LESSON # 19

TWO TYPES OF CELL DIVISION

1- MITOSIS

2- MEIOSIS

The number of chromosomes in the daughter cells is identical to that of the mother cells. It takes place in most cells (somatic cells)

The number of chromosomes in the daughter cells is the half of that of the mother cells. It takes place only in sex cells (gametes)

Somatic Cells

Reproductive Cells or Gametes

Sperm Egg

are Diploid Cells

are Haploid Cells

Somatic human cells are DIPLOID cells because they have 23 pairs of chromosomes ( 46 chromosomes ).

DIPLOID: Two sets of chromosomes.

HAPLOID: One set of chromosomes

Diploid cells: 2n cells. In this case, 2(23) cells.

Haploid cells: 1n cells. In this case, 1(23) cells.

Reproductive human cells are HAPLOID cells because they have 23 chromosomes.

(23)

(23)

Chromosomes in every pair have the same structure and function and are called homologous chromosomes.

MITOSIS

Haploid

Haploid

Haploid

(23)

(23)

(23)

During MITOSIS, the number of chromosomes does NOT change. Therefore, haploid cells will produce haploid cells.

Diploid

Diploid

Diploid

MITOSIS

During MITOSIS, the number of chromosomes does NOT change. Therefore, diploid cells will produce diploid cells.

(2n cell)

Diploid

GametogoniumDuring MEIOSIS I, a single diploid cell divides to produce two haploid repro-ductive cells. The number of chromo-somes is reduced to the half. Sister chromatids remain together.

(23)

Gamete (Haploid) Gamete (Haploid)

(23)(23)

MEIOSIS I

They are the cells that produce the gametes (oocyte and sperm).

MEIOSIS II

Haploid

Haploid

Haploid

(23)

(23)

(23)

During MEIOSIS II, the number of chromosomes does NOT change. Therefore, haploid cells will produce haploid cells.

During MEIOSIS II, each haploid gamete produces two haploid gametes. Sister chromatids separate like in mitosis.

Haploids

+

SpermEgg

1n cell 1n cell

(2n cell)

DiploidDuring FERTILIZATION, two haploid gametes combine to produce a diploid embryo.

Fe

rtilization

Somatic Cells undergo MITOSIS and cell division.

Reproductive Cells or Gametes: They are specialized cells, which come together to produce offspring.

Reproduction: It is the process by which the reproductive cells come together to produce offspring.

+chromosomes chromosomes2323

+

Haploid Haploid Diploid

chromosomes46

Gametogonia: They are cells, which undergo meiosis to produce haploid reproductive cells or gametes.

The cells that give rise to sperm and egg are diploid cells. When the cells that give rise to sperm and egg cells divide, the result is sperm or egg cells that have only half of the usual somatic number of chromosomes.

Haploid (1n) Haploid (1n)Haploid (1n) Haploid (1n)

Diploid (2n)

Spermatogonium

Diploid (2n)

Oogonium

Somatic Human Cells (diploid)

Mitosis

Diploid

Sperma-togonia

Oogonia

(Diploid)

(Diploid)Meiosis

MeiosisHaploid

Haploid

Haploid + Haploid

Diploid zygote

Sex Human Cells (haploid)

It is the process by which a single diploid cell divides to produce haploid reproductive cells.

1n 1n 1n 1n

2n

Meiosis

Homologous means same in size and function.

The Steps in Meiosis

MEIOSIS-I

MEIOSIS-II

One diploid cell gives rise two haploid cells.

Each haploid cell gives rise two haploid cells.

2n1n

1n

1n

1n

1n

1n

1n

1n

Homologous chromosomes

Homologous chromosomes

Meiosis

MetaphaseMetaphase

Paternal chromosome

Maternal chromosome

DNA

I

2n

MEIOSIS I

Homologous chromosomes

2n

MITOSIS

2n

2n

1n

1n

Homologous chromosomes

2n

1n 1n

MEIOSIS-I

MEIOSIS-II

1n 1n 1n 1n

1- Homologous chromosomes links as they condense forming tetrads.

2- Crossing over occurs.

Prophase I

MEIOSIS I

2n2n 2n

Maternal chromosomes

Paternal chromosomes

CROSSING OVER: It is the process by which non-sister chromatids exchange genetic material.

Crossing over is the first important source of genetic variation.

MEIOSIS I

Prophase I

1- Homologous chromosomes align at metaphase plate.

MEIOSIS I

or

Metaphase I

2- Independent assortment occurs.

MEIOSIS I

Green eyes & blond hair

Black eyes & black hair

Green eyes & black hair

Black eyes & blond hair

It is the random distribution of homologous chromosome pairs during the metaphase of meiosis I.

Independent Assortment:

Independent Assortment is the second important source of genetic variation.

Crossing over is the first important source of genetic variation.

Independent assortment is the second important source of genetic variation.

SOURCES OF GENETIC VARIATION

MEIOSIS I

2n2n

Microtubules separate the homologous chromosomes (sister chromatids remain together).

MEIOSIS I

1n

1n

Anaphase I

2n

1n

1n

Two haploid cells result from Cytokinesis

MEIOSIS I

Telophase I

MEIOSIS I

GENETIC VARIATION

1- Homologous chromosomes links as they condense forming tetrads.2- Crossing over occurs.

Prophase I

Crossing over is the process by which non-sister chromatids exchange genetic material Crossing over is the first important source of genetic variation.

Metaphase I1- Homologous chromosomes align at metaphase plate.

2- Independent assortment occurs. Independent assortment is the second important source of genetic variation.

Anaphase I1- Microtubules separate the homologous chromosomes (sister chromatids remain together).

Telophase I1-Two haploid cells result from Cytokinesis.

MEIOSIS I

Meiosis and Sex Outcome

DNA replication

DiploidDiploid

DiploidDiploid

HaploidHaploid

HaploidHaploid

The Y chromosome has about 50 genes.

The X chromosome has about 1500 genes.

MEIOSIS I

XX

X X

X XX X YX X Y

X Y

XY

MEIOSIS II

XX XYFemale Male

Diploid

Haploid Haploid

FERTILIZATION

MOTHER FATHERGAMETOGONIA

Meiosis and Sex Outcome

Diploid