Cell Division

• Multicellular organisms– Important in growth and development

• Single celled organisms– Important for reproduction

• Goals– To create 2 identical daughter cells

Genetic Material Equally Distributed

• What has to occur in order for genetic material to be equally distributed?– Each new cells contains the total number of

chromosomes as the parent cell– DNA has to replicate

• Before we talk about replication we need to understand more about DNA

• Double Helix

• Sugar Phosphate Backbone

• Made of nucleotides

sugar

base base

base base

sugar

sugar

sugar

p

p

p

p

BasesGuanine – CytosineThymine - Adenine

NUCLEOTIDES

Before we go on to cell division:

• What is the structure of DNA?

• How are the bases paired?

• What is the function of DNA?

• Replicate the following chain:

– TAC TTG AAA TGA CCC ACG ACT – AUG AAC TTT ACT GGG TGC TGA

Back to Cell Division

• Cells pass through a life cycle of 5 phases

• In single-celled eukaryotes, – cell cycle is the major mechanism for asexual

reproduction

• In multicellular eukaryotes,– Development – Growth

Cell Cycle Overview

• Key Events– Cell grows (interphase)– DNA doubles (interphase)– Nucleus divides (prophasetelophase)– Cyotplasm divides (telophase)

A. The Eukaryotic Cell Cycle

• Stages of the cell cycle– G1 (gap-1) is the period before chromosomal

replication takes place– S (synthesis) is the period when the chromosomes

are replicated– G2 (gap-2) is the period following chromosomal

replication– M (mitosis) is the period of nuclear division

Mitosis an Overview

• Interphase each chromosome is uncoiled and in an unreplicated state

Mitosis an Overview

• interphase, each chromosome is replicated. The replicated chromosome consists of two sister chromatids attached at the centromere.

Mitosis an Overview

• At the beginning of mitosis, the chromosomes coil and become visible (still Interphase). Note that the chromosome is still in a replicated state, with a sister chromatids attached at the centromere.

Mitosis an Overview

• Anaphase: the centromere splits, the sister chromatids separate and become daughter chromosomes, the daughter chromosomes move to opposite sides of the cell.

Mitosis an Overview

• At the end of mitosis, the chromosomes on each side of the cell uncoil and form two new, identical nuclei. (telophase)

A.     Prophase• 1. Chromosomes coil up and become

individually visible. • 2. The nuclear membrane disappears• 3.  The mitotic spindle will form

between the centrosomes.• 5. microtubules attach to the

centromere of each sister chromatid

A.     Metaphase• 1.  Chromosomes line

up on the middle of the spindle.

• 2.  Each chromosome has microtubules attached to the centromere of each of its sister chromatids.

B.     Anaphase• 1.  Chromosomes move to

opposite ends of the cell. One chromatid from each chromosome goes to each end of the cell.

• Use spindel fibers to do this– a)  Why can’t DNA

fragments without a centromere be passed on to further generations?

– b) Why can’t chromosomes have more than one centromere?

A.     Telophase• 1. Chromosomes reach the opposite ends

of the cell.• 2.  Chromosomes uncoil• 3. The spindle disintegrates, • 4.   The nucleolus reappears• 5.  The cytoplasm divides in a process

known as cytokinesis. – a)  using either a cell plate or cleavage furrow – b). This produces two cells each identical to

the original cell (except, of course for errors in DNA duplication).

Mitosis an Overview

• During what stage of the cell cycle do the drawings take place?

• What is a chromosome? A chromatid? A sister chromatid? A daughter chromosome? And in what stage of the cell cycle do we find each?