Recap on Cell Size

• cells are limited in their size because of 3 things:– 1. Diffusion– 2. DNA– 3. Food Requirements

• Therefore, Cells have 2 methods of dealing with cell size:– Altered Shape– Cell Division

• We are going to learn how cells Divide!

Cell Theory & Chromosomes

• Recall that Cell theory states “all cells come from preexisting cells.”

• Cell Division results in two cells that are identical to the original, parent cell

• Chromosomes – “colored bodies” that contain genetic material.

• It is the identity of the cell• Accurate transmission of

chromosomes during cell division is critical.

The Cell Cycle • Cell Cycle – sequence of

growth and division of a cell• Broken into 2 Parts:

– Interphase : Growth– Mitosis : Division

• A cell spends most of its life in INTERPHASE

• Cell division is during mitosis• One final process, division of the

cytoplasm, cytokinesis, takes place after mitosis.

Cell Cycle - Interphase• During Interphase…

– Cell grows in size– Carries on metabolism– Replicates chromosomes

• Composed of 3 Phases:– G1 : Chromosomes are not

yet visible, rapid protein synthesis due to growth

– S : Chromosomes are replicated in the nucleus (sister chromatids)

– G2 : Chromosomes shorten & coil, protein synthesis prepares for Mitosis, centrioles prepare to make the mitotic spindle, cell organelles are replicated

Cell Cycle: Mitosis - Prophase

• There are 4 Stages of Mitosis• 1.) PROPHASE

– First and longest stage– Chromatin coils up into visible

chromosomes– Sister Chromatids are visible (2 exact

copies of DNA for the 2 new cells) from interphase

– Sister Chromatids are held together by a centromere

– Nucleus begins to disappear– Nucleolus disintegrates– Animal Cells – Centrioles Migrate to the

poles for chromatin separation– Spindle forms – for sister chromatid

separation. In Plant cells the spindle forms without centrioles.

Chromatin – long strands of DNA wrapped around proteins, it only becomes chromosomes for replication and protein synthesis.Centromere – Structure that holds sister chromatids together. Plays a role in chromosome movement during mitosis. By their characteristic location, centromeres also help scientists identify and study chromosomes.

Cell Cycle: Mitosis - Metaphase• During Metaphase…

– Short second phase– Doubled chromosomes become

attached to the spindle fibers by their centromeres

– Chromosomes are pulled by the spindle fibers

– Begin to line up on the midline or equator of the spindle

– Each sister chromatid is attached to its own spindle fiber

– One spindle fiber extends to one pole, the other to the opposite pole

– Ensures that each new cell receives an identical and complete set of chromosomes.

Cell Cycle: Mitosis - Anaphase• During Anaphase…

– Third phase of mitosis– Separation of sister

chromatids at the beginning

– Centromeres split apart– Chromatid pairs from each

chromosomes separate from each other

– Chromatids are pulled apart by the shortening of the microtubles in the spindle fibers

Cell Cycle: Mitosis - Telophase• In Telophase…

– The fourth and final stage– Begins as chromatids reach the

opposite poles of the cell– Many of the Mitotic changes that took

place are reversing for the two new cells

– Chromosomes unwind for protein synthesis

– Spindle breaks down– Nucleolus reappears– New nuclear envelope forms

around each set of chromosomes– A new double membrane begins to

form around the 2 new nuclei

Mitosis - Cytokinesis• Following telophase, the

cell’s cytoplasm divides in a process called Cytokinesis

• In Animal Cells – plasma membrane pinches in along the equator

• In Plant Cells – a cell plate is laid down across the cell’s equator. A cell membrane forms around each cell, and new cell walls form on each side of the cell plate until separation is complete.

Your Turn!Place the following images in order of Mitosis with numbers, and give the name of the cycle. Let interphase be step 5.

How did you do?

2 - Metaphase4 - Telophase 1 - Prophase

5 - Interphase3 - Anaphase

Results of Mitosis• Mitosis gives genetic

continuity• Daughter cells will carry out

the same process or job as the parent cell

• Results in groups of cells– Atoms form Molecules– Molecules form Cells– Cells form Tissues– Tissues form Organs– Organs form Organ systems– Organ Systems form an

Organism

• Remember! The Cell is STILL the most basic unit of organization.