THE BIRDS and BEES LESSON

Everything you didn’t already know about sexual reproduction

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)

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.

Egg Sperm Zygote

from mom from dad child

toomuch!

46 chromosomes + 46 chromosomes = 92 Chromosomes

Why Meiosis is Needed

Meiosis reducesgenetic content by half

23 chromosomes + 23 chromosomes = 46 Chromosomes

Egg Sperm 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)

http://sps.k12.ar.us/massengale/genetics%20tutorial.htm

NOT IDENTICAL

AB

C

D

E

F

AB

C

D

E

F

abc

de

f

a

bc

de

f

What is a Homologous Chromosome?

Chromosomes are made of sister chromatids.Same genes with the identical alleles (versions)

MeiosisDuring 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)

1st 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 1st 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”

2nd 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 CellsResult: 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 Spindlefibers

Nuclearenvelope

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 alignalong the equator of the cell.

Anaphase I

Homologues/tetrads separate andmove to opposite poles.

Sister chromatids remain Attached at their centromeres.

Telophase I

Nuclear envelopes reassemble.Spindle disappears.Chromosomes decondenseCytokinesis 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 CondenseNucleus disappearsSpindle apparatus forms

Haploid Daughter cells

Metaphase II

Chromosomes line-upalong equator of cell.Two sister chromatids attached at the centromeres

Anaphase II

Sister chromatids separateand 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

Mitosis Meiosis

Number of divisions 12

Number of daughter cells

2 4

Genetically identical?

Yes No

Chromosome # Same as parent Half of parent

Where Somatic cells Germ cells

When Throughout life At sexual maturity

Role Growth and repair Sexual reproduction

____________________________:Change in the ______________ or ____________ of chromosomes

CHROMOSOMAL MUTATIONS

structurenumber

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

http://www.tokyo-med.ac.jp/genet/anm/domov.gif

http://web.udl.es/usuaris/e4650869/docencia/gen_etica/meioferti2.html

Nondisjunction

NondisjunctionSince 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 chromosomeexcept one pair

= Monosomy (2n-1)How many chromosomes?

45

Body cells have 2 of each chromosomeexcept 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: http://members.tripod.com/~yenial/chromosome.html

MISSING/EXTRAGENDER

DELETIONS/INSERTIONSsingle gene

SEX DETERMINATIONXX =

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 y44

http://www.angelbabygifts.com/

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 = girlIf dad give y with mom’s X = boy

X X

X

y

X X X X

X y 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

http://medgen.genetics.utah.edu/photographs/diseases/high/611.gif

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