THE BIRDS and BEES LESSON Everything you didn’t already know about sexual reproduction

Overview of Meiosis Overview of Meiosis Meiosis-form of cell division where there are two successive (back to back) rounds of cell division. In meiosis a…… Diploid (parent) cell with (2n) or “double set” of chromosomes produces 4 haploid cells with one (n) or “single set” of chromosomes. Diploid # in humans is 46 double (2n), the haploid # is 23 (n) The chromosome # is reduced by ½ in two divisions: After 1 division - 23 double stranded chromosomes (n) (interphase skipped) After 2nd division - 23 single stranded chromatids -Meiosis occurs in our germative cells (cells that produce our gametes ) Females- Oocyte  eggs (ovum) Males- Spermatocyte  sperm

Body cells have 2 of each chromosome = diploid (2n)diploid (2n) How many total chromosomes? 46

Sperm/egg cells have 1 of each chromosome = haploid (1n) How many chromosomes? 23

Why do we need meiosis? -Process keeps the chromosomal number in organisms from doubling from one generation to the next during sexual reproduction. EggSperm Zygote from momfrom dadchild too much! 46 chromosomes + 46 chromosomes = 92 Chromosomes

Why Meiosis is Needed Meiosis reduces genetic content by half 23 chromosomes + 23 chromosomes = 46 Chromosomes EggSperm Zygote - to bring two haploid gametes together to form a diploid zygote. -n (mom) + n (dad) = 2n (offspring)

HOMOLOGOUS CHROMOSOMES The members of each pair are called homologous because they are similar in genetic content Each member of each pair came from a parent during sexual reproduction One chromosome is maternal (from the mother) One is paternal (from the father) Homologous chromosomes are equivalent and contain the same kinds of genes in the same order. They may have different versions of genes or “alleles” because they come from different parents Overview of Meiosis continued..

SAME SIZE SAME SHAPE CARRY GENES for the SAME TRAITS BUT ______________! Don’t have to have the same versions(alleles) NOT IDENTICAL A B C D E F A B C D E F a b c d e f a b c d e f What is a Homologous Chromosome? Chromosomes are made of sister chromatids. Same genes with the identical alleles (versions)

Meiosis During Meiosis, three important things occur that did not occur in mitosis: Chromosomes pair up prior to division forming Homologous pairs of chromosomes Homologous chromosomes exchange genetic information in an event called “Crossing Over” The newly scrambled chromosomes separate and go into different daughter cells so that each contains only one chromatid of each pair of homologous chromosomes The result is 4 cells each having unique DNA with gene combinations

Metaphase of Mitosis Metaphase of Meiosis I Homologous chromosomes “pair-up” prior to division forming a (grouping 4 chromatids) known as a TETRAD (homologous pairs of chromosomes) 1 st Major Differences between Mitosis in Meiosis Chromosomes line up individually along the equatorial plane

What happens to the pairs? Homologous chromosomes separate in meiosis I and therefore different alleles also separate. During Meiosis II individual chromatids separate In meiosis the homologous pairs of chromosomes will separate in anaphase of the 1 st division (Meiosis I), and in the second division (Meiosis II) individual chromatids will separate Maternal Chromosome Paternal Chromosome Single Paternal Chromatid

Prophase I of Meiosis Dyad pair or “tetrad” 2 nd Major Differences between Mitosis in Meiosis During Meiosis I homologous chromosomes “pair up” forming a Tetrad (grouping of 4 chromatids) Homologous chromosomes exchange genetic information in an event called “Crossing Over”

CROSSING OVER (recombination) The resulting Chromatids have DNA that is unique or “recombinant” a b c d e f A B C D E F A B C D E F a b c d e f Occurs in prophase of meiosis I Generates diversity by reordering the alleles of genes within homologous chromosomes

CROSSING OVER (recombination) Occurs in prophase of meiosis I Generates diversity by reordering the alleles of genes within homologous chromosomes Letters denote genes Case denotes alleles (versions) Creates chromosomes with new combinations of alleles for genes A to F. A B C D E F a b c d e f c d e f A B a b C D E F

CROSSING OVER (recombination) Occurs in prophase of meiosis I Generates diversity by reordering the alleles of genes within homologous chromosomes Letters denote genes Case denotes alleles A B C D E F a b c d e f c d e f A B a b C D E F The resulting Chromatids have DNA that is unique or “recombinant”

What is different between Meiosis and Mitosis? TWO divisions resulting in Four Cells Result: Each new daughter cell contains only One chromatid (DNA) of each of the original homologous pairs. Diploid germ cell Meiosis I Meiosis II Haploid gametes PMATC Single chromosome chromatid PMATC Homologous pairs

Meiosis I : the reduction division Prophase I (early) (diploid) Prophase I (late) (diploid) Metaphase I (diploid) Anaphase I (diploid) Telophase I (diploid) Nucleus Spindle fibers Nuclear envelope Tetrad

Prophase I Early prophase Chromosomes condense. Spindle forms. Nuclear envelope/membrane fragments and disappears. Late prophase Homologous chromosomes pair Tetrads form with four chromatids. Crossing over occurs- an event unique only to meiosis.

Metaphase I Homologous pairs/Tetrads align along the equator of the cell.

Anaphase I Homologues/tetrads separate and move to opposite poles. Sister chromatids remain Attached at their centromeres.

Telophase I Nuclear envelopes reassemble. Spindle disappears. Chromosomes decondense Cytokinesis divides cell into two daughter cells.

Meiosis II : the equational division Prophase II *(haploid) Metaphase II (haploid) Anaphase II (haploid) Telophase II (haploid) Four nonidentical haploid daughter cells

Prophase II Chromosomes Condense Nucleus disappears Spindle apparatus forms Haploid Daughter cells

Metaphase II Chromosomes line-up along equator of cell. Two sister chromatids attached at the centromeres

Anaphase II Sister chromatids separate and move to opposite poles.

Telophase II Nucleus reorganizes and nuclear envelope/membrane reforms. Chromatids which are now called “chromosomes” decondense. Spindle disappears. Cytokinesis divides each cell into two forming four haploid cells.

Results of meiosis Gametes Four haploid cells One copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome

MitosisMeiosis Number of divisions1 2 Number of daughter cells 24 Genetically identical? YesNo Chromosome #Same as parentHalf of parent WhereSomatic cellsGerm cells WhenThroughout lifeAt sexual maturity RoleGrowth and repairSexual reproduction

____________________________: Change in the ______________ or ____________ of chromosomes CHROMOSOMAL MUTATIONS structure number

Deletion Duplication Inversion Translocation Figure 12–20 Chromosomal Mutations Section 12-4

Homologous chromosomes ________________ during MEIOSIS = _________________________ One cell gets 2 copies of the chromosome the other cell gets none. NONDISJUNCTION fail to separate

Mistakes in Meiosis

Normal Meiosis Nondisjunction

Nondisjunction

Since it happens to a sperm or egg, the new baby can end up with _____________ of a chromosome = __________________ OR only ___________ of a chromosome = ___________________ TRISOMY MONOSOMY 3 copies one copy

Body cells have 2 of each chromosome except one pair = Monosomy (2n-1) How many chromosomes? 45

Body cells have 2 of each chromosome except one pair has 3 copies = Trisomy (2n+1) How many chromosomes? 47

Human Abnormalities caused by Non-Disjunction __________________ __________________ __________________ Down syndrome Klinefelter syndrome Turner syndrome

A __________ is a picture of an organism’s chromosomes KARYOTYPE

Karyotype ( need cells from baby) Can tell __________________ chromosomes __________ Some _____________________ Can’t see _______________ mutations Image from: MISSING/EXTRA GENDER DELETIONS/INSERTIONS single gene

SEX DETERMINATION XX = Xy = female male

Chromosomes that determine the sex of an organism = _________________ All other chromosomes = _________________ Sex chromosomes autosomes Humans have two sex chromosomes and _____ autosomes X y 44

Who decides? Mom can give X Dad can give X or y SO ____ determines sex of the baby. If dad gives X with mom’s X = girl If dad give y with mom’s X = boy X X X y X X y Dad

Nondisjunction of Autosomal chromosomes Normal female Normal male

Down syndrome (= ____________) TRISOMY 21

Nondisjunction of Autosomal Chromosomes Male with trisomy 21 (Downs syndrome)

Down syndrome (Trisomy 21) 1 in 800 births Similar facial features Slanted eyes Protruding tongue

Down syndrome (Trisomy 21) Simian line on palm

Down syndrome (Trisomy 21) Most common chromosomal abnormality 50% have heart defects that need surgery to repair Mild to severe mental retardation Increases susceptibility to many diseases Risk of having a child with Down syndrome increases with age of mom

Sex Chromosomal Nondisjunction Failure of chromosomes to separate properly during meiosis Can lead to cells with extra genetic material (chromosomes) Can lead to cells without any chromosomes (bar bodies)

If having extra chromosomes causes genetic problems, how come having two X chromosomes in females and one X in males is not a problem?

Turner syndrome

Turner syndrome ____ 1 in 5000 births Females have only one X chromosome Small size Slightly decreased intelligence 35% have heart abnormalities Hearing loss common Broad chest Reproductive organs don’t develop at puberty Can’t have children XO

Klinefelter syndrome XXy

Klinefelter syndrome 1 in 1000 births Males have extra X chromosomes (Can be XXy, XXXy, or XXXXy) Average to slight decrease in intelligence Small testes/can’t have children Usually not discovered until puberty when don’t mature like peers