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Meiosis and Sexual Reproduction

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Fertilization and meiosis

A life cycle is a generation to generation sequence of stages in the reproductive history of an organism from conception to production of its own offspring.

 n      In humans, each somatic cell has 46 chromosomes.

n      The sizes of the chromosomes and the positions of their centromeres differ.

n      Each chromosome has a distinctive pattern of bands.

n      The chromosomes that make up a pair have the same length, centromere position, and banding are called homologous chromosomes or homologues. 

n      The two chromosomes of each pair carry genes controlling the same inherited characters.

n      The important exception are the sex cells. 

n      Sperm cells and ova are distinct are referred to as gametes because they have 22 autosomes and a single sex chromosome.

n      A cell with a single chromosome set is called a haploid cell.

n      After fertilization or syngamy a zygote is formed.  This zygote contains half of the chromosomes from its mother and half from its father.  Now it is diploid because it contains double the amount of chromosomes.

n      The gametes are the only cells in the body that are not produced by mitosis.

 The Variety of Sexual Life Cycles

n      Although the alternation of meiosis and fertilization is common to all organisms that reproduce sexually, the timing of these two events in the life cycle varies depending on the species.

n      These variations can be grouped into three main types.

n      The human life cycle is one type and characteristic of most animals.  Gametes are the only haploid cells and then become diploid after fertilization.

n      A second type occurs in fungi and some protists including some algae.  After gametes fuse to form a diploid zygote, meiosis occurs before offspring develop.  This produces haploid cells which then divide by mitosis to give rise to multicellular adult organisms that are haploid.  Gametes are produced by the haploid organism by mitosis rather than meiosis.  The only diploid stage is the zygote.

n      Plants and some species of algae exhibit the third type called alternation of generations.  In this life cycle there are both diploid and haploid multicellular stages.  The multicellular diploid stage is called the sporophyte.  Meiosis in the sporophyte produces haploid cells called spores.  A spore gives rise to a multicellular individual without fusing with another cell.  A spore divides mitotically to generate a multicellular haploid stage called the gametophyte  The haploid gametophyte makes gametes by mitosis.  Fertilization results in a diploid zygote which develops into the next sporophyte generation.

 Meiosis reduce Chromosome number

 n      Meiosis is preceded by the replication of chromosomes.  However, this single replication s followed by two consecutive cell divisions called meiosis I and meiosis II.  These divisions result in four daughter cells each with only half as many chromosomes as the parent.

 INTERPHASE I

n      Meiosis is preceded by interphase during which each of the chromosomes replicate. 

n      The result is two genetically identical sister chromatids attached at their centromeres. 

n      The centriole pairs (in animal cells) replicate to form the two pairs. 

PROPHASE I

n      Phase lasts longer and is more complex than prophase in mitosis.

n      The chromosomes begin to condense.

n      During synapsis, homologous chromosomes, each made up of two chromatids, come together as pairs.  Each pair is now referred to as a tetrad.  At numerous places along their length, chromatids of homologous chromosomes are criss-crossed.  These crossings are called chiasmata.

n      The centriole pairs move away from each other.

n      Spindle fibers form between the centrioles.

n      The nuclear envelop and nucleoli disperse.

n      Chromosomes begin their migration to the metaphase plate

n      This phase can last for days and even longer, typically occupying more than 90% of the time required.

 METAPHASE I

n      Chromosomes are now arranged on the metaphase plate still in homologous pairs.  Spindle fibers from one pole of the cell attach to one chromosome of each pair, while the spindle fibers from the other pole attach to the other chromosome of each pair.

 ANAPHASE I

n      The spindle apparatus moves the chromosomes toward the pole.  Sister chromatids remain attached at their centromeres and move as a single unit towards the poles.

 TELOPHASE I AND CYTOKINESIS

n      The spindle apparatus continues to separate the homologous chromosome pairs until the chromosomes reach the poles of the cell. 

n      Each pole now has a haploid chromosome set but each chromosome still has two chromatids.

n      Usually cytokinesis occurs simultaneously with telophase I forming two daughter cells.

n      Cleavage furrows form in animal cells and cell plates appear in plant cells.

n      In some species, the nuclear membranes and nucleoli reform and there is a period of time before meiosis II. 

 PROPHASE II

n      A spindle apparatus forms and the chromosomes progress toward the metaphase II plate.

 METAPHASE II

n      The chromosomes align on the metaphase plate with the kinetochores of sister chromatids of each chromosome pointing toward opposite poles.

ANAPHASE II

n      The centromeres of the sister chromatids finally separate.

n      Sister chromatids of each pair move toward the opposite poles of the cell. 

TELOPHASE II AND CYTOKINESIS

n      Nuclei begin to form at opposite poles of the cell.

n      Cytokinesis occurs

n      Now, four daughter cells are present with the haploid number of chromosomes.

 Sexual Life Cycles Produce Genetic Variation Among Offspring

 Independent assortment of chromosomes

n      One way sexual reproduction generates genetic variation is independent assortment.

n      The orientation of the homologous pairs relative to the two poles is random.

n      Because each homologous pair of chromosomes orients independently of the other pairs at metaphase I, it is as random as the flip of a coin.

n      The number of combinations possible for gametes formed by meiosis starting with tow homologous pairs of chromosomes is four.  If there are six chromosomes, there are eight combinations.

n      More generally, the number of combinations possible when meiosis packages chromosomes into gametes by independent assortment is 2n, where n is the haploid number. 

Crossing Over

n      A process that produces individual chromosomes that combine genes inherited from the two parents.

n      It occurs during prophase I.

n      A protein apparatus called the synaptonemal complex functions something like a zipper to bring the chromosomes into close association but the exact mechanism is unknown.  The pairing is precise and the homologues align with each other gene by gene.

n      Crossing over occurs when homologous portions of the two nonsister chromatids trade places.

n      In humans, an average of two or three such crossover events occur per chromosome pair.  

Random Fertilization

n      The random nature of fertilization adds to the genetic variations arising from meiosis.

n      In a human, an ovum representing nearly 8 million different possibilities of trait is fertilized by a sperm that represents the same number of possible traits.

n      The zygote may have any of the 64 trillion possible combinations.

 Evolutionary adaptation depends on a populations genetic variation

 Darwin recognized the importance of genetic variation in the evolutionary mechanism called natural selection.  On average, the best suited individuals to a local environment leave the most offspring, transmitting their genes in the process.  The natural selection  results in adaptation, the accumulation of those genetic variations that are favored by the environment.  As the environment changes, the population moves, the population may survive if in each generation, at least some of the members can cope effectively with the new conditions.  Different genetic variations may work better than those that prevailed in the old time or place.  Sex and mutations are the two sources of this variation.