Meiosis Chapter 8 Part II

Agenda Textbook: Pages Meiosis and crossing over, Pages The structure of genetic material. Textbook: Pages Meiosis and crossing over, Pages The structure of genetic material. Powerpoint Powerpoint

Quiz dates 12/7 Germ cell vs Somatic cell, Meiosis, crossing over, Homologous chromosomes. 12/7 Germ cell vs Somatic cell, Meiosis, crossing over, Homologous chromosomes. 12/9 TBA 12/9 TBA

Octopus Sex Male reaches under his mantle with tentacle, removes packet of sperm, and insert it into female’s egg chamber Male reaches under his mantle with tentacle, removes packet of sperm, and insert it into female’s egg chamber Eggs are fertilized and give rise to new octopuses Eggs are fertilized and give rise to new octopuses

Limpet Sex Limpet Sex Larva can become adult of either sex Larva can become adult of either sex Depends on whether other limpets are present and what their sex is Depends on whether other limpets are present and what their sex is Adults can change sex in response to new arrivals Adults can change sex in response to new arrivals

Slipper snails Hermaphroditic Hermaphroditic Stacked on top of each other Stacked on top of each other Young are always male Young are always male Can stay male or change to female Can stay male or change to female Cannot go back to male Cannot go back to male

Sexual Reproduction Chromosomes are duplicated in germ cells Chromosomes are duplicated in germ cells Germ cells undergo meiosis and cytoplasmic division Germ cells undergo meiosis and cytoplasmic division Cellular descendents of germ cells become gametes Cellular descendents of germ cells become gametes Gametes meet at fertilization Gametes meet at fertilization

Meiosis

Asexual Reproduction Single parent produces offspring Single parent produces offspring All offspring are genetically identical to one another and to parent All offspring are genetically identical to one another and to parent

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

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

Alleles- Different forms of a gene

What is a gene? Chemical factors that determine traits Chemical factors that determine traits

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

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

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

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

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

92 46x2 23x4

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

Crossing-Over

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

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

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

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

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

Pre-Natal Tests - help to determine if an at risk baby is healthy

Sex chromosomes Human Karyotype

Sex Chromosomes

Down Syndrome (trisomy 21), 47,XY, : the total number of chromosomes (46 is normal). XY: the sex chromosomes (male). +21: designates the extra chromosome as a 21.

Klinefelter’s Syndrome 47, XXY

Turner’s Syndrome Only one complete X Only one complete X The second could also be incomplete The second could also be incomplete In almost all cases, unable to repro. Or menstruate In almost all cases, unable to repro. Or menstruate

Hermaphrodite XX/XY or XO XX/XY or XO Known cases Known cases

Gene Therapy - an absent or faulty gene is replaced by a working gene Has not been very successful