BIO 2/ REVIEW 4 TEST 1

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Meiosis

is a special type of cell division that produces haploid cells and compensates for the doubling of chromosome number that occurs at fertilization.

23 chromosomes.

Meiosis in humans produces sperm cells and ova, which contain ?

(2n = 46)

When fertilization occurs, the diploid condition ________ is restored in the zygote.

4 daughter cells

in meiosis the cell division produce how many daughter cells

2 daughter cells

in mitosis the cells division produce how many daughter cells

meiosis

- The resulting daughter cells have half the number of chromosomes as the parent cell.

interphase 1

what preceded meiosis

interphase 1

Chromosomes replicate.

interphase 1

- Each duplicated chromosome consists of two identical sister chromatids attached at their centromere.

interphase 1

-Centriole pairs in animal cells also replicate into two pairs.

interphase 1

-Chromosomes replicate.
-Each duplicated chromosome consists of two identical sister chromatids attached at their centromere.
-Centriole pairs in animal cells also replicate into two pairs.

Meiosis I

This cell division segregates the two chromosomes of each homologous pair and reduces the chromosome number by one-half. It includes the following four phases:

Prophase I

-Chromosomes condense

Prophase I

-Synapsis occurs. During this process, homologous chromosomes come together as pairs. Since each chromosome has two chromatids, each homologous pair in a synapsis appears as a complex of four chromatids or a tetrad.

Prophase I

-In each tetrad, sister chromatids of the same chromosome are attached at their centromeres. Nonsister chromatids are linked by X-shaped chiasmata, sites where homologous strand exchange or crossing over occurs.

prophase 1

-Chromosomes condense.
-Synapsis occurs. During this process, homologous chromosomes come together as pairs. Since each chromosome has two chromatids, each homologous pair in a synapsis appears as a complex of four chromatids or a tetrad.
-In each tetrad, sister chromatids of the same chromosome are attached at their centromeres. Nonsister chromatids are linked by X-shaped chiasmata, sites where homologous strand exchange or crossing over occurs.

Prophase I

-Centriole pairs move apart and spindle microtubules form between them.

Prophase I

-Nuclear envelope and nucleoli disperse

Prophase I

-Chromosomes begin moving to the metaphase plate.

Prophase I

- Prophase I typically occupies more than 90% of the time required for meiosis.

Prophase I

-Centriole pairs move apart and spindle microtubules form between them.
-Nuclear envelope and nucleoli disperse.
-Chromosomes begin moving to the metaphase plate.
-Prophase I typically occupies more than 90% of the time required for meiosis.

Metaphase I:

-Tetrads are aligned at the metaphase plate.

Anaphase I:

-Homologues separate and are moved to the opposite poles of the cell by the spindle apparatus.

Anaphase I:

-Sister chromatids remain attached at their centromeres and move as a unit towards the same pole, while the homologue movers towards the opposite pole.

Anaphase I

-Homologues separate and are moved to the opposite poles of the cell by the spindle apparatus.
-Sister chromatids remain attached at their centromeres and move as a unit towards the same pole, while the homologue movers towards the opposite pole.

Telophase I and cytokinesis

-The spindle apparatus continues to separate homologous chromosome pairs until the chromosomes reach the poles.

Telophase I and cytokinesis

-Each pole now has a haploid set of chromosomes that are each still composed of two sister chromatids attached at the centromere

Telophase I and cytokinesis

-Usually cytokinesis occurs simultaneously with telophase I, forming two haploid daughter cells. Cleavage furrows form in animal cells, and cell plates form in plant cells.

Telophase I and cytokinesis

-In some species, nuclear membranes and nucleoli reappear, and the cell enters a period of interkinesis before meiosis II. In other species, the daughter cells immediately prepare for meiosis II

Telophase I and cytokinesis

-Regardless of whether a cell enters interkinesis or not, no DNA replication occurs before Meiosis II.

Telophase I and cytokinesis

-The spindle apparatus continues to separate homologous chromosome pairs until the chromosomes reach the poles.
-Each pole now has a haploid set of chromosomes that are each still composed of two sister chromatids attached at the centromere.
-Usually cytokinesis occurs simultaneously with telophase I, forming two haploid daughter cells. Cleavage furrows form in animal cells, and cell plates form in plant cells.
-In some species, nuclear membranes and nucleoli reappear, and the cell enters a period of interkinesis before meiosis II. In other species, the daughter cells immediately prepare for meiosis II.
-Regardless of whether a cell enters interkinesis or not, no DNA replication occurs before Meiosis II.

Meiosis II

This second meiotic division separates sister chromatids of each chromosome.

Prophase II

-If the cell entered interkinesis, the nuclear envelope and nucleoli disperse.

Prophase II

-Spindle apparatus forms and chromosomes move towards the metaphase II plate.

Metaphase II

-Chromosomes (two daughter chromatids attached at the centromere) align singly on the metaphase plate.

Anaphase II

-Sister chromatids separate

Anaphase II

-Sister chromatids of each pair (now individual chromosomes) move toward opposite poles of the cell.

Telophase II and cytokinesis:

-Nuclei form at opposite poles of the cell.

Telophase II and cytokinesis:

-Cytokinesis occurs producing 4 haploid daughter cells.

Sexual life cycles

produce genetic variation among offspring

Meiosis and fertilization

are the primary sources of genetic variation in sexually reproducing organisms

Sexual reproduction

provides genetic variation by three methods:

1.Independent assortment
2.Crossing over during prophase I of meiosis
3.Random fusion of gametes during fertilization

Sexual reproduction 3 methods

At metaphase I,

each homologous pair of chromosomes aligns on the metaphase plate. Each pair consists of one maternal and one paternal chromosome.

Independent assortment of chromosomes

-The orientation of the homologous pair to the poles is random, so there is a fifty-fifty chance that a particular daughter cell produced by meiosis I will receive the maternal chromosome of a homologous pair, or the paternal chromosome.

Independent assortment of chromosomes

-Each homologous pair of chromosomes orients independently of the other pairs at metaphase I; thus the first meiotic division results in independent assortment of maternal and paternal chromosomes

Independent assortment of chromosomes

-A gamete produced by meiosis contains just one of all the possible combinations of maternal and paternal chromosomes

Independent assortment =

The random distribution of maternal and paternal homologues to the gametes. (In a more specific sense, assortment refers to the random distribution of genes located con the different chromosomes.)

Independent assortment

-Since each homologous pair assorts independently from all the others, the process produces 2n possible combinations of maternal and paternal chromosomes in the gametes, where n in the haploid number.

Independent assortment

-In humans, the possible combinations would be 223 or about eight million.

Independent assortment

-Thus, each human gamete contains one of eight million possible assortments of chromosomes inherited from that person's mother and father.

Independent assortment

-Genetic variation results from this reshuffling of chromosomes, because the maternal and paternal homologues will carry different genetic information at many of their corresponding loci.

Crossing Over

another mechanism that increases genetic variation is the process of crossing over, during which homologous chromosomes exchange genes.

Crossing over

= The exchange of genetic material between homologues, occurs during prophase I of meiosis I

Crossing Over

-Occurs when homologous portions of two nonsister chromatids trade places. During prophase I, X-shaped chiasmata becomes visible at places where this homologous strand exchange occurs.

Crossing Over

-Produce chromosomes that contain genes from both parents.

Crossing Over

-In humans, there is an average of two or three crossovers per chromosome pair.

Random Fertilization

In humans, an egg cell that is one of eight million different possibilities will be fertilized by a sperm cell that is also one of eight million possibilities. Thus, the resulting zygote can have one of 64 trillion possible diploid combinations.

1.Sexual reproduction

Independent assortment, crossing over, and random fusion.

2.Mutation

which is a rare structural change in a gene.

Incomplete dominance

Pattern of inheritance in which the dominant phenotype is not
fully expressed in the heterozygote, resulting in a phenotype intermediate between the
homozygous dominant and homozygous recessive

Incomplete dominance

-For example when red snapdragons (RR) are crossed with white snapdragons (rr), all F1 hybrids (Rr) have pink flowers.

are color blindness, Duchenne muscular dystrophy and hemophilia.

-Examples of sex-linked traits in humans are

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