The Chromosomal Basis of Heredity

Lecture 2

Bio 106

TOPICS

A. The Cell

B. The Chromosome Structure

C. Cell Division

1. Mitosis 2. Meiosis

D. Life Cycles

1. Terminal or Gametic Meiosis

2. Initial or Zygotic Meiosis

3. Intermediary or Sporic Meiosis

THE CELL

The Chromosome Structure

Components of a Nucleosome :

Histone octomer: H2A, H2B, H3, H4 (2 copies each) 146 bp of superhelical DNA

PARTS OF THE CHROMOSOME

CELL CYCLE

CELL DIVISION

ASPECT/FEATURE MITOSIS MEIOSIS

Number of divisions

1 2

Number of cells produced per division

2 4

Type of cells produced

Diploid (2N) Haploid (N)

Genetically identical to parent cell

Different from parent cell;

varied

Type of cells involved

Somatic cells Sex cells

MITOSIS

MITOSIS

- nuclear division of a cell which produces two daughter nuclei that are genetically identical to the parent nucleus. - cytoplasmic division usually follows nuclear division.

MITOSIS

P

R

O

P

H

A

S

E

Chromosomes become more coiled and thicker, the 2 sister chromatids being connected by their CENTROMERES.

Nucleolus disappears towards the later part.

The 2 pairs of centrioles move apart and spindle fibers start to form between them.

Nuclear

envelope

Condensed

chromosomes

Mitotic spindle

Separating centrosomes

microtubules

Centrosome with 2 centrioles

MITOSIS

P

R

O

P

H

A

S

E Plant cell Animal cell

MITOSIS P

R

O

M

E

T

A

P

H

A

S

E

Breakdown of nuclear envelope

Kinetochore MTs attach to kinetochores

Polar MTs push against each other, moving centrosomes apart

MITOSIS

M

E

T

A

P

H

A

S

E

Centrosomes now at opposite poles of the cell.

Chromosomes, now maximally condensed , are lined up at the metaphase plate (equatorial plate). They are held in place by spindle fibers coming from the centrosomes (kinetochore MTs).

centrosome

Kinetochore

MT

Polar MT

Astral MT

Metaphase plate

MITOSIS

M

E

T

A

P

H

A

S

E

Plant cell

Animal cell

MITOSIS

A

N

A

P

H

A

S

E

Separation of sister chromatids and their migration to opposite poles

At the end of this stage, a complete set of chromosomes has assembled at each pole of the cell.

Single-strand daughter

chromosomes

Increasing separation of spindle

poles

MITOSIS

A

N

A

P

H

A

S

E

Plant cell

Animal cell

MITOSIS

T

E

L

O

P

H

A

S

E

Chromosomes assemble in sets at the 2 poles. They uncoil and eventually look like they did at interphase. Nuclear envelope

reforms around each set of chromosomes.

Spindle fibers

disappear.

Nucleolus reforms.

Nuclear division by mitosis is now complete.

MITOSIS

T

E

L

O

P

H

A

S

E

Plant cell

Animal cell

MITOSIS

C

Y

T

O

K

I

N

E

S

I

S

Animal cytokinesis

Plant cytokinesis

In animal cells, a CLEAVAGE FURROW forms.

In plant cells, a CELL PLATE forms.

centrosome

Complete nuclear

envelope

MITOSIS

No variation in chromosome number

No genetic variation

Formation of 2 identical diploid

daughter cells

Consequences of MITOSIS:

Why is MITOSIS important?

Regeneration

Growth

Cell replacement

Vegetative reproduction

MEIOSIS

- two successive divisions of a diploid (2N) eukaryotic cell of a sexually reproducing organism that result in four haploid (N) progeny cells, each with half of the genetic material of the original cell.

MEIOSIS

1

2

LEPTOTENE

ZYGOTENE (synapsis)

3

(Crossing over)

PACHYTENE

Formation of TETRAD / BIVALENT

4

DIPLOTENE

5

DIAKINESIS

PROGRESSION FROM INTERPHASE to PROPHASE I

Alignment of bivalents/tetrads at the metaphase plate

Separation of bivalents/tetrads and movement of double-strand chromosomes to opposite poles

The 2 sets of chromosomes complete their migration to opposite poles.

Nuclear envelope reforms around each chromosome set.

Cytoplasm divides.

44

46

51

How meiosis generates GENETIC VARIABILITY:

the exchange of genetic material between homologous chromosomes during Meiosis I

the random alignment and orientation of maternal and paternal chromosomes in Meiosis I

the random alignment of the sister chromatids at Meiosis II

MEIOSIS:

Prophase I

3

(Crossing over)

PACHYTENE

Formation of TETRAD / BIVALENT

LIFE CYCLES: 3 major kinds Terminal or gametic meiosis

Terminal or gametic

Ex: animals

Only one multicellular stage: diploid (2n)

Initial or zygotic

Ex: most fungi & protists

Often a dominant, multicellular haploid stage (n), transitory diploid stage (2n)

Zygotic and gametic life cycles are called HAPLOBIONTIC.

Intermediary or sporic

Alternation of generations: both haploid (n) and diploid (2n) are multicellular.

has two mitosis events (DIPLOBIONTIC)

Ex: algae and plants

moss