Chromosomes

By the end of this class you should understand:

• The structure and function of human chromosomes

• The purpose of the different types of cell division

• The pattern of chromosome movements in mitosis and meiosis

Chromosomes

Remember that chromosomes are the packages that our DNA is organized into

Normally spread out Only condensed

during cell division

Chromatin & Centromere Chromosomes are made of a

material called chromatin A mixture of DNA and protective

protein The core in the center is

referred to as the centromere Not always in the center!

Sometimes off to one side Divides the chromosome into two

arms which may be uneven (long arm and short arm)

Why the X? Remember chromosomes

are only compacted during mitosis Mitosis doubles the

chromosome from a I to an X

A visual image of the chromosome is only possible at this stage

This visual stain is called a karyotype

Blood Karyotype

Actual Karyotype Result

Chromosomal duplication Chromosomes are typically

spread out in a huge mass of active DNA

Only when the cell is about to divide are they packaged tightly Imagine those blueprints in a

busy library, then imagine them packed into books before being shipped away

Telomere The tips of chromosomes

have long repeated sequences of DNA that are noncoding but protect the rest of the DNA DNA replication typically fails

to completely duplicate these telomeres

They function as the aglets on your shoelaces

Telomere Shortening Every time a cell undergoes

mitosis it loses a bit of its telomeres

Adult cells have shorter telomeres and telomere shortening may be linked to aging Dolly the sheep was cloned

from an adult cell and so may have showed premature aging

Telomerase Immortal cells such as stem

cells and cancer cells express an enzyme called telomerase Lengthens telomeres and

extends life of daughter cells Research in stem cells and

telomerase may provide clues into aging process

Cell Replication One cell can become two identical cells through

a process called mitosis One cell can also become four haploid cells

through a process called meiosis These are the only two ways to make more cells in

your body Picture:cytoskeletal stainof cell in mitosis

Mitosis & Meiosis

MITOSIS: Produces two cells

identical to original Many stem cells in

your body constantly perform mitosis to grow new tissue

This is how you grow

MEIOSIS: Produces four haploid

cells Only certain stem cells

in the gonads perform meiosis

This is how you make sex cells (gametes)

Mitosis & Cell Cycle Mitosis is actually

only a part of the cell replication cycle

Cells that are amitotic (such as brain cells) are in G0 phase and never self-replicate

Interphase

Actually several major stages of cell growth G1: “Okay, guess I'll get ready to divide” S: “Gonna need to copy my DNA” G2: “And I'll need spindle fibers to do the job”

Phases of the Cell Cycle

Phase control Each phase of the cell cycle has controls

The cell can only move to the next phase when given “permission”

What is the name for when these controls break?

Steps of Mitosis

A video is worth a thousand words! http://www.youtube.com/watch?v=VlN7K1-

9QB0 Key detail: the microtubule spindles that pull

the chromosomes apart come from the centromere and are a type of cytoskeleton structure

Mitosis Diagram part 1

Mitosis Diagram part 2

Mitosis Summary!

Meiosis Meiosis is a specialized type

of cell replication It produces gametes (sex

cells) Sperm and eggs in humans

and animals Pollen and seeds in plants Bacteria cannot undergo

meiosis

Steps of Meiosis

Meiosis is similar to a double mitosis Chromosomes are duplicated 46 x 2 = 92 chromosomes

At the end there are four cells with only one set of chromosomes 23 chromosomes x 4 gametes

Double Mitosis??

Yes! They even have the same steps: Prophase I, Metaphase I, Anaphase I, Telophase I Prophase II, Metaphase II, Anaphase II, Telophase

II In Meiosis I, the homologues are separated In Meiosis II, the sister chromatids are pulled

apart as in Mitosis

Meiosis I

Meiosis II

To Summarize:

See you in lab!