Cell Cycle and Mitosis

a.k.a. Cell Reproduction

Biology is the only subject in

which multiplication is the same

thing as division

Introduction• All organisms start

with a single cell.• All new cells are

produced from other, already existing cells.

• Millions of cell divisions take place in our bodies every day to replace the cells that have died

Cell Type Approximate Life Span

Skin Cells 2 weeks

Red Blood Cells 4 months

Liver Cells 300-500 days

Intestine-internal lining 4-5 days

Intestine-muscles and other tissue 16 years

Source: Spaulding et al., Cell 122:1.

Type of Cell Division Type of Cells it occurs in Function

Binary Fission Prokaryotes Asexual Reproduction

Mitosis EukaryotesAsexual Reproduction Growth of Individual

Repair/Maintenance of Tissues

Meiosis Eukaryotes Sexual Reproduction

3 Types of Cell Division

Binary fission• Asexual reproduction of a Prokaryotic cell• Bacteria (Ex E. coli = every 20 minutes)

• How many bacteria will result from 1 bacteria 7 hours (optimal conditions)? 1 million!

Binary Fission• Begins with the

duplication of the genetic material

• A plasma membrane splits the cell in two, with each side containing a single copy of the DNA

• The resulting two daughter cells replace the single parent cell and are genetically identical

• Multiphase process in which the cell divides from 1 parent cell to 2 identical daughter cells.

• Occurs in all cells except reproductive cells

• Mitosis is how asexual cell are replicated.

• Meiosis is how sex cells are produced (we will look at this process later in the term)

Eukaryotes Use Mitosis to Generate Identical Daughter Cells

The Cell Cycle• Interphase is not part of mitosis!!• This is when the cell is doing its

“everyday” jobs and getting ready for mitosis

• Interphase is divided into 3 phases:

• G1 = 1st Gap– cell doing its “everyday

job”– cell grows

• S = DNA Synthesis– copies chromosomes

• G2 = 2nd Gap– prepares for division– cell grows– produces organelles, proteins,

membranes

• Many adult cells enter a non-dividing phase called G0

• The G0 phase can last from a few days to the lifetime of the organism

• Ex: liver cells stay in G0 for approximately a year, then reenter the cycle

The Cell Cycle

Organization of Genetic Material• For equal division of the DNA to

occur it has to be organized • In interphase each DNA double helix

is packaged with special proteins to form long strands of chromatin

• Chromatin is further compacted during mitosis to form

chromosomes

DNA Packaging

DNA during Interphase

DNA during Mitosis

Organization of Genetic Material

• DNA is replicated during S phase, resulting in two identical pieces of DNA, known as sister chromatids

• Each human cell has twice the usual amount of DNA at the end of S phase and the start of mitosis

• The identical sister chromatids are held together at a region called the centromere

One Chromosome

The Karyotype Describes Allthe Chromosomes in a Nucleus

• Every species has its own characteristic number of chromosomes

• The display of all the chromosomes in a somatic (non-sexual) cell is called the karyotype

• Each of us has two of each chromosome, one from mom and one from dad.

• This pair of chromosomes, one from mom and one from dad, are similar but not identical

• They are called homologous chromosomes

Karyotypes Before and After S-phase of Cell Cycle

Before S-Phase After S-Phase

Karyotype of The Human GenomeLet’s not get Chromosomes, Homologous Chromosomes, and Sister Chromatids Confused!!

• This is part of a karyotype looking at just chromosome 1

Copied Homologous

Chromosomes

Sister Chromatids

Homologous Chromosomes

ProphaseMetaphaseAnaphaseTelophase

Video

4 Stages of Mitosis:

• Beginning of mitosis• DNA compacts into

chromosomes• Centrosomes appear and

begin to move to opposite ends of cell making the poles of the cell

• The nuclear envelope breaks down

• The mitotic spindle forms. It is made up of microtubules attached to each centrosome

Prophase

• Sister chromatids attach to the spindle fibers at the centromere through the kinetochore

• The kinetochores are patches of proteins that attaches to the microtubules

• Each centromere has two kinetochores that attach to the spindle fibers, one from each centrosome

• This causes the chromosomes to line up on the metaphase plate

Metaphase

• Sister Chromatids separate and begin to move to opposite ends of the cell

• The centrosomes shorten the spindle fibers dividing the sister chromosomes and bringing them to opposite side of the cell

• Each chromatid is now considered a new chromosome

Anaphase

• Begins when the new chromosomes reach the centrosome

• The spindle fibers break down

• Two new nuclei form and the nuclear envelope forms around the chromosomes at each pole of the cell

• Chromosomes appear as chromatin

• After telophase cytokinesis occurs

Telophase

• Not considered a phase in mitosis

• Follows telophase• The cell

membrane moves inward to create two daughter cells

• Occurs at the metaphase plate

Cytokinesis in Animal Cells

• Since plants have cell walls they also have to build this between daughter cells.

• Vesicles containing cell wall components move to the metaphase plate where the cell wall forms

Cytokinesis in Plant Cells

How can you remember the order????

• Answer this question....Where would you like your cat or dog to pee?

• On a PMAT of course

The Cell Cycle Is Highly Regulated• Cells divide in response to

internal and external signals• In humans, cell cycle

regulatory proteins are activated by external signals to divide, advancing the cell from G1 to S phase and initiating DNA replication

• Cell cycle regulatory proteins can stop or pause the cell cycle if conditions are not favorable for cell division

What happens when the cell does not stop at a checkpoint?

What is this condition called?

Cancer 6 key mutations• Turn on cell division

– Turn on promoter genes• Ignore checkpoints

– Turn off suppressor genes• Avoid programed cell death

– Turn off “suicide” genes• Unlimited divisions

– Turn on chromosome maintenance genes• Promotes blood vessel growth

– Turn on blood vessel genes• Cell attachment, anchor and density dependence genes

– Turn off touch censor gene

Some people love mitosis a little too much!