Lecture 14 (Eukaryotic cell division - meiosis)

Question 1

Clonal reproduction

Answer 1

Single-celled organisms usually reproduce by binary fission

Multicellular plants and animals can also use vegetative means - runners, bulbs, tubers, rhizomes; anemone, sponge, starfish

Question 2

Sexual cycle

Answer 2

In this cycle there is an alternation between cells that are diploid and cells that are haploid.

Found in almost all eukaryotes

Meiosis- fertilisation - mitosis and development and then repeat

Sexual reproduction - two parents contribute genetic information to produce unique offspring

Question 3

n

Answer 3

The letter n symbolises the haploid number of chromosomes

Question 4

Haploid

Answer 4

Haploid is the term used when a cell has half the usual number of chromosomes. (n)

Question 5

Diploid

Answer 5

Diploid is a cell or organism that has paired chromosomes, one from each parent. (2n)

Question 6

Why do gametes use meiosis and not mitosis?

Answer 6

If gametes were produced by mitosis, they would be 2n, and resulting embryos would be 4n. Meiosis is a process of cell division that halves the number of chromosomes going into gametes (n), so that diploid number (2n) is retained in the zygote

Question 7

Meiosis

Answer 7

Cell division that creates gametes which contain half the number of chromosomes (to a somatic cell)

After the chromosomes duplicate in interphase, the diploid cell divides twice, yielding four haploid daughter cells.

Meiosis produces four daughter cells that contain unreplicated chromosomes

Question 8

What cells does meiosis occur in?

Answer 8

Only in the reproductive cells/gametes (the ovaries and testes in humans)

Question 9

Interphase

Answer 9

Chromosomes duplicated to form two sister chromatids. Forms a diploid cell with duplicated chromosomes

Question 10

Meiosis 1 summary

Answer 10

This cell division separates homologous chromosomes. Produces haploid cells with duplicated chromosomes.

Question 11

Meiosis 2 summary

Answer 11

This cell division separates sister chromatids. Produces haploid cells with unduplicated chromosomes

Question 12

Prophase 1

Answer 12

Homologous chromosomes align and synapse (Chromatids are still attached by the centromere)

Crossing over between non-sister chromatids occurs at chiasmata (involves the exchange of genetic material between non-sister chromatids)

Crossing over results in chromatids now being a mix of pieces from each homologous chromosome

Question 13

Metaphase 1

Answer 13

Paired homologous chromosomes move to the metaphase plate

Chiasmata (not kinetochores, as in mitosis) line up on metaphase plate

Both chromatids of one homolog are attached to kinetochore microtubules from one pole whilst the chromatids of the other homolog are attached to microtubules from the opposite poles

Question 14

Anaphase 1

Answer 14

Recombined homologous chromosomes seperate (disjoin)

Sister chromatids remain attached (in anaphase of mitosis, sister chromosomes seperate)

Question 15

Telophase 1 and cytokinesis

Answer 15

Haploid cells with duplicated chromosomes (the pairs of sister chromatids) form

Haploid because only half the genetic information is in each new cell

Cytokinesis (division of the cytoplasm) usually occurs simultaneously

Question 16

Prophase 2

Answer 16

Start off with haploid cells, haploid because the two sister chromatids are identical

Spindles start to form

Question 17

Metaphase 2

Answer 17

The chromosomes are positioned at the metaphase plate as in mitosis

Because of crossing over in meiosis 1, the two sister chromatids of each chromosome are not genetically identical

The kinetochores of sister chromatids are attached to microtubules extending from opposite poles

Question 18

Anaphase 2

Answer 18

Sister chromatids seperate

The chromatids move toward opposite poles as individual chromosomes

Kinetochore microtubules shorten and the non-kinetochore microtubules lengthen

Question 19

Telophase 2 and cytokinesis

Answer 19

Nuclei form, the chromosomes begin decondensing and cytokinesis occurs. The meiotic division of one parent cell produces four daughter cells, each with a haploid set of (unduplicated) chromosomes. The four daughter cells are genetically distinct from one another and the parent cell

Question 20

Main differences between mitosis and meiosis 1

Answer 20

Mitosis 
Chromosomes align independently 
No chiasmata 
Centromeres on metaphase plate 
Chromatids disjoin 
2n to 2n 

Meiosis 
Homologous chromosomes synapse (which allows for crossing over) 
Chiasmata 
Chiasmata on metaphase plate 
Chromosomes disjoin 
2n to n

Question 21

The structures that move to the spindle poles at anaphase of meiosis 1 usually differ from those that move to the poles at anaphase 2 with respect to …

Answer 21

chromatid number
Gene copy number
The ratio of maternal to paternal genes

Question 22

Daughter cells of mitosis vs meiosis

Answer 22

Mitosis - the daughter cells have the exact same genetic constitution as the parent cells

Meiosis - the daughter cells are different to the parent cells that they started with as they contain half the number of chromosomes and an assortment of maternal and paternal genes. Every single product is different

Question 23

Asexual reproduction

Answer 23

Asexual reproduction generates offspring that are genetically identical to a single parent. This form of reproduction often creates more offspring that sexual reproduction. Sexual reproduction provides more diversity.

Question 24

Crossing over

Answer 24

Crossing over occurs when homologous pairs line up on the centre of the cell. During crossing over, sections of non-sister chromatids lie across each other, break and join to the other chromatid, causing alleles to be swapped from one homologous chromosome to the other of the homologous pair. Crossing over creates genetic variation in the gametes as it causes new combinations of alleys to form in gametes of each parent

Crossing over results in chromatids now being a mix of pieces from each homologous chromosome

Larger chromosomes are likely to have more crossing over therefore chromosome one is likely to have more crossing over events

The DNA molecule for non sister chromatids are broken (by proteins) and are rejoined to each other

Make new combinations of linked genes. Genes on the same chromosome are linked, they don’t assort independently but can be shuffled by crossing over

Occurs during prophase of meiosis 1

Question 25

What does crossing over mean for the products of meiosis?

Answer 25

It means that the products of meiosis do not just contain combinations of chromosomes from mum and chromosomes from dad, but they consist of chromosomes with part mum and part dad all the way through

Question 26

Chiasma/Chiasmata

Answer 26

A chiasma is the point of contact, between two chromatids belonging to homologous chromosomes. At a given chiasma, an exchange of genetic material can occur between both chromatids, crossing over.

Question 27

Sexual reproduction produces genetic diversity through...

Answer 27

independent assortment of chromosomes Crossing over And random fertilisation of gametes

Question 28

Random fertilisation

Answer 28

Fertilisation is the process where the male and female gametes combine to form a fertilised egg called a zygote. This is a random event and is not governed by any other previous fertilisation events

Question 29

Genetic diversity allows selective responses to things such as...

Answer 29

Spatially variable environments (e.g. climate, ecology) Changing environments (e.g. parasites, seasons) Sib-sib competition

Question 30

Segregation

Answer 30

The process where two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, seperate from each other and migrate to opposite poles. Each allele is distributed to a different gamete therefore increasing genetic variation A parent has 2 alleles for any trait but only one goes into its gamete

Question 31

Independent assortment

Answer 31

When cells divide during meiosis, homologous chromosomes are randomly distributed during anaphase 1, separating and segregating independently of each other. This is called independent assortment. This results in gametes that have unique combinations of chromosomes Occurs in anaphase 1 of meiosis When homologous pairs line up on the metaphase plate it is random which way they line up either maternal:paternal or paternal:maternal Makes new combinations of unlinked genes

Question 32

Gamete diversity and chromosome number

Answer 32

Number of chromosome pairs = n Number of possible gametes = 2^n

Question 33

The presence of the sexual cycle in a species has the potential to...

Answer 33

Decrease competition among offspring Allow the species to respond better to parasites Increase evolutionary flexibility

Question 34

Unreplicated chromosomes contain ...

Answer 34

no chromatids

Question 35

If you have 4 replicated chromosomes...how many homologous pairs and chromatids do you have?

Answer 35

2 homologous pairs | 8 chromatids

Question 36

Non-sister chromatids

Answer 36

Two chromatids on different replicated chromosomes in the same homologous pairs

Question 37

Why is meiosis I referred to as reduction division....

Answer 37

During meiosis I, the chromosome number is reduced from diploid (2n) to haploid (n). Meiosis I is therefore referred to as reduction division

Question 38

Meiosis II is referred to as mitotic division why...

Answer 38

It is similar to the process of meiosis however meiosis II starts and finishes with a haploid cell and meiosis II produces four genetically unique cells

Question 39

Are chromosomes replicated or unreplicated at the end of meiosis I?

Answer 39

replicated

Question 40

How many chromosomes are present in the daughter cells at the end of meiosis I?

Answer 40

Half the number of chromosomes present in the parent cell

Question 41

Are the chromosomes replicated or unreplicated at the end of meiosis II?

Answer 41

Unreplicated

Question 42

Non-disjunction

Answer 42

Non-disjunction is an error in cell division in which members of a pair of homologous chromosomes (in meiosis I) or a pair of sister chromatids (in meiosis II or mitosis) fail to seperate properly from each other. This leads to chromosomes not being distributed equally in the daughter cells

Question 43

How are the chromosomes in a cell at metaphase of mitosis similar to, and different from the chromosomes in a cell at metaphase of meiosis II?

Answer 43

Similarities - Chromosomes are comprised of two sister chromatids and are aligned along the cell equator Differences- In the mitotically dividing cell, sister chromatids are identical, whereas in meiotically dividing cells, they are genetically distinct because of crossing over. The metaphase cell of mitosis is diploid whereases the metaphase cell of meiosis II is haploid