Meiosis and sexual life cycles
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Meiosis and Sexual Life Cycles. The process of creating sex cells i.e.: Eggs (females) Sperm (males). Cell Reproduction. Asexual (vegetative) reproduction A form of duplication using only mitosis. Example, a new plant grows out of the root or a shoot from an existing plant.

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Meiosis and Sexual Life Cycles

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Meiosis and sexual life cycles

Meiosis and Sexual Life Cycles

The process of creating sex cells

i.e.: Eggs (females)

Sperm (males)


Cell reproduction

Cell Reproduction

  • Asexual (vegetative) reproduction

    • A form of duplication using only mitosis.

    • Example, a new plant grows out of the root or a shoot from an existing plant.

    • Produces only genetically identical offspring since all divisions are by mitosis.

      • Offspring called clones meaning that each is an exact copy of the original organism

      • This method of reproduction is rapid and effective allowing the spread of an organism

      • Since the offspring are identical, there is no mechanism for introducing diversity.


Sexual reproduction

Sexual reproduction

  • Formation of new individual by a combination of two haploid sex cells (gametes).

  • Fertilization- combination of genetic information from two separate cells that have one half the original genetic information

  • Gametes for fertilization usually come from separate parents

    • Female- produces an egg

    • Male produces sperm

  • Both gametes are haploid, with a single set of chromosomes

  • The new individual is called a zygote, with two sets of chromosomes.

  • Meiosis is a process to convert a diploid cell to a haploid gamete, and cause a change in the genetic information to increase diversity/variation in the offspring.


Chromosomes

Chromosomes

Chromosomes

  • In humans, each somatic cell has 46 chromosomes.

  • Made up of 23 pairs of homologous chromosomes chromosomes that make up a pair – that have the same length, centromere position and pattern of coded genes.

    • The two chromosomes of each pair carry genes controlling the same inherited characters. (i.e. if a gene for eye colour is situated at a particular spot (locus) on a certain chromosome, then the homologue of that chromosome will also have a gene specifying eye colour at the equivalent locus.

      • Except for the two sex chromosomes – the chromosomes that determine an individual’s sex (all other chromosomes are called autosomes.

How Do Scientists Read Chromosomes?


Meiosis and sexual life cycles

Make a Karyotype

Karyotype: a visual representation of the organization of the chromosomes in the cell of an organism.


Ploidy number of sets of chromosomes in a cell

Ploidy: Number of sets of chromosomes in a cell

Haploid (n)-- one set chromosomes. In humans n=23

  • Diploid (2n)-- two sets chromosomes. In humans 2n=2x23=46

  • Most plant and animal adults are diploid (2n)

  • Eggs and sperm are haploid (n)

    • Each gamete has a single set of the 22 autosomes plus a single sex chromosome, either X or Y. These are the only cells in the body not produced by mitosis.


Chromosomes in a diploid cell

Chromosomes in a Diploid Cell

  • Summary of chromosome characteristics

    • Diploid set for humans; 2n = 46 (2 sets of 23)

    • Autosomes; homologous chromosomes, one from each parent (humans = 22 sets of 2)

    • Sex chromosomes (humans have 1 set of 2)

      • Female-sex chromosomes are homologous (XX)

      • Male-sex chromosomes are non-homologous (XY)


What is meiosis

What is Meiosis?

Figure: Sexual Life cycle

  • Meiosis involves two successive nuclear divisions that produce four haploid cells. The first division (meiosis I) is the reduction division; the second division (meiosis II) separates the chromatids.

  • Occurs in the ovaries or testes.

  • Fertilization restores the diploid condition.

Gametes

n

2n

ZYGOTE

2n

Diploid multicellular organism


Advantages of meiosis

Advantages of Meiosis

  • Mitotic cell division produces new cells genetically identical to the parent cell.

    VS

  • Meiosis increases genetic variation in the population. – exchange of information can occur during meiosis I while mutations can also occur

Unique Features of Meiosis

Comparison of Meiosis and Mitosis


Interphase

Interphase

  • Interphase is an important stage preceding meiosis. Without this stage meiosis would not occur.

  • During this stage, each individual chromatid replicates, similar to mitosis..

  • At this stage, the chromosomes are long and stringy and are not visible.

  • **Remember: All somatic cells are diploid in number (2n), therefore for each chromatid there also exists its homologue, which also replicates during interphase.


Prophase i

Prophase I

  • Prophase I is one of the most important stages of meiosis. The chromotid threads begin to twist and condense, creating chromosomal structures which are visible to the microscope.

  • In a process called synapsis, homologous chromosomes, each made up of two sister chromatids, come together as pairs.

  • After the homologous chromosomes pair, the structure is referred to as a tetrad (four chromatids). The point at which two non-sister chromatids intertwine is known as a chiasmata (sing. = chiasma).

  • Sometimes a process known as crossingover occurs at this point. This is where two non-sister chromatids exchange genetic material. This exchange does not become evident, however, until the two homologous pairs separate.


Meiosis and sexual life cycles

Meanwhile, centrosomes move away from each other and spindle fibres form between them.

The nuclear membrane disappears.

Animations


Metaphase i

Metaphase I

  • At metaphase, each chromosome has reached its maximum density.

  • The homologous pairs and their sister chromatids also prepare for separation.

  • They interact with spindle fibers which form from either side of the nuclear envelope of the cell.

  • There is a centriole at opposite ends of the cell, which is referred to as poles.

  • During metaphase, the chromosomes are lined by the spindle fibers at what is known as the metaphase plate.


Anaphase i

Anaphase I

  • Spindle fibres pull apart the tetrad, separating each homologous chromosome. Sister chromatids still remain attached to each other and move as a single unit toward the same pole.

  • It is by random chance that a certain chromosome is pulled to a certain pole.


Telophase i

Telophase I

  • Telophase I varies from species to species.

  • Sometimes Telophase I is skipped and meiosis starts its second division immediately.

  • In general, however, two nuclear envelopes begin to surround the separate chromosomes and cytokinesis (splitting of the cytoplasm into two separate entities) will sometimes occur.

  • Each pole now has a haploid chromosome set, but each chromosome still has two sister chromatids.

  • Then a phase called interkinesis will follow, which essentially is a resting period from Telophase I to Prophase II.

  • This differs from mitosis because DNA replication does not occur again.


Prophase ii

Prophase II

  • During Prophase II, each dyad (1/2 a tetrad) is composed of a pair of sister chromatids and they are connected by a centromere.

  • The centrioles (replicated during Telophase I) which produce the spindle fibers also start to move toward the poles of the cell.


Metaphase ii

Metaphase II

  • Metaphase II is similar to Metaphase I in that the dyads are lined up at a metaphase plate by the spindle fibers.


Anaphase ii

Anaphase II

  • The centromeres of sister chromatids finally separate, and the sister chromatids of each pair, now individual chromosomes, move toward opposite poles of the cell.

  • Each sister chromatid ends up on one side of the cell.


Telophase ii

Telophase II

  • At the end of Telophase II, the nuclear envelopes forms around each set of DNA at opposite poles of the cell and the cytoplasm divides once again (cytokinesis).

  • As a result, four haploid daughter cells have formed from one diploid cell.

  • The chromosomal content of a haploid cell in one-half the chromosomal content of a diploid cell (n as opposed to 2n)

Animations


Cell photograph of telophase ii the chromosomes are the number in a somatic cell

Cell Photograph of Telophase IIthe chromosomes are ½ the number in a somatic cell

Stages of Meiosis

  • End result of meiosis  gametogenesis = production of gametesSpermatogenesis = process of male gamete production, one diploid cell gives rise to 4 sperm cellsOogenesis = process of female gamete production, one diploid cell gives rise to 1 viable egg cell and three polar bodies, occurs in the ovaries once a month starting at puberty.


Meiosis and sexual life cycles

Cdck- cell division activity

Meiosis


Genetic variation

Genetic Variation

Independent Assortment and Gamete Diversity

Random Orientation of Chromosomes During Meiosis

Independent Assortment of Alleles

  • Independent Assortment of Chromosomes: Arrangements of chromosomes are sorted out/moved to opposite poles by chance into gametes; maternal and paternal (Metaphase I)

  • Crossing Over: Combining DNA inherited from two parents into a single chromosome. (Prophase I)

  • Random Fertilization: An egg cell has 1/8 million possible chromosomes combinations, is fertilized by a single sperm cell, which represents 1/8 million different possibilities. (Potentially producing a zygote with any of 64 trillion diploid combinations).


Errors in meiosis

Errors in Meiosis

  • Mutations can be inherited if an error occurs in the formation of one gamete and that gamete fertilizes another to form a zygote  all the cells in the zygote will carry the mutation.

  • Nondisjunction: members of a pair of homologous chromosomes do not move apart properly during meiosis I, or in which sister chromatids fail to separate during meiosis II.

    • One gamete receives 2 of the same type of chromosome and another gamete receives no copy

    • Anueploidy = abnormal chromosome number

    • Trisomy= if the chromosome is present in triplicate in the fertilized egg (2n+1 chromosomes) e.g. double in egg cell, normal in sperm cell; e.g.:Trisomy 21 = Down Syndrome

    • Monosomic = if a chromosome is missing (2n-1)

    • Polyploidy = when organisms have more than 2 complete chromosome sets (triploidy – 3n, tetraploidy – 4n) occurs if the cell fails to divide after replicating its chromosomes OR nondisjunction of all its chromosomes.


Down syndrome karyotype

Down Syndrome Karyotype

Using Karyotypes to Predict Genetic Disorders

3 of chromosome 21


Other errors

Other Errors


More errors

More errors:


Alterations of chromosome structure

Alterations of Chromosome Structure

  • Breakage of a chromosome:

    • Deletion: when a chromosomal fragment lacking a centromere is lost during cell division  missing genes; usually lethal b/c missing essential genes

Ex. Crie du chat

Cri du Chat Syndrome ("Cry of the cat" in French) is a genetic disorder caused by the loss or misplacement of genetic material from the fifth chromosome. It was first identified in 1963 by Professor Lejeune, who also identified the genetic cause of Downs Syndrome. He described the syndrome after the sound that many of the babies and young children make when crying.


Meiosis and sexual life cycles

  • Duplication: broken fragment may rejoin but to the homologous chromosome instead


Meiosis and sexual life cycles

  • Inversion: broken fragment may rejoin the correct chromosome, but in the reverse orientation

  • Translocation: broken fragment joins a nonhomologous chromosome

Mistakes in Meiosis

Animations


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