E SSENTIAL Q UESTION : W HAT ARE THE FOUR STAGES OF MITOSIS ?

T HINK -P AIR -S HARE Why are cells small? Why would it be bad for them to grow too large?

C ELL C YCLE R E - CAP 1. G 1 2. S phase 3. G 2 4. M phase Remember that G 1, S, and G 2 together are known as interphase and a cell spends most of it’s life in interphase.

R EASONS FOR CELL DIVISION 1. To prevent them from being inefficient in getting nutrients surface area to volume ratio 2. To reproduce For example: bacteria can reproduce asexually 3. To repair damaged tissue For example: if you get a paper cut 4. To grow an organism

M ITOSIS IN THE C ELL

There are many words that describe DNA in it’s various forms… A. DNA is usually found as chromatin while the cell is doing its normal job. 1. Chromatin is the granular material visible within the nucleus that consists of DNA tightly coiled around proteins.

B. B EFORE A CELL DIVIDES, CHROMATIN (DNA) COILS UP TO FORM CHROMOSOMES. 1. C HROMOSOMES ARE MADE UP OF DNA AND PROTEINS.

2. A LL ORGANISMS DO NOT HAVE THE SAME NUMBER OF CHROMOSOMES. 3. H UMAN CELLS HAVE 46 CHROMOSOMES. 4. C HROMOSOMES ARE ONLY VISIBLE DURING CELL DIVISION.

C. B EFORE EACH CELL DIVISION, EACH CHROMOSOME IS REPLICATED, OR COPIED. D. F OR THIS REASON, EACH CHROMOSOME CONSISTS OF TWO IDENTICAL SISTER CHROMATIDS.

E. E ACH PAIR OF SISTER CHROMATIDS IS ATTACHED AT AN AREA CALLED THE CENTROMERE. A HUMAN BODY CELL ENTERING CELL DIVISION CONTAINS 46 CHROMOSOMES OR 92 CHROMATIDS.

N OW LET ’ S TALK ABOUT MITOSIS … II. T HE TWO MAIN STAGES OF CELL DIVISION ARE MITOSIS AND CYTOKINESIS.

Cyto = Cell Kinesis = Movement

A. T HE FIRST STAGE, DIVISION OF THE CELL NUCLEUS, IS CALLED MITOSIS. B. T HE SECOND STAGE, DIVISION OF THE CYTOPLASM, IS CALLED CYTOKINESIS.

C. R EPRODUCTION BY MITOSIS IS CLASSIFIED AS ASEXUAL. 1. U NICELLULAR ORGANISMS REPRODUCE IN THIS MANNER. 2. M ITOSIS IS ALSO THE SOURCE OF NEW CELLS WHEN A MULTICELLULAR ORGANISM GROWS AND DEVELOPS.

D. A S YOU REMEMBER, A CELL SPENDS MOST OF ITS TIME IN I NTERPHASE. D URING INTERPHASE, A CELL DOES ITS NORMAL JOB, GROWS LARGER, COPIES ITS DNA, AND PREPARES FOR CELL DIVISION. DRAW THIS!

III. M ITOSIS A. B IOLOGISTS DIVIDE THE EVENTS OF MITOSIS INTO 4 PHASES : PROPHASE, METAPHASE, ANAPHASE, AND TELOPHASE (PMAT).

D URING PROPHASE, THE CHROMOSOMES BECOME VISIBLE, THE CENTRIOLES SEPARATE TO OPPOSITE SIDES OF THE CELL, THE CHROMOSOMES ATTACH TO THE SPINDLE, THE NUCLEOLUS DISAPPEARS, AND THE NUCLEAR ENVELOPE BREAKS DOWN. DRAW THIS!

B. T HE CENTRIOLES ARE TWO TINY STRUCTURES LOCATED IN THE CYTOPLASM NEAR THE NUCLEAR ENVELOPE THAT HELP ORGANIZE THE SPINDLE. C. T HE SPINDLE FIBERS ARE A FANLIKE MICROTUBULE STRUCTURE THAT HELPS SEPARATE THE CHROMOSOMES.

D URING METAPHASE, THE CHROMOSOMES LINE UP ACROSS THE CENTER OF THE CELL ( M IDDLE ). D URING ANAPHASE, THE CENTROMERES THAT JOIN THE SISTER CHROMATIDS SPLIT AND THE CHROMOSOMES MOVE UNTIL THEY HAVE SEPARATED INTO TWO GROUPS NEAR THE POLES OF THE SPINDLE ( A WAY ). DRAW THIS!

D URING TELOPHASE, THE CHROMOSOMES BEGIN TO DISPERSE INTO A TANGLE OF DENSE MATERIAL, THE NUCLEAR ENVELOPE REFORMS AROUND EACH CLUSTER OF CHROMOSOMES, THE SPINDLE BREAKS APART, AND THE NUCLEOLUS BECOMES VISIBLE IN EACH NUCLEUS. DRAW THIS!

P ROPHASE M ETAPHASE A NAPHASE T ELOPHASE PMAT Always remember…

D. C ELL DIVISION IS NOT COMPLETE AFTER TELOPHASE. C YTOKINESIS IS DIVISION OF THE CYTOPLASM.

I N ANIMAL CELLS, CYTOKINESIS OCCURS WHEN THE CELL MEMBRANE IS DRAWN INWARD UNTIL THE CYTOPLASM IS PINCHED INTO TWO EQUAL PARTS. I N PLANT CELLS, CYTOKINESIS OCCURS WHEN A CELL PLATE FORMS CLEAVAGE FURROW BETWEEN THE DIVIDED NUCLEI GRADUALLY DEVELOPING INTO A SEPARATING MEMBRANE.

M ITOSIS IN THE C ELL

L ET ’ S PRACTICE … On the back of your page, number 1-5. On the next slide, you will see 5 pictures. You need to figure out what phase each one is in and write it down.