Meiosis and Sexual Life Meiosis and Sexual Life CyclesCycles

The process of creating sex cellsThe process of creating sex cells

i.e.: Eggs (females)i.e.: Eggs (females)

Sperm (males)Sperm (males)

Cell ReproductionCell ReproductionAsexual (vegetative) reproduction Asexual (vegetative) reproduction A form of duplication using only mitosis. A form of duplication using only mitosis. Example, a new plant grows out of the root or a Example, a new plant grows out of the root or a

shoot from an existing plant. shoot from an existing plant. Produces only genetically identical offspring Produces only genetically identical offspring

since all divisions are by mitosis.since all divisions are by mitosis. Offspring called clones meaning that each is an exact Offspring called clones meaning that each is an exact copy of the original organism copy of the original organism

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

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

Sexual reproductionSexual reproduction Formation of new individual by a combination of two Formation of new individual by a combination of two haploidhaploid sex cells (gametes). sex cells (gametes). Fertilization- combination of genetic information from two Fertilization- combination of genetic information from two separate cells that have one half the original genetic separate cells that have one half the original genetic information information Gametes for fertilization usually come from separate Gametes for fertilization usually come from separate parents parents

Female- produces an egg Female- produces an egg Male produces spermMale produces sperm

Both gametes are haploid, with a single set of Both gametes are haploid, with a single set of chromosomes chromosomes The new individual is called a zygote, with two sets of The new individual is called a zygote, with two sets of chromosomes. chromosomes. Meiosis is a process to convert a diploid cell to a haploid Meiosis is a process to convert a diploid cell to a haploid gamete, and cause a change in the genetic information to gamete, and cause a change in the genetic information to increase diversity/increase diversity/variationvariation in the offspring. in the offspring.

ChromosomesChromosomesIn humans, each somatic cell has 46 chromosomes.In humans, each somatic cell has 46 chromosomes.

Made up of 23 pairs of homologous chromosomesMade up of 23 pairs of homologous chromosomes chromosomes that make up a pair – that have the chromosomes that make up a pair – that have the same length, centromere position and pattern of same length, centromere position and pattern of coded genes.coded genes. The two chromosomes of each pair carry genes controlling The two chromosomes of each pair carry genes controlling

the same inherited characters. (i.e. if a gene for eye colour the same inherited characters. (i.e. if a gene for eye colour is situated at a particular spot (locus) on a certain is situated at a particular spot (locus) on a certain chromosome, then the homologue of that chromosome will chromosome, then the homologue of that chromosome will also have a gene specifying eye colour at the equivalent also have a gene specifying eye colour at the equivalent locus.locus.

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

Chromosomes

How Do Scientists Read Chromosomes?

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

Make a Karyotype

Ploidy: Number of sets of Ploidy: Number of sets of chromosomes in a cellchromosomes in a cell

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

Diploid (2n)-- two sets chromosomes. In humans Diploid (2n)-- two sets chromosomes. In humans 2n=2x23=462n=2x23=46

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

Eggs and sperm are haploid (n) Eggs and sperm are haploid (n) Each gamete has a single set of the 22 autosomes plus Each gamete has a single set of the 22 autosomes plus

a single sex chromosome, either X or Y. These are the a single sex chromosome, either X or Y. These are the only cells in the body not produced by mitosis.only cells in the body not produced by mitosis.

Chromosomes in a Diploid Cell Chromosomes in a Diploid Cell

Summary of chromosome Summary of chromosome characteristics characteristics Diploid set for humans; 2n = 46 (2 sets of 23)Diploid set for humans; 2n = 46 (2 sets of 23) Autosomes; homologous chromosomes, one Autosomes; homologous chromosomes, one

from each parent (humans = 22 sets of 2) from each parent (humans = 22 sets of 2) Sex chromosomes (humans have 1 set of 2) Sex chromosomes (humans have 1 set of 2)

Female-sex chromosomes are homologous (XX) Female-sex chromosomes are homologous (XX)

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

What is What is MeiosisMeiosis?? Meiosis involves two Meiosis involves two successive nuclear successive nuclear divisions that produce four divisions that produce four haploid cells. The first haploid cells. The first division (division (meiosis Imeiosis I) is the ) is the reduction division; the reduction division; the second division (second division (meiosis meiosis IIII) separates the ) separates the chromatids. chromatids. Occurs in the ovaries or Occurs in the ovaries or testes.testes.Fertilization restores the Fertilization restores the diploid condition.diploid condition.

FERTILIZATION

2n

MEIOSIS

2n

nn

ZYGOTE

Diploid multicellular organism

Gametes

2n 2n

n

Figure: Sexual Life cycle

Advantages of MeiosisAdvantages of Meiosis

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

VSVS

MeiosisMeiosis increases genetic variation in the increases genetic variation in the population. – exchange of information can population. – exchange of information can occur during occur during meiosis Imeiosis I while mutations while mutations can also occurcan also occur

Unique Features of Meiosis

Comparison of Meiosis and Mitosis

InterphaseInterphase

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

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

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

**Remember: All somatic cells are diploid in number **Remember: All somatic cells are diploid in number (2n), therefore for each chromatid there also exists its (2n), therefore for each chromatid there also exists its

homologue, which also replicates during interphasehomologue, which also replicates during interphase..

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

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

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

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

Meanwhile, centrosomes move away from each other and Meanwhile, centrosomes move away from each other and spindle fibres form between them.spindle fibres form between them.The nuclear membrane disappears.The nuclear membrane disappears. Animations

Metaphase IMetaphase I

At metaphase, each chromosome At metaphase, each chromosome has reached its maximum density. has reached its maximum density. The homologous pairs and their The homologous pairs and their sister chromatids also prepare for sister chromatids also prepare for separation. separation. They interact with spindle fibers They interact with spindle fibers which form from either side of the which form from either side of the nuclear envelope of the cell. nuclear envelope of the cell. There is a centriole at opposite There is a centriole at opposite ends of the cell, which is referred ends of the cell, which is referred to as to as polespoles. . During metaphase, the During metaphase, the chromosomes are lined by the chromosomes are lined by the spindle fibers at what is known as spindle fibers at what is known as the metaphase plate. the metaphase plate.

Anaphase IAnaphase I Spindle fibres pull apart Spindle fibres pull apart the tetrad, separating the tetrad, separating each homologous each homologous chromosome. chromosome. Sister Sister chromatids still remain chromatids still remain attached to each other attached to each other and move as a single and move as a single unit toward the same unit toward the same polepole..It is by random chance It is by random chance that a certain that a certain chromosome is pulled to a chromosome is pulled to a certain pole. certain pole.

Telophase ITelophase I Telophase I varies from species to Telophase I varies from species to species. species. Sometimes Telophase I is skipped and Sometimes Telophase I is skipped and meiosis starts its second division meiosis starts its second division immediately. immediately. In general, however, two nuclear In general, however, two nuclear envelopes begin to surround the envelopes begin to surround the separate chromosomes and separate chromosomes and cytokinesis (splitting of the cytoplasm cytokinesis (splitting of the cytoplasm into two separate entities) will into two separate entities) will sometimes occur. sometimes occur. Each pole now has a haploid Each pole now has a haploid chromosome set, but each chromosome set, but each chromosome still has two sister chromosome still has two sister chromatids.chromatids.Then a phase called interkinesis will Then a phase called interkinesis will follow, which essentially is a resting follow, which essentially is a resting period from Telophase I to Prophase II. period from Telophase I to Prophase II. This differs from mitosis because DNA This differs from mitosis because DNA replication does not occur again. replication does not occur again.

Prophase IIProphase II

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

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

Metaphase IIMetaphase II

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

Anaphase IIAnaphase II The centromeres of sister chromatids The centromeres of sister chromatids finally separate, and the sister finally separate, and the sister chromatids of each pair, now individual chromatids of each pair, now individual chromosomes, move toward opposite chromosomes, move toward opposite poles of the cell. poles of the cell. Each sister chromatid ends up on one Each sister chromatid ends up on one side of the cell.side of the cell.

Telophase Telophase IIII

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

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

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

Animations

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

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

Stages of Meiosis

MeiosisCdck- cell division activity

Genetic VariationGenetic Variation

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

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

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

Independent Assortment and Gamete Diversity

Random Orientation of Chromosomes During MeiosisIndependent Assortment of

Alleles

Errors in MeiosisErrors in MeiosisMutations can be inherited if an error occurs in the formation Mutations can be inherited if an error occurs in the formation of one gamete and that gamete fertilizes another to form a of one gamete and that gamete fertilizes another to form a zygote zygote all the cells in the zygote will carry the mutation. all the cells in the zygote will carry the mutation.NondisjunctionNondisjunction: members of a pair of homologous : members of a pair of homologous chromosomes do not move apart properly during meiosis I, chromosomes do not move apart properly during meiosis I, or in which sister chromatids fail to separate during meiosis or in which sister chromatids fail to separate during meiosis II.II. One gamete receives 2 of the same type of chromosome One gamete receives 2 of the same type of chromosome

and another gamete receives no copyand another gamete receives no copy AnueploidyAnueploidy = abnormal chromosome number = abnormal chromosome number TrisomyTrisomy = if the chromosome is present in triplicate in = if the chromosome is present in triplicate in

the fertilized egg (2n+1 chromosomes) e.g. double in egg the fertilized egg (2n+1 chromosomes) e.g. double in egg cell, normal in sperm cell; e.g.:Trisomy 21 = Down cell, normal in sperm cell; e.g.:Trisomy 21 = Down SyndromeSyndrome

MonosomicMonosomic = if a chromosome is missing (2n-1) = if a chromosome is missing (2n-1) PolyploidyPolyploidy = when organisms have more than 2 = when organisms have more than 2

complete chromosome sets (triploidy – 3n, tetraploidy – complete chromosome sets (triploidy – 3n, tetraploidy – 4n) occurs if the cell fails to divide after replicating its 4n) occurs if the cell fails to divide after replicating its chromosomes OR nondisjunction of all its chromosomes.chromosomes OR nondisjunction of all its chromosomes.

Down Syndrome Down Syndrome KaryotypeKaryotype

3 of chromosome 21

Using Karyotypes to Predict Genetic Disorders

Other ErrorsOther ErrorsSyndromeSyndrome DefectDefect ProblemsProblems

PetauPetau Extra Extra chromosome 13 chromosome 13

Causes numerous physical and mental Causes numerous physical and mental abnormalities, owing mostly to heart abnormalities, owing mostly to heart defectsdefects

Lifespan is measured in daysLifespan is measured in days

EdwardEdward Trisomy 18 extra Trisomy 18 extra chrom 18 chrom 18

Usually naturally aborted or stillbornUsually naturally aborted or stillborn

TurnerTurner Females are Females are missing X missing X chromosome X chromosome X

The most common characteristics of The most common characteristics of Turner syndrome include short stature and Turner syndrome include short stature and lack of ovarian development. A number of lack of ovarian development. A number of other physical features, such as webbed other physical features, such as webbed neck, arms that turn out slightly at the neck, arms that turn out slightly at the elbow, and a low hairline in the back of the elbow, and a low hairline in the back of the head are sometimes seen in Turner head are sometimes seen in Turner syndrome patients. Individuals with Turner syndrome patients. Individuals with Turner syndrome are also prone to syndrome are also prone to cardiovascular problems, kidney and cardiovascular problems, kidney and thyroid problems, skeletal disorders such thyroid problems, skeletal disorders such as scoliosis (curvature of the spine) or as scoliosis (curvature of the spine) or dislocated hips, and hearing and ear dislocated hips, and hearing and ear

disturbances.disturbances.

More errors:More errors:SyndromeSyndrome DefectDefect ProblemProblem

Triplo-XTriplo-X Males have Males have an extra X an extra X chromosomchromosome: XXXe: XXX

The symptoms of this syndrome vary widely.  The The symptoms of this syndrome vary widely.  The one consistent feature is tall stature.  one consistent feature is tall stature.  Some females Some females exhibit no or very few symptomsexhibit no or very few symptoms, while others have , while others have more severe features of developmental delay and/or more severe features of developmental delay and/or

behavioral abnormalities.behavioral abnormalities.

Klinefelter Klinefelter Males have Males have an extra X an extra X chromosomchromosome: XXYe: XXY

XXY males usually have difficulty with expressive XXY males usually have difficulty with expressive language the ability to put thoughts, ideas, and language the ability to put thoughts, ideas, and emotions into words. In contrast, their faculty for emotions into words. In contrast, their faculty for receptive language-understanding what is said-is receptive language-understanding what is said-is

close to normal.close to normal.

JacobsJacobs Males have Males have an extra Y: an extra Y:

XYYXYY

XYY males are fertile, they have testes of normal XYY males are fertile, they have testes of normal size, and they have a normal sexual libido and size, and they have a normal sexual libido and potency. In spite of a somewhat decreased sperm potency. In spite of a somewhat decreased sperm quality with many so-called immature sperm cells, quality with many so-called immature sperm cells, the fertility seems to be normal. In the same way as the fertility seems to be normal. In the same way as for triple-X women, who as a rule do not get children for triple-X women, who as a rule do not get children with an extra X chromosome, males with XYY most with an extra X chromosome, males with XYY most probably only very rarely get sons with two Y probably only very rarely get sons with two Y

chromosomes.chromosomes.

Alterations of Chromosome Alterations of Chromosome StructureStructure

Breakage of a chromosome:Breakage of a chromosome: DeletionDeletion: when a chromosomal fragment lacking : when a chromosomal fragment lacking

a centromere is lost during cell division a centromere is lost during cell division missing missing genes; usually lethal b/c missing essential genesgenes; 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.

DuplicationDuplication: broken fragment may rejoin but : broken fragment may rejoin but to the homologous chromosome insteadto the homologous chromosome instead

InversionInversion: broken : broken fragment may rejoin the fragment may rejoin the correct chromosome, but correct chromosome, but in the reverse orientationin the reverse orientation

TranslocationTranslocation: broken : broken fragment joins a fragment joins a nonhomologous nonhomologous chromosomechromosome

Mistakes in Meiosis

Animations