Top1-Ch2-P24-42EndReverse Flashcards
Number of chromosomes and number of DNA molecules during cell cycle picture?
- One cell to two cells with same genetic information.
- Two cells produced contain a full complement of chromosomes.
- Not all cells produced have identical cytoplasmic content.
Genetic consequences of cell cycle? Name three points.
- Meiosis - leads to gametes in which chromosome number is reduced by half.
- Fertilisation - where two haploid gametes fuse and restore chromosome number to its original diploid value.
Gametes are reproductive cells that unite during sexual reproduction to form a new cell called a zygote.
Name the two processes of sexual reproduction?
- Mitosis single nuclear division and usually a single cell division. Meiosis has two divisions.
- After mitosis chromosome number in new cells is same as original cell. Meiosis chromosome numbers in new cells are halved.
- Mitosis produces genetically identical cells but mieosis produces genetically variable cells.
How does meiosis differ from mitosis?
Like mitosis, meiosis is preceded by a interphase stage that includes G1, S, and G2 phases.
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
Cytokinesis
Interkinesis
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis
Name the phases of Meiosis?
Five:
Leptotene - chromosomes contract and become visible.
Zygotene - chromosomes continue to condense. Homologous chromosomes pair up and begin synapsis, a very close pairing association. Each homologous pair of synapsed chromosomes consists of four chromatids called a bivalent or tetrad.
Pachytene - Chromosomes become shorter and thicker and a three part synaptonemal complex develops between homologous chromosomes.
Diplotene - centromeres of the paired chromosomes move apart. The two homologs remain attached at each chiasma which is the result of crossing over
Diakinesis - chromsome condensation continues and the chiasmata move towards the ends of the chromosomes as the strands slip apart so the homologs remain paired only at the tips.
Near the end of prophase I the nuclear membrane breaks down and the spindle forms setting the stage for metaphase I
Define: chiasmata - a point at which paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands.
Crossing over occurs during prophase I in which homologous chromosomes exchange genetic information. This generates genetic variation and is essential for the proper alignment and separation of homologous chromosomes.
Meiosis Prophase I is divided into how many substages? Explain a bit about each.
when homologous pairs of chromosomes align along the metaphase plate. Microtubules from each pole attach to different chromosomes.
Meiosis metaphase I is?
separation of homologous chromosomes. Two chromosomes of homologous pair are pulled towards opposite poles.
Meiosis anaphase I is?
when the chromosomes arrive at the spindle poles and the cytoplasm divides.
meiosis telophase I is?
Period between Meiosis I and Meiosis II in which the nuclear membrane re-forms around the chromosomes clustered at each pole, the spindle breaks down and the chromosomes relax.
meiosis, interkinesis is?
when events of interkinesis are reversed. chromosomes recondense, spindle re-forms and nuclear envelope breaks down.
Some cell types chromosomes remain condensed in interkinesis. These cells move directly from cytokinesis into metaphase II.
meiosis, prophase II is?
chromosomes line up on the metaphase plate.
meiosis, metaphase II is?
when the kintechores of the sister chromatids separate and the chromatids are pulled to opposite poles. Each chromatid is now a distinct chromosome.
meiosis, anaphase II is ?
chromosomes arrive at spindle poles, nuclear envelope reforms around the chromosomes and cytoplasm divides.
meiosis, telophase II is ?
cytoplasm divides.
meiosis, cytokinesis in Meiosis II is when?
the cytoplasm divides to produce two cells, each having half the original number of chromosomes.
meiosis, cytokinesis in Meiosis I is when?
Meiosis graphic
After each homologous chromosome replicates one chromosome has AB (chromatids are the same) and the other has ab. Then one part of one chromatid from each chromosome crosses over with the same spot from the other chromosome. So, one chromosome has two different chromatids. Ie AB, aB and the other chromosome has Ab, ab. So each chromatid is now different. This shuffles alleles on the same chromosome into new combinations.
Crossing over in meiosis is?
Happens in metaphase I after random alignment of chromosomes in metaphase I. One chromosome in each homologous pair is maternal and the other paternal in origin. But when the line up for the homologous pairs to be separated they line up randomly. So, maternals and paternals can go to either side, with maternals and paternals mixing. This shuffles alleles on different chromosomes into new combinations.
Explain “Random Separation of Homologous Chromosomes”
- crossing over
- random separation of homologous chromosomes
Genetic differences among cells result from two processes that are unique to meiosis. What are the two processes?
Compare differences between mitosis and meiosis
a protein that holds the chromatids together during meiosis and mitosis. Keeps sister chromatids together and during cross over also tethers homologs together at their ends.
In Anaphase I cohesin along chromosome arm is broken pulling two homologs apart but shugoshin prevents the two sister chromatids from separating.
To learn: cohesin and shugoshin
Cohesin is?
Diploid primordial germ cells divide mitotically to produce diploid cells called spermatogonia.
Each spermatagonium can undergo repeated rounds of mitosis producing more. Or, it can enter into prophase I and is called a primary spermatocyte (still diploid).
Primary spermatocyte completes meiosis I, producing two haploid secondary spermatocytes that then undergo meisos II, producing two haploid spermatids.
Therefore, each primary spermatocyte produces a total of four haploid spermatids which mature and develop into sperm.
Meiosis in male animals explain.
is the production of gametes in a male animal and occurs in the testes.
Spermatogenesis is?
2n, where n equals the number of homologous pairs. Ie three homologous pairs produce eight gametes.
What is formula to calculate how many different combinations of gametes can be formed by pairs of chromosomes?
the production of gametes in a female animal
Oogenesis is?
Within ovaries diploid primordial germ cells divide mitotically to produce oogonia which can undergo repeated mitosis or go into meiosis.
Enter prophase I and are called primary oocytes, which completes meiosis I and divides.
During cytokinesis most cytoplasm goes to one of the two haploid cells called the secondary oocyte. The smaller cell (first polar body) may or may not divide further or may disintegrate.
Secondary oocyte completes meiosis II and cytokinesis is again unequal. The larger cell is ovum (mature female gamete) and smaller cell is second polar body. Only the ovum can be fertilised and the polar bodies usually disintegrate.
Meiosis in female animals explain.
Sperm formation occurs continuously during male adult reproductive life.
For females, oogenesis begins before birth giving rise to primary oocytes. Meiosis is then stalled in prophase I. Suspended animation, in humans, may last 30-40 years.
Before ovulation, rising hormones, stimulate one or more primary oocytes to recommence meiosis. Meiosis I is completed and a secondary oocyte is ovulated from the ovary.
In humans and many other species, Meiosis II is delayed until contact with sperm. When sperm penetrates outer layer of secondary oocyte Meiosis II occurs. Second polar body is extruded (forced out) from egg and nuclei of sperm and newly formed ovum fuse to form zygote.
Apart from basic explanation of animal meiosis and differences between male and femal, how else does oogenesis differ from spermatogenesis in mammals?
