Meiosis Flashcards
What is the name of the cell at the beginning of the cell cycle
Parent Cell
What is the parent cell at the start of the cell cycle
A diploid (2n) cell
How many chromosomes are present in a haploid cell
n
Define Homolgous
Chromosomes of a similar length with the same genes at the same loci
What is a diploid cell
A cell containing paired chromosomes one from each parent
Which way round are mother and father chromosomes
Mum on left, Dad on right
What does n refer to when talking about cells
The quantity of DNA within the cell
How does meiosis produce daughter cells that are genetically different from each other and to the parent cell
the processes of independent assortment and crossing over
What is independent assortment
The production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I
How come the allele a gamete received for one gene does not influence the allele received for another gene
Because homologous chromosomes line up in random orientations at the middle of the cell at metaphase as they prepare to separate, meaning that the same parent cell can produce different combinations of chromosomes in the daughter cells
Do paternal and maternal chromosomes in a homologous pair contain the same genes at the same loci
Yes, though they may contain different alleles
What do homologous chromosomes replicate to form
pairs of sister chromatids (genetically identical), that are linked by a specialised DNA sequence known as a centromere.
What are non sister chromatids
chromatids of homologous chromosomes that are not identical and may contain different alleles at the same gene loci.
How can you distinguish between sister and non sister chromatids
Non sister chromatids are not joined by a centromere
Why are non sister chromatids important
They play an important role in one of the key events of meiosis that results in variations
How can we distinguish between the phases in Meiosis I and Meiosis II
Roman numerals are used e.g. There are two Prophases - Prophase I and Prophase II
What is crossing over
The process whereby a chromatid breaks during meiosis and rejoins to the chromatid of its homologous chromosome so that its alleles are exchanged. Resulting in recombinant chromatids.
what is a chiasma
X-shaped points of attachment between two non-sister chromatids of a homologous pair.
What occurs during prophase I
DNA condenses and becomes visible as chromosomes
DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere
The chromosomes are arranged side by side in homologous pairs
As the homologous chromosomes are very close together the crossing over of non-sister chromatids may occur
In this stage centrioles migrate to opposite poles and the spindle is formed
The nuclear envelope breaks down and the nucleolus disintegrates
When does crossing over occur
Meiosis I (Prophase I)
When does chiasma break separating chromosomes
Anaphase I
What occurs during metaphase I
The bivalents line up along the equator of the spindle in a completely random nature, with the spindle fibres attached to the centromeres. on some occasions, the paternal chromosome will be facing a particular pole of the cell and on other occasion, it will be the maternal chromosome that faces that pole
What are bivalents
A pair of homologous chromosomes
What occurs during Anaphase I
The homologous pairs of chromosomes are separated as spindle fibres contract and so microtubules pull whole chromosomes to opposite ends of the spindle
The centromeres do not divide
What occurs during telophase I
Spindle fibres start to break down
Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I
Although there are two nuclear divisions and two sets of cytoplasmic divisions during cytokinesis, interphase only occurs once at the beginning of the cell cycle.
What is cytokinesis
When the division of the cytoplasm occurs
How does cytokinesis occur in animal cells
the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half
How does cytokinesis occur in plant cells
Vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate). The vesicles merge with each other to form the new cell surface membrane and secrete a layer of calcium pectate which becomes the middle lamella. Layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cell
What is the product of cytokinesis in meiosis I:
Two haploid cells - They are haploid because they contain half the number of centromeres
What occurs during prophase II
The nuclear envelope breaks down and chromosomes condense
A spindle forms at a right angle to the old one
What occurs during metaphase II
Chromosomes line up in a single file along the equator of the spindle
What occurs during anaphase II
Centromeres divide and individual chromatids are pulled to opposite poles
This creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell
What occurs during telophase II
Nuclear membranes form around each group of chromosomes
What occurs after Telophase II
Cytokinesis - Cytoplasm divides as new cell surface membranes are formed creating four haploid cells genetically different to each other and to the parent cell which began the cell cycle
What is having genetically different offspring advantageous for
Natural Selection
What mechanisms of meiosis increase the genetic diversity of gametes produced
Both crossing over and independent assortment (random orientation) result in different combinations of alleles in gametes
How does the entanglement in crossing over cause non sister chromatids to swap DNA
The entanglement places stress on the DNA molecules
As a result of this a section of chromatid from one chromosome may break and re-join with the chromatid from the other chromosome
How many chiasmata are usually present in each bivalent during meiosis
At least one, if not more, chiasmata present in each bivalent during meiosis
Where is crossing over most likely to occur
further down the chromosome away from the centromere
What is the number of homologous chromosomes in humans
23
What would the calculation and answer be for the number of possible chromosomal combinations in a human
2^23 = 8 388 608 possible chromosomal combinations
How can cells undergoing meiosis be observed and photographed
Using specialised microscopes
What can the stages of meiosis be identified with and why
The stages can be identified from photomicrographs or diagrams because each stage has distinctive characteristics
How can you distinguish between meiosis I and meiosis II on a photomicrograph or diagram
Homologous chromosomes pair up side by side in meiosis I only. If there are two new cells forming it is meiosis I but if there are four new cells forming it is meiosis II
What does meiosis do in comparison to mitosis
Mitosis contributes to the growth of an organism or to replace dead/dying cells. Meiosis produces genetically different gametes for sexual reproduction
How many daughter cells do does meiosis make in comparison to mitosis
Mitosis = 2
Meiosis = 4
Ploidy (number of chromosomes) of daughter cells in meiosis compared to mitosis
Mitosis = 2n
Meiosis = n
Are the daughter cells genetically identical to each other in mitosis and meiosis
Mitosis = yes
Meiosis = no
Why is it important that mitosis produces 2 daughter cells genetically identical to each other and the parent cell
This is important so that growth and cell replacement can occur within a body continually
What similarity does every cell in our bodies have
Every cell in an organism’s body (other than gametes) contain exactly the same genetic material - the full genome
Why is it important that meiosis produces 4 daughter cells genetically unidentical to each other and the parent cell
This is important for genetic variation within families and the population. Genetic variation can reduce the risk of inheriting genetic diseases
