Mitosis & Meiosis Flashcards
Homologous chromosomes
A pair of chromosomes, one of maternal origin and the other of paternal origin, which have the same genes but may have different alleles at corresponding loci.
They have similar size, shape, centromere position and staining pattern.
Centromeres
Non-coding tandem repeat sequences of nucleotides at one position along chromosomes where sister chromatids adhere to each other which allow kinetochore proteins and thus kinetochore microtubules to attach.
Mitosis
Results in proper alignment of chromosomes in a single row along the equator during metaphase of mitosis and equal separation of sister chromatids when spindle fibres shorten and pull them to opposite poles of the cell during anaphase.
Meiosis
Results in proper alignment of bivalents along equator of cell during metaphase I of meiosis and equal separation of homologous chromosomes when spindle fibres shorten and pull them to opposite poles of the cell during anaphase I.
Subsequently results in proper alignment of chromosomes in a single row during metaphase II and equal separation of sister chromatids when spindle fibres shorten and pull them to opposite poles in anaphase II.
Centrioles
There is a pair of centrioles in every cell and their positions determines the polarity of the cell.
They form part of the microtubule organising centre and are involved in synthesis of spindle fibres which are essential for proper alignment of chromosomes at equator during metaphase and equal separation of sister chromatids after centromere division when spindle fibres shorten and pull…
Mitosis
In G phase of interphase, organelles are replicated and enzymes required for nuclear division are synthesised. In S phase, DNA is replicated by semi-conservative replication where parental strands unwind and act as templates for addition of nucleotides by complementary base pairing to synthesise two complementary, genetically identical sister chromatids.
In prophase, nuclear envelope disintegrates and nucleolus disappears as chromatin condenses into chromosomes to allow kinetochore microtubules to attach to kinterochore proteins on centromeres of chromosomes. Centrosomes move to opposite poles of the cell.
In metaphase, chromosomes align along the equator in a single row.
In anaphase, centromeres divide and spindle fibres shorten, pulling sister chromatids, now called daughter chromosomes, to opposite poles of the cell with centromeres leading.
In telophase, nuclear envelope reforms around to enclose chromosomes to form a nucleus as chromosomes decondense into chromatin, causing nucleolus to reappear.
Telophase is followed by cytokinesis where a cleavage furrow forms and deepens until the cytoplasm divides, splitting the cell into two genetically identical daughter cells.
Daughter cells have the same type and same number of alleles as the parental cell.
Meiosis
In Prophase I, nuclear envelope disintegrates and nucleolus disappears as chromatin condenses into chromosomes. Centrosomes move to opposite poles of the cell. Homologous chromosomes pair up to form bivalents by synapsis and undergo crossing over between non-sister chromatids to form chiasmata where alleles of the same gene are exchanged at random.
In Metaphase I, homologous chromosomes align along the equator in pairs with the aid of kinetochore microtubules attached to kinetochore proteins on respective centromeres of chromosomes.
In Anaphase I, homologous chromosomes are separated as spindle fibres shorten and pull them to opposite poles of the cell.
Genetic variation in cells by meiosis
1) Crossing over between non-sister chromatids of homologous chromosomes during Prophase I results in exchange of alleles giving rise to new combinations of alleles on chromosomes.
Since non-sister chromatids may have different alleles for the same gene, breakage of linkage groups and forming new linkage groups brings about new combinations of alleles.
2) Independent assortment of homologous chromosomes during metaphase I on metaphase plate can result in 2^n possible combinations of maternal and paternal chromosomes in gametes where n is the number of chromosomes.
3) Independent orientation of non-identical sister chromatids of each chromosome in metaphase II and their subsequent separation in anaphase II.
4) Chromosome mutations may occur, such as non-disjunction where homologous chromosomes in anaphase I or sister chromatids in anaphase II fail to separate properly, resulting in abnormal gametes with (2n+1) and (2n-1) chromosomes. Resulting in aneuploidy.
5) Random fusion of gametes during fertilisation results in greater genotypic combinations of a zygote.
Mitosis vs Meiosis
1) In meiosis, homologous chromosomes undergo crossing over in Prophase I and form chiasmata and exchange alleles between non-sister chromatids while in mitosis, chromosomes do not undergo crossing over in Prophase, do not form chiasmata and do not exchange alleles.
2) In meiosis, homologous chromosomes pair up in synapsis to form bivalents in Prophase I while in mitosis, chromosomes do not undergo synapsis and do not pair up in Prophase.
3) Meiosis produces four daughter haploid cells while mitosis produces two daughter diploid cells.
4) Meiosis produces four genetically dissimilar daughter cells while mitosis produces two genetically identical daughter cells.
5) In mitosis, chromosomes are arranged singly at the equator during metaphase while in meiosis, homologous chromosomes are arranged in pairs at equator in metaphase I.
6) In mitosis, anaphase involves division of centromeres and separation of sister chromatids in anaphase while in meiosis, anaphase I involves separation of homologous chromosomes with no centromere division
Reduction division
Reduction division is the production of four haploid cells from one diploid cell.
When fertilisation occurs and two haploid cells fuse to form a zygote, results in restoration of chromosome number.
Ensures that the same number of chromosomes is present in cells on subsequent generations and does not clutter up the cell.
Haploid cells allows for genetic variation during random combination of gametes and for crossing over and independent assortment.
