Unit Three “Cell Proliferation and Genetics”
Unit Three
“Cell Proliferation and
Genetics”
“Meiosis”
History of Meiosis
In the mid 1880’s after Walter Fleming discovered chromosomes, Belgian cytologist Pierre-Joseph van Beneden found that different types of cells from the same organisms had two different numbers of chromosomes
He further documented the “Gametes”
(sperm and egg) each had half the number of chromosomes of the somatic cells (non-sex cells)
History of Meiosis
In 1887, van Beneden proposed an egg and sperm fuse to produce a single cell called the “Zygote”
The Zygote contains two copies of each chromosome
The fusion of the gametes is called
“Fertilization” or “Syngamy”
It was clear to van Beneden and his contemporaries some mechanism must be involved in reducing the number of chromosomes by half in preparation for fertilization
Meiosis
Meiosis is the process by which the
Diploid number of chromosomes (2n) is reduced to the Haploid number of chromosomes (n) in preparation for fertilization
Because Meiosis is the halving of chromosomes, it is sometimes referred to as “Reduction Division”
Meiosis is a process that occurs only in species that participate in “Sexual
Reproduction”
Species that reproduce asexually via cloning, fission etc. participate in
“Asexual Reproduction”
Meiosis
Meiosis produces viable sperm and egg that are ready to participate in fertilization
Upon fertilization, the single cell that is the result of the sperm and egg combining, begins to divide mitotically to produce the entire fully functioning eukaryotic organism
Both somatic cells and primordial gamete cells are diploid, however, the sperm and egg cells are haploid
Meiosis
Meiosis is divided into:
– Meiosis I
Duplicated chromatin (2 sets of 46 chomosomes) from interphase is divided between two daughter cells
– Meiosis II
Each daughter cell divides further to reduce the diploid # of 46 to the haploid # of 23
Meiosis I and II
How Meiosis differs from Mitosis
PMAT occurs twice instead of once
In conjunction with the above statement, the resulting 4 cells in
Meiosis have a Haploid number of chromosomes, whereas the resulting 2 cells in Mitosis have a Diploid number of chromosomes
To encourage variation in individuals of a species, two mechanisms have evolved which are unique to Meiosis:
– “Crossing Over”
– “Independent Assortment”
Synapsis & Crossing Over
During Metaphase I, two similar sized chromosomes with variations of the same gene line up next to each other forming a “Tetrad”
The tetrad is made of two sets of
“Sister Chromatids”
When the tetrad has formed during
Metaphase I, equal segments of two
“Nonsister Chromatids” are exchanged amongst each other
Synapsis & Crossing Over
Independent Assortment
Occurs during Meiosis I
Sister chromatids are pulled to either end of the cell in a random fashion
This situation results in additionally variation in the genetic code each egg or sperm cell contains
Independent Assortment
Consequences of Meiosis and
Sexual Reproduction
Genetic diversity is favored due to an ever changing environment with new abiotic variables coming to the forefront constantly
The best mode of ensuring diversity is via sexual reproduction as the result of
Meiosis
It should be noted, changes in the genetic code are not always beneficial; they can be harmful too
Consequences of Meiosis and
Sexual Reproduction
Main benefit to sexual reproduction is variation between offspring which allows for better chance of survival of species
Main cost is the energy requirement of entire process:
– Nucleic acid and cellular division
– Search for a suitable mate
– Physical act of intercourse
Sex Determination within Sexual
Reproduction
Sex of offspring is determined in a variety of ways
– humans: 23 rd pair/46 th chomosome
– Reptiles: temperature of nest
– Insects: number of chromosomes – haploid or diploid
Meiotic Abnormalities
Errors during Meiosis I and II result in an incorrect number of chromosomes - Nondisjunction
Turner’s Syndrome, Klinefelter’s
Syndrome, Metafemales, Metamales,
Trisomy 21/Down Syndrome etc.
