meiosis and sexual reproduction

Meiosis and Sexual Reproduction

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Formation of Haploid Cells

Meiosis: a form of cell division that halves the number of chromosomes creating haploid cells (gametes or spores)

Involves two divisions of the nucleus

Meiosis I

Meiosis II


reduces the number of chromosomes by half to form reproductive cells

when the reproductive cellsunite in fertilization, the normal diploid number is restored


Meiosis

Meiosis I

four phases:

a. prophase I

b. metaphase I

c. anaphase I

d. telophase I


Longest and most complex phase (90%).

Chromosomes condense.

Synapsis occurs: homologous chromosomes come together to form a tetrad.

Tetrad is two chromosomes or four chromatids (sister and nonsisterchromatids).


Prophase I

Crossing over


Metaphase I

Pairs of homologous chromosomes moved to the middle

Spindle fibers are attached


Anaphase I

homologous chromosomes separate

sister chromatids remain attached

move toward opposite poles

Genetic material has recombined


Telophase I and Cytokinesis

chromatids gather at poles

cytoplasm divides

Now have 2 haploid cells, but still have sister chromatids

no further replication of genetic material


Meiosis II

Includes four phases

Prophase II

Metaphase II

Anaphase II

Telophase II


Prophase II

A new spindle forms around the chromosomes


Metaphase II

Chromosomes line up at the equator

Spindle fibers attached at the centromeres


Anaphase II

Centromeres divide

Chromatids move to opposite poles


Telophase II and Cytokinesis Nuclear envelope forms around each set of chromatids (chromosomes)

Spindle breaks down

Cell cytoplasm divides

RESULT:

4 HAPLOID GENETICALLY

DIFFERENT CELLS


Meiosis and Genetic Variation

Meiosis allows for rapid generation of new genetic combinations

Genetic variation is essential for evolution to occur.

Three things that contribute to genetic variation:

Independent assortment

Crossing-over

Random fertilization


Independent Assortment

the random distribution of homologous chromosomes during meiosis

contributes to genetic variation in sexually reproducing organisms

Occurs in metaphase I

2N = number of combinations possible of chromosomes (N = 23 chromosomes from mom or dad)

223 = 8.4 million possible combinations of gametes


Crossing Over

Produce individual chromosomes that combine genes inherited from parents

frequency of crossing over depends on size of chromosome (larger chromosomes, more crossing over)

Occurs during prophase I and is the exchange of corresponding segments of DNA

contribute to genetic variation within a species

Genetic recombination has occurred at the end


Random Fertilization

zygote formed by random joining of two gametes

one egg cell – 1 of 8.4 million possibilities

one sperm cell – 1 of 8.4 million possibilities

223 x 223 = 70 trillion diploid combinations

This is not including variation from crossing over!!!


Meiosis and Gamete Formation

In sexually reproducing eukaryotic organisms, gametes form through the process of

spermatogenesis in males.

oogenesis in females.


Sexual Reproduction

Two types of reproduction

Asexual: a single parent passes copies to make identical offspring

Sexual: two parents with gametes

Reproduction: process of producing offspring


Asexual Reproduction

the formation of offspring from one parent.

The offspring are genetically identical to the parent

Simplest and most primitive method of reproduction

All types lead to clones of the parent.

Types of Asexual Reproduction

Fission (amoebas), fragmentation (planarea), and budding (hydra) .


Sexual Reproduction

the formation of offspring through the union of gametes from two parents.

The offspring are genetically different from their parents.


Genetic Diversity

• Raw material for evolution• sexual reproduction increases variation in the population by making possible genetic recombination

• asexual reproduction leads to a lack of genetic diversity among offspring. – This lack of diversity is a disadvantage in a changing environment.


Evolution of Sexual Reproduction

• Sexual reproduction may have begun as a mechanism to repair damaged DNA


Sexual Life Cycles in Eukaryotes

Life Cycle: the entire span in the life of an organism from one generation to the next

Three types of sexual life cycles:

Haploid Life Cycle

Diploid Life Cycle

Alteration of generations


Haploid Life Cycle the simplest of all life cycles,

the haploid cell occupies the major portion of the life cycle

Zygote is the only diploid cell and undergoes meiosis to create new haploid cells

Found in protists and fungi and algae

Example = moss plants


Diploid Life Cycle

the adults are diploid

the diploid individual occupies the major portion of the life cycle

Gametes are the only haploid cells

Examples are humans and other mammals


Some organisms have a life cycle that alternates between diploid and haploid phases. (plants, algae, and some protists)

Reproduces by mitosis and meiosis

In plants, the diploid phase produces spores = sporophyte (creates 4 haploid spores)

In plants, the haploid phase produces gametes = gametophyte

Example = roses


Alternation of Generations

meiosis and sexual reproduction

Education

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chromosomes easyshiksha

number of chromosomes

individual chromosomes

recombined easyshiksha

centromeres easyshiksha

different cells easyshiksha

opposite poles easyshiksha