Meiosis

Sex CellsGametes (germ cells)Eggs and Sperm

Somatic CellsAll other cells

Types of Human Cells

Autosomes – same number in males and females

Sex Chromosomes – X or Y

Human females: XX

Human males: XY

Testosterone and other hormones from the Y make a male

Types of Chromosomes

46 chromosomes(23 pairs)

Humans

Describes how many sets of different chromosomes a cell has.

n = number of chromosomes in 1 set (haploid number)

Haploid Cells: 1nDiploid Cells: 2nIn humans, n = 23 (the

number of different chromosomes we have)

Ploidy

32/16

62

Figure 13.5

Key

Haploid (n)Diploid (2n)

Egg (n)

Haploid gametes (n 23)

Sperm (n)

Ovary Testis

Mitosis anddevelopment

Diploidzygote(2n 46)

Multicellular diploidadults (2n 46)

MEIOSIS FERTILIZATION

Figure 13.6

KeyHaploid (n)

Diploid (2n)

Gametes

MEIOSIS FERTILIZATION

Zygote

MitosisDiploidmulticellularorganism

(a) Animals

n

n

n

2n 2n 2n2n

2n

n n

n

n n

nn

n

n

nMEIOSIS

MEIOSIS

FERTILIZATION

FERTILIZATION

Mitosis Mitosis

Mitosis

Mitosis Mitosis

GametesSpores

Gametes

Zygote

Zygote

Haploid multi-cellular organism(gametophyte)

Diploidmulticellularorganism(sporophyte)

Haploid unicellular ormulticellular organism

(b) Plants and some algae (c) Most fungi and some protists

Plants and some algae exhibit an alternation of generations.

In crossing over, nonsister chromatids exchange DNA segments.

Each pair of chromosomes forms a tetrad, a group of four chromatids.

Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred.

Crossing Over

Figure 13.11-1Prophase Iof meiosis

Nonsister chromatidsheld togetherduring synapsis

Pair of homologs

Figure 13.11-2Prophase Iof meiosis

Nonsister chromatidsheld togetherduring synapsis

Pair of homologs

Chiasma

Centromere

TEM

Figure 13.11-3Prophase Iof meiosis

Nonsister chromatidsheld togetherduring synapsis

Pair of homologs

Chiasma

Centromere

TEM

Anaphase I

Figure 13.11-4Prophase Iof meiosis

Nonsister chromatidsheld togetherduring synapsis

Pair of homologs

Chiasma

Centromere

TEM

Anaphase I

Anaphase II

Figure 13.11-5Prophase Iof meiosis

Nonsister chromatidsheld togetherduring synapsis

Pair of homologs

Chiasma

Centromere

TEM

Anaphase I

Anaphase II

Daughtercells

Recombinant chromosomes

Down Syndrome

Klinefelter’s Syndrome

Cause: Nondisjunction