Chapter 9 Sexual Reproduction

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meiosis,

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a type of nuclear division that is important in sexual reproduction

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Meiosis serves two major functions:

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reproducing the chromosome number and shuffling the chromosomes and genes to produce genetically different gametes, called sperm in males and eggs in females

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With random Fertilization of the egg

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by a single sperm even more shuffling occurs--at the end each child has its own unique genetic code

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Karyotype

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are chromosomes that are pictures they appear in pairs both male and females have 23 pairs

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22 pair of these are called

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autosomes they are the same in both males and females

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1 pair

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are called the sex chromosomes because they contain the genes that determine gender The larger gene of this pair is the X the smaller the Y

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Twenty-three pairs of chromosomes or 46 all together

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constitute the diploid(2n) number of chromosomes in humans

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Half this number is the

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hapliod (N) number

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The members of a chromosome pair are called

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homologous chromosomes or homologues because they have the same size, shape, and location of the cetromere

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Alternate versions of a gene for a particular trait are called

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alleles --alleles occur at the same location on each homologue

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The term life cycle is sexually reproducing organisms refers to

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all the reproductive events that occur from one generation to the next

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The human life cycle involves two types of nuclear division

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mitosis (regular cell division) and meiosis (sexual Cell division)

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During development and after birth

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mitosis is involved in the continued growth of the child and repair of tussiues at any time as a result of mitosis each somatic (body) cell had a diploid number of chromosomes

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During sexual reproduction,

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meiosis reduces the chromosome number from the diploid to the hasploid number in such a way that6 that gametes have one chromosome from each pair

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In males meiosis is part of

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spermatogenesis which occurs in the testes and produces sperm

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In females meiosis is part of

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oogenesis which occurs in the ovaries and produces eggs

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After sperm and egg joijn during fertilization

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the resulting cell is called a zygote it now had homologous pairs of chromosomes--and now under goes mitosis

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Meiosis results in four daughter cells because it consists of two divisions

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Meiosis I and Meiosis II before meiosis I begins each chromosome has duplicated and consists of two sis chromatids During Meiosis I the homologous chromosomes of each pair come together and line up side by side

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In an event called synapsis

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synapsis results in a tetrad an association of four choromatids

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the chromosomes of a titrad stay close together until they seperate

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when they separate no rules restrict which chromosome goes to which daughter nucleus--therefore all possible combinations of choromsomes may occur within the gametes

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No duplication of chromosomes is needed between

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meiosis I and Meiosis II

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Following meiosis I the daughter nuclei each have half the number of chromosomes

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but the chromosomes are still duplicated-- the chromosomes are called dyads because each on is composed of two sister chrimatids

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During Meiosis II the sister chromatids of each dyad

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separate and become daughter chromosomes the result is four daughter cells have the haploid number

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The non sister chromatids

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may exchange genetic material, an event called crossing over this happens in the tetrad--this leads to different alleles shuffling between sister chromatids

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The importance of Meiosis

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1 helps keep the chromosome number constant makeing hapoid daughter cells
2. it introduce genetic variations because 1 crossing over 2 every possible combination of chromosomes

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Meiosis I

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four phases: Prophase I
Metaphase I
Anaphase I
Telophase I

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Prophase I

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tetrads form and crossing over occurs as chromosomes condense the nuclear envelope fragments

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Metaphase I

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tetrads aligh at the spindle equator. either homologue can face either pole

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Anaphase I

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homologues separate and dyads move to poles

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Telephase I

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Daughter nuclei are hapoid having recieved one duplicated chromosome from each homologous pair

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No replication of DNA occurs between meiosis I and Meiosis II

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a period called interkinesis

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Phases of Meiosis II

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Prophase II
Metaphase II
Anaphase II
Telophase II

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Prophase II

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Chromosomes condense and the nuclear envelope fragments

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Metaphase II

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the dyads align at the spindle equator

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Anaphase II

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sister chromatids separate, becoming daughter chromosomes that move to the poles

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Telophase II

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four hapliod daughter cells are genetically different from each other and from the parent cell

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Meiosis I compared to Mitosis

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Prophase I pairing of homologous chromosomes crossing over
Prophase No pairing of chromosomes no crossing over

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Meiosis I compared to Mitosis

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Mataphase I Tetrads at spindle equator

Metaphase Dyads at spindle equator

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Meiosis I compared to Mitosis

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Anaphase I Homologues of each tetrad sperate, and dyads move to poles
Anaphase sister chromatids separate becoming daughter chromosome that move to the poles

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Meiosis I compared to Mitosis

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Telophase I Two haploid daughter cells not identical to parent cell
Telophase two diploid daughter cells identical to the parent cell

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Meiosis II compared with Mitosis

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Porphase II no pairing of choromosomes
Prophase no pairing of chromosomes

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Meiosis II compared with Mitosis

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Metaphase II haploid number of dyads at spindle equator
Metaphase Diploid number of dyads at spindle equator

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Meiosis II compared with Mitosis

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Anaphase II Sister chomatids separate becoming daughter chromosomes that move to poles
Anaphase Sister chromatids separate becoming daughter chromosomes that move to the poles

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Meiosis II compared with Mitosis

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Telophase II four haploid daughter cells, different from each other genetically and from the parent cell
Telophase Two diploid daughter cells genetically identical to the parent cell

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The normal number of chromosomes in a human cell is 46

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but once in a while humans are born with an abnormal number of chromosomes because they failed to separate correctly called nondisjuntion

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If an egg ends up with 24 chromosomes and it is fertilized with a normal sperm this is called

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trisomy there will be three chromosome of that type in the cell

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If an egg ends up with 22 chromosomes and is fertilized by a normal sperm this is called

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monosomy because one type of chromosome is present in a single copy

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Down Syndrome is trisomy 21

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the individual has 3 copies of chromosome 21

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Normal females have only one functioning X chromosome just like males

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the other is inactive mass and called a Barr Body

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Turner Syndrome

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(45 XO) is a female the O represents a missing chromosome females with this syndrome have broad chests and webbed necks

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Klinefelter syndrome

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(47, XXY) is a male. in this man the testes and the prostrate gland are under developed--generally the affected have large hands and feet