Meiosis (D2.1) Flashcards
(39 cards)
Is Meiosis considered reduction division?
Meiosis is considered a reduction division because it produces haploid gametes and it reduces the number of chromosomes in half.
What is Meiosis I?
The first meiotic division is called reduction division and it halves the chromosomes number in each nuclei
What is Meiosis II?
The second meiotic division is pretty much the same at mitosis (mitotic division) as it separates the sister chromatids
What is the goal of meiosis?
Meiosis is meant to produce haploid gametes that are genetically diverse
The four haploid nuclei result in 4 sperm or three polar bodies and 1 ovum (the egg is much larger than the sperm)
What are homologous chromosomes?
Homologous chromosomes are pairs of chromosomes that must be the same length, have centromeres in the same location, have genes in the same location, may have alleles of the genes and one of the copies must come from the father and the other from the mother.
They are from each the maternal and paternal chromosomes.
What is a chromosome?
Chromosome is a package of DNA containing part of the genetic material. Chromosomes are made up of two chromatids
What is reduction division?
Reduction division functions to separate the homologous chromosomes into seperate nuclei - cutting the chromosome number in half. The resulting cells are known as haploid cells and will develop into sex cells to be used in reproduction
Reduction division results in half the number of chromosomes.
What are haploid cells?
Haploid cells are have one set of chromosomes (Half of what sexually reproducing cells have as diploid cells). Haploid cells result in the formation in gametes to be used in sexual reproduction
Why is reduction division important?
Reduction division ensures that the when two sex cells are fused together during fertilization the offspring has the same number of chromosomes as the parents
What happens prior to meiosis?
Replication of the DNA occurs during S phase of the cell cycle (before meiosis)
Duplicated chromosomes (two sister chromatids) are attached at the centromere
The sister chromatids are virtually identical to one another after replication (low amount of errors are made
What is synapsis?
The pairing of homologous chromosomes and this forms the tetrad
What is a tetrad?
A pair of homologous chromosomes, each with two sister chromatids
What is prophase I?
Meiotic prophase I lasts longeer and is more complex that mitotic prophase
The chromosomes thicken and condense, becoming visible
Centrioles move to the poles and nuclear envelope begins to disappear
Homologous chromosomes come together during a process known as Synapsis to form a Tetrad
At numerous places along their lengths, chromatids of homologous chromosomes criss-crossed. These crossing help hold the chromosomes togther and the MAY trade segments (Cross over)
What is Metaphase I?
Chromosomes move to the equator as a tetrad
Nuclear membrane is completely dissolved
Spindle fibers attach to chromosomes
The chromosomes are now arranged on the metaphase plate and are still homologous pairs
What is Anaphase I?
Spindle fibers shorten and move the homologous chromosomes towards the poles
The sister chromatids remain attached at their centromeres and move as a single unit toward the same pole
What is telophase I?
The spindle apparatus continues to separate the homologous chromosomes until they are at the opposite poles of the cell
Each pole has a haploid chromosome set but the chromosomes still has two sister chromatids
Cell membrane begins to pinch in
Nuclear membrane reappears
What is cytokinesis? (For Meiosis I)
Cytoplasm divides
Formation of the cell membran
Results in two daughter cells
What is Meiosis II?
Identical to mitosis but the DNA is not replicated
Occurs immediately following cytokinesis and Meiosis I
Prophase II
The spindle fibers form and begin to attach to the chromosomes
Centrioles move to opposite poles
nuclear membrane begins to begins to dissolve
Metaphase II
In metaphase II the chromosomes are positioned on the metaphase plate in mitosis-like fashion
The sister chromatids of each chromosome pointing toward opposite poles
Anaphase II
The centromeres of sister chromatids finally separate
The sister chromatids of each pair move toward opposite poles of the cell
Telophase II
The nuclear membranes form at the opposite poles of the cells and the chromosomes uncoil into chromatin
What is cytokinesis? (For meiosis II)
There are four daughter cells, each with the haploid number of unreplicated chromosomes
What is crossing over, how does it work and how does it affect genetic variation?
Crossing over may occur during prophase I of meiosis. Crossing over results in new combinations of genes.
During cross over there is a ‘break’ in the DNA and then a swap of the broken DNA between the non-sister chrosmatids which remain attached at these locations
The location of the attachment is known as the chiasmata (can have several chiasmata in a homologous pair)
The chromosome can experience repelling and twisting around which can result in different shapes
