Meiosis How to divide the chromosome number in half

Sexual Reproduction Sexual Reproduction Involves Involves Meiosis Meiosis Gamete production Gamete production Fertilization Fertilization Produces genetic variation among offspring Produces genetic variation among offspring

Sexual Reproduction Shuffles Alleles Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits This variation in traits is the basis for evolutionary change This variation in traits is the basis for evolutionary change

Variation Sexual reproduction results in greater variation among offspring than does asexual reproduction. Sexual reproduction results in greater variation among offspring than does asexual reproduction. Two parents give rise to offspring that have unique combinations of genes inherited from the parents. Two parents give rise to offspring that have unique combinations of genes inherited from the parents. Offspring of sexual reproduction vary genetically from their siblings and from both parents. Offspring of sexual reproduction vary genetically from their siblings and from both parents.

Chromosome Number Sum total of chromosomes in a cell Sum total of chromosomes in a cell Germ cells are diploid (2n) Germ cells are diploid (2n) Gametes are haploid (n) Gametes are haploid (n) Meiosis halves chromosome number Meiosis halves chromosome number

Chromosome Number Sum total of chromosomes in a cell Sum total of chromosomes in a cell Germ cells are diploid (2n) Germ cells are diploid (2n) Gametes are haploid (n) Gametes are haploid (n) Meiosis halves chromosome number Meiosis halves chromosome number

Human Chromosome Number Diploid chromosome number (n) = 46 Diploid chromosome number (n) = 46 Two sets of 23 chromosomes each Two sets of 23 chromosomes each One set from father One set from father One set from mother One set from mother Mitosis produces cells with 46 chromosomes--two of each type Mitosis produces cells with 46 chromosomes--two of each type

In humans, each somatic cell has 46 chromosomes. In humans, each somatic cell has 46 chromosomes. Each chromosome can be distinguished by its size, position of the centromere, and by pattern of staining with certain dyes. Each chromosome can be distinguished by its size, position of the centromere, and by pattern of staining with certain dyes. A karyotype display of the 46 chromosomes shows 23 pairs of chromosomes, each pair with the same length, centromere position, and staining pattern. A karyotype display of the 46 chromosomes shows 23 pairs of chromosomes, each pair with the same length, centromere position, and staining pattern. These homologous chromosome pairs carry genes that control the same inherited characters. These homologous chromosome pairs carry genes that control the same inherited characters. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Karyotypes are often prepared with lymphocytes. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 13.3

Gamete Formation Gamete Formation Gametes are sex cells (sperm, eggs) Gametes are sex cells (sperm, eggs) Arise from germ cells Arise from germ cells testes ovaries anther ovary

Meiosis: Two Divisions Two consecutive nuclear divisions Two consecutive nuclear divisions Meiosis I Meiosis I Meiosis II Meiosis II DNA is NOT duplicated between divisions DNA is NOT duplicated between divisions Four haploid nuclei are formed Four haploid nuclei are formed

Homologous Chromosomes Carry Different Alleles Cell has two of each chromosome Cell has two of each chromosome One chromosome in each pair from mother, other from father One chromosome in each pair from mother, other from father Paternal and maternal chromosomes carry different alleles Paternal and maternal chromosomes carry different alleles

Meiosis I Each homologue in the cell pairs with its partner, then the partners separate

Meiosis II The two sister chromatids of each duplicated chromosome are separated from each other The two sister chromatids of each duplicated chromosome are separated from each other one chromosome (duplicated) two chromosomes (unduplicated)

Stages of Meiosis Stages of Meiosis Meiosis I Prophase I Prophase I Metaphase I Metaphase I Anaphase I Anaphase I Telophase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II

Meiosis I - Stages Prophase IMetaphase IAnaphase ITelophase I

Prophase I Each duplicated, condensed chromosome pairs with its homologue Each duplicated, condensed chromosome pairs with its homologue Homologues swap segments Homologues swap segments Each chromosome becomes attached to microtubules of newly forming spindle Each chromosome becomes attached to microtubules of newly forming spindle

Metaphase I Chromosomes are pushed and pulled into the middle of cell Chromosomes are pushed and pulled into the middle of cell Sister chromatids of one homologue orient toward one pole, and those of other homologue toward opposite pole Sister chromatids of one homologue orient toward one pole, and those of other homologue toward opposite pole The spindle is now fully formed The spindle is now fully formed

Anaphase I Homologous chromosomes segregate from each other Homologous chromosomes segregate from each other The sister chromatids of each chromosome remain attached The sister chromatids of each chromosome remain attached

Telophase I The chromosomes arrive at opposite poles The chromosomes arrive at opposite poles The cytoplasm divides The cytoplasm divides There are now two haploid cells There are now two haploid cells This completes Meiosis I This completes Meiosis I

Meiosis II - Stages Prophase IIMetaphase IIAnaphase IITelophase II

Prophase II Microtubules attach to the kinetochores of the duplicated chromosomes Microtubules attach to the kinetochores of the duplicated chromosomes Motor proteins drive the movement of chromosomes toward the spindle’s equator Motor proteins drive the movement of chromosomes toward the spindle’s equator

Metaphase II All of the duplicated chromosomes are lined up at the spindle equator, midway between the poles All of the duplicated chromosomes are lined up at the spindle equator, midway between the poles

Anaphase II Sister chromatids separate to become independent chromosomes Sister chromatids separate to become independent chromosomes Motor proteins interact with microtubules to move the separated chromosomes to opposite poles Motor proteins interact with microtubules to move the separated chromosomes to opposite poles

Telophase II The chromosomes arrive at opposite ends of the cell The chromosomes arrive at opposite ends of the cell A nuclear envelope forms around each set of chromosomes A nuclear envelope forms around each set of chromosomes The cytoplasm divides The cytoplasm divides There are now four haploid cells There are now four haploid cells

Crossing Over Each chromosome becomes zippered to its homologue All four chromatids are closely aligned Non-sister chromosomes exchange segments

Effect of Crossing Over After crossing over, each chromosome contains both maternal and parental segments After crossing over, each chromosome contains both maternal and parental segments Creates new allele combinations in offspring Creates new allele combinations in offspring

Random Alignment During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes Initial contacts between microtubules and chromosomes are random Initial contacts between microtubules and chromosomes are random

Random Alignment Either the maternal or paternal member of a homologous pair can end up at either pole Either the maternal or paternal member of a homologous pair can end up at either pole The chromosomes in a gamete are a mix of chromosomes from the two parents The chromosomes in a gamete are a mix of chromosomes from the two parents

Possible Chromosome Combinations As a result of random alignment, the number of possible combinations of chromosomes in a gamete is: 2 n (n is number of chromosome types)

Possible Chromosome Combinations or 123

Spermatogenesis Growth Mitosis I, Cytoplasmic division Meiosis II, Cytoplasmic division spermatids (haploid) secondary spermatocytes (haploid) primary spermatocyte (diploid) spermato- gonium (diploid male reproductive cell)

Oogenesis Growth Mitosis I, Cytoplasmic division Meiosis II, Cytoplasmic division ovum (haploid) primary oocyte (diploid) oogonium (diploid reproductive cell) secondary oocyte haploid) first polar body haploid) three polar bodies haploid)

Fertilization Male and female gametes unite and nuclei fuse Male and female gametes unite and nuclei fuse Fusion of two haploid nuclei produces diploid nucleus in the zygote Fusion of two haploid nuclei produces diploid nucleus in the zygote Which two gametes unite is random Which two gametes unite is random Adds to variation among offspring Adds to variation among offspring

Factors Contributing to Variation Among Offspring Crossing over during prophase I Crossing over during prophase I Random alignment of chromosomes at metaphase I Random alignment of chromosomes at metaphase I Random combination of gametes at fertilization Random combination of gametes at fertilization

Mitosis Functions Functions Asexual reproduction Asexual reproduction Growth, repair Growth, repair Occurs in somatic cells Occurs in somatic cells Produces clones Produces clones Mitosis & Meiosis Compared Mitosis & Meiosis Compared Meiosis Function Sexual reproduction Occurs in germ cells Produces variable offspring

Prophase vs. Prophase I Prophase (Mitosis) Prophase (Mitosis) Homologous pairs do not interact with each other Homologous pairs do not interact with each other Prophase I (Meiosis) Prophase I (Meiosis) Homologous pairs become zippered together and crossing over occurs Homologous pairs become zippered together and crossing over occurs

Anaphase, Anaphase I, and Anaphase II Anaphase, Anaphase I, and Anaphase II Anaphase I (Meiosis) Anaphase I (Meiosis) Homologous chromosomes are separated from each other Homologous chromosomes are separated from each other Anaphase/Anaphase II (Mitosis/Meiosis) Anaphase/Anaphase II (Mitosis/Meiosis) Sister chromatids of a chromosome are separated from each other Sister chromatids of a chromosome are separated from each other

Results of Mitosis and Meiosis Mitosis Mitosis Two diploid cells produced Two diploid cells produced Each identical to parent Each identical to parent Meiosis Meiosis Four haploid cells produced Four haploid cells produced Differ from parent and one another Differ from parent and one another