besides the sex chromosomes

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8
Q

How can pairs of chromosomes be distinguished?

A

By size, shape and the distinctive bonding pattern when stained with certain stains

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9
Q

Each chromosomes has a characteristic set of…?

A

Genes

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10
Q

Where are genes found?

A

The gene for a particular characteristic is always found at the same locus on a chromosome

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11
Q

Describe the genes in homologous chromosomes

A
  • Each member of a homologous pair possesses genes controlling the same characteristics
  • A gene for a given characteristic may exist in different forms (alleles) which are expressed differently
  • Homologous chromosomes carry the same genes on the same loci
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12
Q

What is a gene?

A

A length of DNA that codes for a particular protein or polypeptide

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13
Q

What is an allele?

A

A particular variety of gene

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14
Q

What is a locus?

A

The position at which a particular gene is found on a particular chromosome
- the same gene is always found on the same locus

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15
Q

What is a diploid cell?

A

A cell that contains two complete set of chromosomes

- represented as 2n, where n = the number of chromosomes in one set of chromosomes

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16
Q

What is a haploid cell?

A

A cell that contains one complete set of chromosomes (n)

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17
Q

What are human body cells?

A

Diploid

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18
Q

What are human gametes?

A

Haploid

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19
Q

Explain the need for a reduction division (meiosis) prior to fertilisation in sexual reproduction

A
  • For the life cycle to contain sexual reproduction, there must be a point before fertilisation takes place where the number of chromosomes is halved
  • This results in the gametes containing only one set of chromosomes, rather than two sets
  • If there were no point in the life cycle where the number of chromosomes halved, then the number of chromosomes would double with every generation
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20
Q

What is meiosis?

A
  • A type of nuclear division in which daughter cells are produced that have half the normal amount of DNA and each one is genetically different
  • The process of nuclear division in which the number of chromosomes in a cell halved
  • Gametes are always haploid as a result of meiosis
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21
Q

What is meiosis sometimes described as and why?

A

A reduction division - because the number of chromosomes is reduced

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22
Q

How can genetic variation arise in gametes and therefore zygotes?

A

Through mitosis and mutation

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23
Q

What is meiosis 1?

A

A reduction division, resulting in two parent daughter nuclei with half the number of chromosomes of the parent nucleus - members of homologous pairs of chromosomes are separated

24
Q

What happens during meiosis 2?

A

The chromosomes behave as in mitosis, so that each of the two haploid daughter nuclei divide again - the twin chromatids are separated

25
Q

What does meiosis result in?

A

A total of 4 haploid nuclei

26
Q

What two events that take place during meiosis help to produce genetic variation in the daughter cells?

A

1) Independent assortment of the homologous chromosomes
2) Crossing over, which happens between the chromatids of homologous chromosomes
- These form new combinations of alleles

27
Q

What else produces more variation amongst offspring?

A

When these genetically different gametes fuse randomly at fertilisation

28
Q

What happens during early prophase 1 of meiosis?

A
  • Chromosomes start to appear as chromatin condenses (same as mitosis prophase 1)
29
Q

What happens during middle prophase 1 of meiosis?

A
  • Centrosomes move to opposite ends of the nucleus

- Synapsis - homologous chromosomes pair up, each pair is called a bivalent

30
Q

What happens during late prophase 1 of meiosis?

A
  • The nuclear envelope and nucleolus disappear/break up

- The crossing over of chromatids occur, forming recombinant chromatids

31
Q

What happens during crossing over of homologous chromosomes?

A
  • Chromatids break and reconnect to another chromatid

- Crossing over occurs at a chiasma or many chiasmata along the length

32
Q

What is the point where crossing over occurs called?

A

A chiasma

33
Q

What happens during metaphase 1 of meiosis?

A

Bivalents line up across the equator, attached to the spindle by centromeres

34
Q

What happens during anaphase 1 of meiosis?

A
  • Centromeres do not divide, unlike in mitosis

- Whole chromosomes move towards opposite ends of the spindle, centromeres first, pulled by microtubules

35
Q

What happens during telophase 1 of meiosis?

A

(Same as mitosis telophase)

  • Nuclear envelope and nucleolus reform
  • Cytokinesis occurs
  • Chromosomes reach the poles of the spindle
36
Q

What is the difference between animal and plant cells in meiosis 1?

A
  • Animal cells usually divide before meiosis 2

- Plant cells go straight into meiosis 2 with no reformation of nuclear envelopes or nucleoli

37
Q

What happens in prophase 2 of meiosis?

A

(Occuring in the two cells formed from meiosis 1)

  • Nuclear envelope and nucleolus disperse
  • Centrosomes and centrioles replicate and move to opposite poles of the cell
38
Q

What happens in metaphase 2 of meiosis?

A

Chromosomes line up separately across the equator of the spindle

39
Q

What happens in anaphase 2 of meiosis?

A

Centromeres divide and spindle microtubules pull the chromatids to opposite poles

40
Q

What happens in telophase 2 of meiosis?

A

Same as telophase in mitosis, but 4 haploid daughter cells are produced

41
Q

When does meiosis occur in humans?

A

As gametes are formed inside the testes and ovaries

42
Q

What is spermatogenesis?

A

The formation of male gametes

43
Q

What is oogenesis?

A

The formation of female gametes

44
Q

Where does sperm production take place?

A

Inside tubules in the testes

45
Q

Describe the process of spermatogenesis

A

1) Diploid cells in tubules in the testes divide by mitosis to produce numerous spermatogonia, which grow to form diploid primary spermatocytes
2) Meiosis 1 takes place, forming 2 haploid secondary spermatocytes
3) Meiosis 2 produces haploid spermatids which mature into spermatozoa

46
Q

What is the difference between oogenesis and spermatogenesis?

A

In oogenesis, many fewer gametes are made and the process takes much longer with long ‘waiting stages’

47
Q

Where does oogenesis take place?

A

Inside the ovaries

48
Q

Describe the process of oogenesis

A

1) Diploid cells divide by mitosis in the ovaries to produce many diploid oogonia
2) These begin to divide by meiosis by stop at prophase 1 - they are diploid primary oocytes
3) At puberty, some of the primary oocytes proceed from prophase 1 to the end of meiosis forming 2 haploid cells
4) However, the division is uneven - one cell gets most of the cytoplasm (secondary oocyte) while the other is a little more than a nucleus (polar body)
5) Each month, one secondary oocyte is released into the oviduct from one of the ovaries

49
Q

What can the polar body be thought of as?

A

Simply a way to get rid of 1/2 of the chromosomes and has no further role in reproduction

50
Q

What happens when a secondary oocyte is fertilised?

A

1) It continues its division by meiosis and can now be called an ovum
2) The chromosomes of the spermatozoan and the ovum join together to form a single diploid nucleus and the cell made by this process is called a zygote
3) The zygote can now divide repeatedly by mitosis to form first an embryo, then a fetus

51
Q

Describe male gametogenesis in plants

A

1) Inside the anthers diploid pollen mother cells divide by meiosis to form 4 haploid cells
2) The nuclei of each of these haploid cells then divide by mitosis, but the cell itself does not divide (i.e. cytokinesis does not take place), resulting in cells that contain two haploid nuclei
3) These cells mature into pollen grains, each surrounded by a protective wall (made of a tough exine and thinner intine)
4) One of the haploid nuclei is called the tube nucleus and the other the generative nucleus - these are the male gametes

52
Q

Describe female gametogenesis in plants

A

1) Inside each ovule, a large, diploid spore mother cell develops
2) This cell divides by meiosis to produce 4 haploid cells
3) All but one of these degenerates, and the surviving haploid cell develops into an embryo sac
4) The embryo sac grows larger, and its haploid nucleus divides by mitosis 3 times, forming 8 haploid nuclei
5) One of these becomes the female gamete

53
Q

When does fertilisation occur in plants?

A
  • When a male gametes from a pollen grain fuses with a female gamete inside an ovule
  • This forms a diploid zygote, which grows into an embryo plant
54
Q

How is the formation of gametes different in plants from animals?

A
  • Gametes are not formed directly by mitosis
  • Instead, meiosis is used in the production of pollen grains and the embryo sac and the gametes are then formed inside these structures by mitotic divisions
55
Q

What is the process between meiosis 1 and meiosis 2 called?

A

Interkinesis - when there is no replication of DNA

56
Q

What has happened before meiosis 1?

A

The chromosomes have replicated

57
Q

Why does no crossing over occur in prophase 2?

A

Because there are no homologous pairs left together as each homologous chromosome was separated from its former pair during meiosis 1

Biology A-Level (44 decks)

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