Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell...

16
Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal number producing genetically diverse cells. Diploid cells are cells that contain pairs of chromosomes called "homologous chromosomes". The number of chromosomes in these cells is called the diploid number and is equal to 2n (n is the number of homologous pairs). Haploid organisms/cells have only one set of chromosomes. The haploid number and is equal to n. These are sperm and egg cells.

Transcript of Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell...

Page 1: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

Meiosis - Cell DivisionMeiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal number producing genetically diverse cells.

Diploid cells are cells that contain pairs of chromosomes called "homologous chromosomes". The number of chromosomes in these cells is called the diploid number and is equal to 2n (n is the number of homologous pairs).

Haploid organisms/cells have only one set of chromosomes. The haploid number and is equal to n. These are sperm and egg cells.

Page 2: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

Meiosis - Cell DivisionMeiosis:- The chromosome numbers of a cell produced by meiosis are reduced by 1/2 - Haploid nuclei are formed which contain only one chromosome of the original homologous pair and are gentically different. - Meiosis ONLY occurs in the gamete producing cells of the gonads (ovaries & testes). - One of the main goals of meiosis is to mix up the combination of maternal chromosomes (from the mother) and paternal chromosomes (from the father) in a process called Random Assortment.

Page 3: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

Meiosis - Cell Division

The Process of Meiosis:

· involves two divisions · the first divisions is a called Meiosis I · the second divisions is a called Meiosis II· creates gametes (eggs or sperm)

Page 4: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

Homologous chromosomes = a pair of chromosomes in which one member of the pair is obtained from the organism's maternal parent and the other from the paternal parent.

*homologous chromosomes are found in diploid cells.

Tetrads: the two pairs of homologous chromosomes

Page 5: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

First Meiotic Division:

· Each single stranded chromosome is replicated during interphase (S phase).· Before meiosis, there are double stranded chromosomes formed. · Chromosomes shorten and thicken and spindle fibers are formed. · Homologous chromosomes pair up side by side in a double line at the equator (center of the cell). · Homologous chromosome pairs are called tetrads. · It is at the synapsis (homologous chromosome pairs overlap) that crossing over occurs.

Meiosis II

Page 6: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

Meiosis - Cell DivisionCrossing over: the exchange of chromosome pieces in the tetrad during prophase I - Crossing over increases the variability of the offspring. - This is why the offspring of sexual reproduction show many variations.

Synapsis: the location where two homologous chromosome pairs overlap

Page 7: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

Meiosis - Cell DivisionHomologous pairs of chromosomes separate & move toward opposite poles of the cell.

Disjunction: the separation of the homologous chromosome pairs during the first division of meiosis.

Nondisjunction: the failure of homologous chromosomes to separate

Page 8: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

Meiosis - Cell DivisionSecond Meiotic Division · similar to mitosis but no chromosome replication occurs · diploid number of chromosomes is reduced to the haploid number · homologous chromosomes line up at the equator.· cytoplasmic division of the cell occurs after telophase.

fromMeiosis I

Page 9: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

Meiosis - Cell DivisionResults of Meiosis: - Diploid number (2n) of chromosomes is reduced by 1/2 to the haploid number (n). - Specialized reproductive cells (eggs and sperms) are formed. - Distribution of homologous chromosomes is random so variations frequently occur. - Meiosis is associated with sexual reproduction.

** Mitosis is associated with growth and asexual reproduction.** Variations rarely occur in MITOSIS (mutations).

Page 10: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

Meiosis - Cell Division

Page 11: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

Meiosis - Cell DivisionSpermatogenesis: the production of sperm - male gamete(one successful meiotic division produces 4 usable sperm cells)

Page 12: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

Oogenesis: the production of ova or eggs - female gamete (1 good/usable egg and three polar bodies are produced in one meiotic division)

Meiosis - Cell Division

Page 13: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

Gamete Production and Fertilization Overview

Page 14: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

Fertilization: egg and sperm unite to form a diploid cell (the tail of sperm disintegrates after chromosomes are delivered)

egg (n) + sperm (n) = zygote (2n)

Meiosis and Reproduction

When errors occur in meiosis we can get the wrong number of chromosomes in a cell.· a missing chromosome · an extra chromosome

Page 15: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

Errors in Meiosis

In mitosis or meiosis, there can be an error in the processes that results in the change in the number of chromosomes that the daughter cells receive.

There are two types of this error condition:

Polyploidy – is a condition of having too many sets (pairs) of chromosomes. For example, having one pair of a chromosome makes a cell diploid (2n), while having two pairs of a chromosome makes a cell that is tetraploid (4n) – ie: domestic blueberries Aneuploidy – the gain or loss of whole chromosomes (this is the most common chromosome abnormality). It is caused by nondisjunction, the failure of chromosomes to correctly separate.

Monosomy = having only one copy of a chromosome when you should have a pair.

Trisomy = having three copies of a chromosome when you should have a pair. Examples include: - Downs’ Syndrome (extra chromosome #21) - Turner Syndrome (only one X chromosome in a female) - Klinefelter Syndrome (extra X chromosome in a male -> XXY)

Page 16: Meiosis - Cell Division · 2020. 1. 22. · Meiosis - Cell Division Meiosis: the process of cell division which involves a reduction of the chromosome number to one half the normal

Errors in Meiosis

The consequences of an error in the number of chromosomes passed on to daughter cells shows up during fertilization (egg and sperm cells join).