Chapter 8 – Section 3

Bell WorkWhat would children be like if humans

reproduced using the process of mitosis?List some reasons why it is good for a species

overall for every individual to be a little bit different.

Chromosomes are matched in homologous pairsHuman cells have

46 chromosomes, making up 23 pairs of homologous chromosomes

MEIOSIS

Chromosomes

Centromere

Sister chromatids Figure 8.12

Gametes have a single set of chromosomes

Cells with two sets of chromosomes are said to be diploid (2n = 46 for humans)

Gametes are haploid, with only one set of chromosomes (n = 23 for humans)

Sexual Life Cycle• At fertilization, a sperm fuses with an egg,

forming a diploid zygote• Repeated mitotic divisions lead to the

development of a mature adult• The adult makes haploid gametes by meiosis• All of these processes make up the sexual life

cycle of organisms

The human life cycle

Figure 8.13

MEIOSIS FERTILIZATION

Haploid gametes (n = 23)

Egg cell

Sperm cell

Diploidzygote

(2n = 46)Multicellular

diploid adults (2n = 46)

Mitosis anddevelopment

Meiosis reduces the chromosome number from diploid to haploid

Meiosis, like mitosis, is preceded by chromosome duplication (during interphase)

However, in meiosis the cell divides twice to form four daughter cells, each of which is haploid (n = 23).

Meiosis IIn the first division, meiosis I, homologous

chromosomes are pairedWhile they are paired, they cross over and

exchange genetic informationThe homologous pairs are then separated,

and two daughter cells are produced, which at this point are haploid (n = 23).

But because each chromosome has double the genetic info (2 sister chromatids), another division is necessary.

Figure 8.14, part 1

MEIOSIS I: Homologous chromosomes separate

INTERPHASE PROPHASE I METAPHASE I ANAPHASE I

Centrosomes(withcentriolepairs)

Nuclearenvelope

Chromatin

Sites of crossing over

Spindle

Sisterchromatids

Tetrad

Microtubules attached tokinetochore

Metaphaseplate

Centromere(with kinetochore)

Sister chromatidsremain attached

Homologouschromosomes separate

Meiosis IIMeiosis II is essentially the same as mitosis

The sister chromatids of each chromosome separate

The result is four haploid daughter cells, each of which are haploid (n = 23).

Figure 8.14, part 2

MEIOSIS II: Sister chromatids separate

TELOPHASE IAND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II

Cleavagefurrow

Sister chromatidsseparate

TELOPHASE IIAND CYTOKINESIS

Haploiddaughter cellsforming

Review: A comparison of mitosis and meiosis

For both processes, chromosomes replicate only once, during interphase

Figure 8.15

MITOSIS MEIOSIS

PARENT CELL(before chromosome replication)

Site ofcrossing over

MEIOSIS I

PROPHASE ITetrad formedby synapsis of homologous chromosomes

PROPHASE

Duplicatedchromosome(two sister chromatids)

METAPHASE

Chromosomereplication

Chromosomereplication

2n = 4

ANAPHASETELOPHASE

Chromosomes align at the metaphase plate

Tetradsalign at theMetaphase plate

METAPHASE I

ANAPHASE ITELOPHASE I

Sister chromatidsseparate duringanaphase

Homologouschromosomesseparateduringanaphase I;sisterchromatids remain together

No further chromosomal replication; sister chromatids separate during anaphase II

2n = 4 2n = 4

Daughter cellsof mitosis

Daughter cells of meiosis II

MEIOSIS II

Daughtercells of

meiosis I

Haploidn = 2

n n n n

Haploidn = 2

Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring

Each chromosome of a homologous pair comes from a different parentEach chromosome thus differs at many points from the other member

of the pair

The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes (Independent Assortment)

• Random fertilization also increases variation in offspring (Which sperm will fertilize the egg?)

Figure 8.16

POSSIBILITY 1 POSSIBILITY 2

Two equally probable

arrangements of chromosomes at

metaphase I

Metaphase II

Gametes

Combination 1 Combination 2 Combination 3 Combination 4

Independent Assortment

Crossing over further increases genetic variability

Crossing over is the exchange of corresponding segments between two homologous chromosomes

Genetic recombination results from crossing over during prophase I of meiosis

This increases variation further

What leads to variability/diversity?

Why are we not all identical?Independent assortmentCrossing overRandom fertilization