Ch9 Meiosis; Sex Cell Cycle

asexual reproduction

1 parent organism replicates its DNA and splits the contents into 2 genetically identical offspring. Ex. Bacteria, archaea, protists

Sexual reproduction

DNA of the offspring comes from 2 parents. An egg fuses with a sperm producing genetically diverse offspring

Diploid

2n, cells with 2 sets of chromosomes

Haploid

n, cells with 1 set of chromosomes

Humans have how many homologous sets of chromosomes

23

autosomes

numbered chromosomes, 1 to 22

Sex chromosomes 

23rd, X or Y for haploid, XX or XY for diploid 

germline cell

only cell capable of meiosis; we have 1

somatic cells

Body cells, are diploid

Karyotype

all chromosomes in your nucleus, stained under a microscope to be seen

Homologous chromosomes

same size and type and carry the same sets of genes, carry different alleles, DNA sequence is not identical

allele

alternative versions of the same gene

when members of a homologous pair are replicated

identical alleles are on sister chromatids

gametes are produced by

meiosis

gamete

haploid sex cells, ex sperm and egg

fertilization 

2 haploid sex cells fuse, 1 diploid zygote is formed

zygote

1st cell of a new organism 

tetrad

homologous pair

germ cells

specialized diploid cells that divide by meiosis to form haploid gametes; male is spermatogonia, female oocytes

sperm cells form in

testes

egg cells form in

ovaries

Spermatogenesis 

1 spermatogonium produces 4 sperms 

spermatogonia 

male germ cells 

Oogenesis

1 oogonium produces 1 egg and 3 polar bodies 

Spermatogenesis step 1

1 spermatogonium splits by mitosis into a spermatogonium and a primary spermatocyte

Spermatogenesis step 2

the primary spermatocyte splits by meiosis 1 into 2 haploid secondary spermatocytes

Spermatogenesis step 3

The secondary spermatocytes both split into 2 spermatids using meiosis 2.  

Spermatogenesis step 4 

The 4 spermatids mature into 4 sperm 

Oogenesis step 1 

a diploid oogonium splits by mitosis into an oogonium and a primary oocyte.

Oogenesis step 2

The primary oocyte splits by meiosis 1 into a haploid polar body and a haploid secondary oocyte. 

Oogenesis step 3

The polar body haploid splits by meiosis 2 into 2 polar bodies. The Haploid secondary oocyte splits by meiosis 2 into an Ovum and a polar body 

ovum

egg

meiosis halves the 

chromosome number

meiosis produces

4 genetically distinct gametes

in meiosis

DNA replicates once, the nucleus divides twice

Meiosis generates variability because of 

crossing over, independent assortment, random fertilization, mutations, and nondisjunction’s  

Prophase I

homologous chromosomes pair up and attach to the spindle, condensing. Crossing over shuffles genes. Results in 4 different sister chromatids

metaphase I

homologous chromosomes line up in 2 rows. Independent assortment occurs.

independent assortment

chromosome pairs align randomly, scrambling the combination of chromosomes for every gamete

anaphase I

homologous chromosomes separate haploid daughter cells

Telophase I

sister Chromatids remain together

Prophase II

chromosomes attach to a spindle

Metaphase II

chromosomes line up in a row single file

Chiasmata

site of crossing over

Anaphase II

sister chromatids separate into nonidentical haploid cells

Telophase II

4 new cells have 1 set of chromosomes

Monozygotic

an embryo split in 2, each embryo develops independently. 1 zygote develops into 2 individuals. Identical twins.

Dizygotic

2 sperm cells fertilize 2 separate egg cells. Fraternal twins.

Mitosis prophase

chromosomes condense

mitosis metaphase

chromosomes line up single file

Mitosis anaphase/telophase

centromeres dissolve, sister chromatids separate into identical daughter cells

requires 2 nuclear divisions

Meiosis

requires 1 nuclear division

mitosis

chromosomes synapse and cross over

meiosis 

centromeres survive anaphase I

meiosis

centromeres dissolve in anaphase

mitosis

halves chromosome number

meiosis 

preserves the chromosome number 

mitosis

produces 4 daughter nuclei

meiosis

produces 2 daughter nuclei

mitosis

produces daughter cells genetically different from parent and each other

meiosis

produces genetically identical daughter cells

mitosis

used only for sexual reproduction

meiosis

used for asexual reproduction and growth

mitosis 

nondisjunction

when chromosomes fail to separate properly; forms abnormal games leading to zygotes with too many or too little chromosomes

nondisjunction can occur in anaphase I

all 4 zygotes have too many or too little chromosomes

nondisjunction can occur in anaphase II

only 2 zygotes will be normal 

down syndrome 

3 copies of chromosome 21 instead of 2; caused by nondisjunction

nondisjunction of sex chromosomes

sex disorders; XXX, XXY, XYY, XO

trisomy

extra copy of a chromosome, causes genetic disorders

mutation in chromosome structure effects

the organism

Meiosis also produces

spores

fungi, some protists, and some plants produce haploid spores using meiosis

the spores undergo mitosis to form haploid gametophytes which produce sperm and eggs 

Step 1 of spore life

Mature Sporophyte produces spores using meiosis

Step 2 of spore life

the spores undergo mitosis becoming male and female gametophytes

Step 3 of spore life

the gametophyte undergoes mitosis becoming gametes

Step 4 of spore life

the gametes fertilize producing a zygote

Step 5 of spore life

The zyogtye undergoes mitosis becoming a sporophyte

interphase

DNA replicates, cell produces proteins needed for cell division 

early prophase comes after interphase 

chromosomes condense and become visible 2n=4

Late prophase

crossing over occurs, spindle forms, nuclear envelope breaks up

metaphase 1 is after

prophase

anaphase 1 is after 

metaphase 1

telophase and cytokinesis is after 

anaphase 

Sex cell cycle

interphase, Meiosis I, Interkinesis, Meiosis II

Interphase consists of

G0, G1, S, G2

Interkinesis consists of

G1, G2

Sex cell cycle detailed

G0, G1, S, G2, Prophase I, Metaphase I, anaphase I, Telophase I, cytokinesis I, G1, G2, prophase II, metaphase II, anaphase II, telophase II, cytokinesis II