The Cell Cycle & Cell DivisionThe Cell Cycle & Cell Division

MitosisMitosis

DNA and Cell Division:• First, a look at Chromosomes:

• Chromatin: DNA in long, thin,

loose strands

• Visible and seen when the cell is NOT Dividing

• Just before cell division, the DNA replicates, then coils and condenses into thicker rod like structures called chromosomes

DNA, or deoxyribose nucleic acid, contains all of the genetic instructions to run an organism. DNA has a unique double helix structure, which resembles a twisted ladder. In eukaryotes, DNA is stored in the nucleus. In prokaryotes it is twisted into a nucleoid.

Most of the time your DNA is in a loose pile called chromatin (Think of a plate of spaghetti). During certain processes your DNA condenses into tight structures called chromosomes (when you wrap a noodle around your fork). Every species has a unique number of chromosomes.

• Chromatin

Chromosome

Chromosomes

• Chromosomes: DNA coils and condenses around small proteins called histoneshistones

• Visible and seen when the cell is actively dividing.

A closer look at chromosomes:A closer look at chromosomes:

This basically shows one copy of DNA

Centromere

Sister Chromatids

This basically shows the two copies of DNA side by side

Un-replicated Chromosome

Replicated Chromosome

• Every organism has a “normal” total Every organism has a “normal” total number of Chromosomes in each cell.number of Chromosomes in each cell.

• In all sexually reproducing organisms, In all sexually reproducing organisms, the chromosomes occur in the chromosomes occur in HomologousHomologous Pairs: Pairs:

– Each pair contains similar genetic Each pair contains similar genetic information, and you get one from information, and you get one from each of your parents.each of your parents.

OrganismTypical

Chromosome Number

Human 46

Chimp 48

Bat 44

Goldfish 94

Amoeba 50

Cat 38

Dog 78

Fruit Fly 8

Mold 4

The Importance of Chromosome Number

Humans: 46 chromosomes, 23 pairsFruit flies: 8 chromosomes, 4 pairsAdder’s Tongue Fern: 1262 chromosomes, 631 pairs

*** The number of chromosomes has NO relationship to complexity! ***

The number of chromosomes is ALWAYS an even number because “Normal” cells have two copies of each chromosome. You received one copy from mom and the other from dad. For example, Mom gave you a copy of her 14th chromosome, and so did Dad. Now you walk around with two Chromosome #14s in every cell. Since the pairs code for the same information they are called homologous. We call a cell with two copies of each chromosome diploid, and represent them with a 2n notation.

Diploid and Haploid Cells

• Any cell that contains all paired chromosomes is called a diploid cell

• Any cell that contains only half or unpaired chromosomes is called a Haploid cell

Chromosome Number Symbolism and Terminology:

• N = Symbol indicating number of

different types of chromosome in

a species

Humans have 23 types of chromosomes;

Therefore N in Humans = 23 In Chimps N = 24

Diploid Haploid

• 2N = A diploid cell; A cell with pairs of chromosomes

• In humans 2N = 46; In Chimps 2N = 48

• Body Cells or Somatic Cells are diploid

• 1N = A haploid cell; A cell with unpaired chromosomes

• In humans 1N = 23; In Chimps 1N = 24

• Sex Cells or gametes are haploid

There are two reasons why cells divide, instead of continuously grow: A. Demands on the genetic material

a. Similar to a library serving more and more people, but not buying more books.

Cell Growth

B. Difficult for osmosis and diffusion to occur across the cell membrane

a. Depends on surface area to volume ratio. b. As size increases the volume

increases faster than the surface area.

The Cell Cycle

When a cell becomes too large, it needs to divide. Cells follow a predictable life cycle, called the cell cycle where the cell grows, prepares to divide, and actually divides, over and over and over, etc. This cell division process is happening in your body right now, in all of your somatic, or body cells (all of your cells except those inside of your testes or ovaries).

Cell division has two predictable steps, first mitosis copies and divides the nucleus in half, and second cytokinesis divides the cytoplasm in half. The two newly formed cells are called daughter cells.

The Cell Cycle: • Cells go through phases or a

cell cycle during their life before they divide to form new cells 

• The cell cycle includes 2 main parts --- interphase, and cell division

• Interphase is the longest part of a cell's life cycle and is called the "resting stage" because the cell isn't dividing

• Cell division includes mitosis (nuclear division) and cytokinesis (division of the cytoplasm)

We use the following diagram to represent the Cell Cycle.

The Cell Cycle

Cell Cycle Phases in Detail

Interphase is the period of time between cell divisions. Somatic cells spend 94% of their life in this phase.

**This is NOT a phase where NOTHING happens.**

This is a phase is when normal life occurs (protein building)!

Interphase:

• G1 (Gap 1) = Cell grows and doubles in size, organelles doubled– Any cell that will never

divide again such as nerve cells stay “arrested” here (G0)

• S (Synthesis) = DNA is replicated or synthesized here.

• G2 (Gap 2) = Cell makes structures like organelles in preparation for division

Other features of Interphase:

• DNA is in chromatin form (loose thread-like)

• Nucleus and nucleolus are visible

• Centrioles are non-active

Mitosis Phases Mitosis has four phases: prophase, metaphase, anaphase, and telophase (Often called PMAT.)

Prophase:

• DNA begins to condense to form chromosomes

• Nucleolus and nuclear membrane begin to disappear

• Centrioles begin migration to opposite ends of the cell

• Spindle fibers form from the centrioles

Chromosomes

Spindle

Centrioles

Metaphase:

• Chromosomes attach to the spindle

• Microtubules connect the centromere to the spindle fibers.

• Chromosomes align in the middle (equator) of the cell

Equator

Anaphase:

Sister Chromatids

Events:1. Chromosomes separate into

individual sister chromatids and move to opposite poles

2. Spindle fibers begin to break down

Telophase

Events:1. Chromosomes loosen into Chromatin.2. Nuclear envelope reappears.3. Spindle fibers disappear completely.4. Cleavage furrow forms in animal cells, or Cell plate forms in plant cells to BEGIN cytokinesis. Daughter Cells Forming

Cytokinesis• Spindle begins to

disappear

• Chromosomes un-condense and revert to chromatin

Two Identical Daughter Cells – Back in Interphase

• Animal cells simply pinch off at the cleavage furrow.

• In plant cells the cell plate builds a new cell wall.

Cytokinesis is not a step of mitosis or interphase. It overlaps both mitosis and the G1 phase.

Mitosis Animation

Mitosis Video

Cell Division Controls

All cells grow and divide at different rates. For example, skin cells rapidly and continuously divide throughout life, while nerve cells almost never divide after development. Cyclins are proteins which regulate division and growth in eukaryotic cells. They respond to internal or external cell conditions, such as the amount of DNA, or the loss of contact with neighboring cells. Cancer, an uncontrolled growth of cells, is caused by the failure of cells to respond to cyclins.

Breast Cancer Cell Brain Cancer Cell