Mitosis

How Cells Reproduce

Types of Cells• The body contains two different types of

cells– Somatic cells

• Reproduce by mitosis (asexual reproduction)• They are diploid, that is they contain two complete

sets of genetic information, (one from each parent)

– Gametes• Reproduce by meiosis (sexual reproduction)• They are haploid. Contain only on set of genetic

information.

Mitosis

• Aged cell transforms itself into twin youthfu cells

• Happens millions of times every second in the human body.

• It gradually replaces every cell in your body except muscle and nerve cells

• In the body, what cells do you think replace themselves the fastest?

Fig. 9.3, p. 151

The Cell Cycle

• The life of a cell can be divided into recurring different time periods called the cell cycle.

• Two main parts to the cell cycle.– Interphase – mitosis

Fig. 9.3, p. 151

Interphase

• The first part of interphase is devided into 4 different parts

1. G1

2. S

3. G2

4. Cytokinesis or the division of the cytoplasm

Most cells spend most their time (90%) in interphase

Fig. 9.3, p. 151

in-text, p. 163

centromere

one chromosome(in duplicated state)

one chromatid

its sister chromatid

Mitosis

• The division of the nucleus is divided into four distinct phases

• Prophase

• Metaphase

• Anaphase

• Telephase

Prophase

• Chromosomes start condensing• The spindle starts forming

– Centroles start to move to opposite ends of the cells– Microtubules extend out from the centrioles to the kinetochores

on each sister chromotid

• Nuclear envelope begins to break apart

pair of centrioles microtubules

kinetochores

nucleus

plasmamembrane

pair of centrioles

cell chromosomes

nuclear envelope

microtubules

CELL AT INTERPHASE EARLY PROPHASE LATE PROPHASE PROMETAPHASE

Fig. 9.5a, p. 152

Metaphase

• Spindle fibers interact to orient chromatids and aligne them at the cell’s equator, halfway between the poles.

Anaphase

• The microtubules that are attached to the kinetochores begin to shorten and pull the sister chromotids apart.

• One half of each sister chromotide goes to each side of the cell

Telophase

• Telophase is the opposite of prophase– Nuclear envelope is

reformed– Microtubules are

wound up in the centrioles

– Chromosomes unwind into chromotine

Cytokinesis (The division of the cytoplasm)

• Contractile microfilaments at the cleavage furrow pull the plasma membrane inward.

• The membranes come together and pinch off to form new daughter cells

Questions

• How many DNA molecules are present in a duplicated chromosome?

• What is being separated in anaphase?

• What type of cells undergo mitosis?

• What is the chromosome number at G1? What is the chromosome number after cytokinesis?

METAPHASE ANAPHASE TELOPHASE INTERPHASE

Fig. 9.5b, p. 153

Meiosis

The Making of gametes

Meiosis

• Meiosis can be broken up into two different parts: Meiosis I and Meiosis II

• In meiosis the nucleus devides 2 times, once in meiosis I and again in meiosis II

• Meiosis I is broken up into four parts– Prophase I, Metaphase I, Anaphase I and telophase I

• Meiosis II also has four different stages.– Prophase I, Metaphase I, Anaphase I and telophase I

Prophase I

• The exact same steps that occur in prophase of mitosis occur here.

• There are additional steps– Synapsis: Homologous chromosomes come

together.– Crossing over takes place

Homologous chromosomes

• Homologous chromosomes are chromosomes that code for the same genes

• We inherit one homolog from each parent

• There are 23 pairs of homologous chromosomes in human somatic cells.

Fig. 10.4a p. 164

newly forming microtubules in the cytoplasm

PROPHASE IMEIOSIS I

Homologous Chomosomes come together in a process called synapsis

Crossing over

• During synapsis homologous chromosomes exchange segments of DNA in a process called Crossing over

Meiosis I

• In Meiosis I the cell is separating homologous chromosomes.

• In Meiosis II the cell is like mitosis in that the cell is separating sister chromotids

Fig. 10.4a p. 164

plasma membrane

newly forming microtubules in the cytoplasm

spindle equator (midway between the two poles)

one pair of homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I TELOPHASE I

MEIOSIS I is separating homologous chromosomes

Fig. 10.4b p. 165

PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II

MEIOSIS II