Lecture 14 (Eukaryotic cell division - meiosis) Flashcards
Clonal reproduction
Single-celled organisms usually reproduce by binary fission
Multicellular plants and animals can also use vegetative means - runners, bulbs, tubers, rhizomes; anemone, sponge, starfish
Sexual cycle
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
n
The letter n symbolises the haploid number of chromosomes
Haploid
Haploid is the term used when a cell has half the usual number of chromosomes. (n)
Diploid
Diploid is a cell or organism that has paired chromosomes, one from each parent. (2n)
Why do gametes use meiosis and not mitosis?
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
Meiosis
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
What cells does meiosis occur in?
Only in the reproductive cells/gametes (the ovaries and testes in humans)
Interphase
Chromosomes duplicated to form two sister chromatids. Forms a diploid cell with duplicated chromosomes
Meiosis 1 summary
This cell division separates homologous chromosomes. Produces haploid cells with duplicated chromosomes.
Meiosis 2 summary
This cell division separates sister chromatids. Produces haploid cells with unduplicated chromosomes
Prophase 1
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
Metaphase 1
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
Anaphase 1
Recombined homologous chromosomes seperate (disjoin)
Sister chromatids remain attached (in anaphase of mitosis, sister chromosomes seperate)
Telophase 1 and cytokinesis
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
Prophase 2
Start off with haploid cells, haploid because the two sister chromatids are identical
Spindles start to form
Metaphase 2
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
Anaphase 2
Sister chromatids seperate
The chromatids move toward opposite poles as individual chromosomes
Kinetochore microtubules shorten and the non-kinetochore microtubules lengthen
Telophase 2 and cytokinesis
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
Main differences between mitosis and meiosis 1
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
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 …
chromatid number
Gene copy number
The ratio of maternal to paternal genes
Daughter cells of mitosis vs meiosis
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
Asexual reproduction
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.
Crossing over
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
What does crossing over mean for the products of meiosis?
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
Chiasma/Chiasmata
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.
Sexual reproduction produces genetic diversity through…
independent assortment of chromosomes
Crossing over
And random fertilisation of gametes
Random fertilisation
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
Genetic diversity allows selective responses to things such as…
Spatially variable environments (e.g. climate, ecology)
Changing environments (e.g. parasites, seasons)
Sib-sib competition
Segregation
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
Independent assortment
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
Gamete diversity and chromosome number
Number of chromosome pairs = n
Number of possible gametes = 2^n
The presence of the sexual cycle in a species has the potential to…
Decrease competition among offspring
Allow the species to respond better to parasites
Increase evolutionary flexibility
Unreplicated chromosomes contain …
no chromatids
If you have 4 replicated chromosomes…how many homologous pairs and chromatids do you have?
2 homologous pairs
8 chromatids
Non-sister chromatids
Two chromatids on different replicated chromosomes in the same homologous pairs
Why is meiosis I referred to as reduction division….
During meiosis I, the chromosome number is reduced from diploid (2n) to haploid (n). Meiosis I is therefore referred to as reduction division
Meiosis II is referred to as mitotic division why…
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
Are chromosomes replicated or unreplicated at the end of meiosis I?
replicated
How many chromosomes are present in the daughter cells at the end of meiosis I?
Half the number of chromosomes present in the parent cell
Are the chromosomes replicated or unreplicated at the end of meiosis II?
Unreplicated
Non-disjunction
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
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?
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
