8. CELL DIVISION: Mitosis

Last update (edited J): 15.II.2003


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Students are required to read the textbook for the instructor to be able to teach the concepts the course encompasses. (From FOUNDATION SKILLS in the Biology Department Master Syllabus.)
Students, do not just read these notes, to succeed in this class -- at least if you wish to have a high grade, you MUST use your textbook as a LEARNING TOOL!


MAIN TOPICS OUTLINE

8.1 CELL DIVISION PURPOSE
8.2 CHROMOSOMES
8.3 THE CELL CYCLE
8.4 MITOSIS (Karyokinesis)
8.5 CELL DIFFERENTIATION
8.6 ABNORMAL CELL DIVISION (CANCER)




LECTURE OBJECTIVES


1. List the purposes of cell division.
2. Describe a chromosome.
3. Recognize that through the process of mitosis two new daughter cells form with a complete set of chromosomes identical to the chromosome set of the original mother (parental) cell.
4. State the processes that occur during interphase.
5. Understand what is meant by cell cycle and be able to visualize where mitosis fits into the cell cycle.
6. Differentiate between karyokinesis and cytokinesis.
7. Name the four (4) stages of mitosis, and explain what is happening during each stage.
8. Define selected key terms.


Key Terms:

def. Anaphase: The stage in mitosis when the sister chromatids of each chromosome separate and move to opposite poles of the cell.
def. Cancer: a malignant tumor (the term is often used for large group of diseases with uncontrolled growth and spread of abnormal cells).
def. Cell cycle: Nuclear division (see mitosis).
def. Cell differentiation: The process in which cells in multicellular organisms activate and / or suppress genes and become specialized in composition, structure, and function.
def. Cell division: The splitting of a parent cell into two daughter cells, consisting of a nuclear division and a cytoplasmic division.
def. Centriole (ÅAster): A structure (organelle) in animal cells, which are involved in cell division.
def. Centromere (ÅKinetochore): Small constricted region where the two sister chromatids are held together.
def. Chromosome: long molecule of DNA & proteins, that contains the genes, found in the nucleus of eukaryotic cells
def. Cytokinesis: Cytoplasmic division.
def. Gene: Unit of heredity, the portion of DNA that determines characteristics (traits)
def. Karyokinesis: Nuclear division (see mitosis).
def. Kinetochore: Part of the centromere where spindle fibers (microtubules) become attached at cell division (each chromatid of a duplicated chromosome has its own kinetochore).
def. Meiosis: A process of nuclear division in eukaryotic sex cells that reduces the chromosome number of sex cells to the haploid number.
def. Metaphase: The stage in mitosis when each chromosome has become positioned midway between the poles of the cell.
def. Mitosis: A process of nuclear division in eukaryotic somatic cells that maintains the parental chromosome number by equally allocating chromosomes to each of the daughter cells, conventionally divided into four stages: prophase, metaphase, anaphase, and telophase.
def. Nucleus: a chromosome containing membranous organelle of eukaryotic cells.
def. Prophase: The stage in mitosis when chromosomes condense, spindle apparatus forms, and the nuclear membrane starts to break up.
def. Sex cells: Sperms or eggs (including the germ cells, which give rise to the sex cells).
def. Sister chromatid: one of the two DNA molecules in a duplicated chromosome. (The two chromatids held together at the centromere, and separated during mitosis.)
def. Somatic cells: Cells that are not sex cells (see sex cells).
def. Telophase: The stage in mitosis when chromosomes decondense, and two daughter nuclei form.
def. DNA (Deoxyribonucleic Acid): Double stranded, helical nucleic acid molecule capable of replicating and determining the (inherited) structure of a proteins



8.1 CELL DIVISION PURPOSE


Growth of the body

We all began as a single cell, resulting from the union of a sperm and an egg.

– As we grow, cell division results in an increase number of cells that make up our body.

– Many cells (nerve & muscle cells) stop dividing in early childhood.

– Other cells (blood & skin) continue to divide throughout life.

All growing organisms are capable of cell division: ‘
Mitosis

Maintenance of the body

replacement of old cells
repair (broken bone, sores) possible through cell division: ‘
Mitosis

Reproduction

All organisms attempt to perpetuate (reproduce), make more of themselves.

Meiosis. (Meiosis is discussed in next chapter!)



8.2 CHROMOSOMES

def. Chromosome: long molecule of DNA & proteins, that contains the genes, found in the nucleus of eukaryotic cells.
def. Nucleus: a chromosome containing membranous organelle of eukaryotic cells.
def. Gene: One of many units ("hereditary information units") on DNA.



• Chromosomes are hereditary units on DNA containing genetic language:

-- directions for making structural & regulatory proteins required for life.



Review question: What is DNA?

def. Nucleic acid: Large, complex molecules that store and transfer information.
[Information = amino acid sequence ‡ kinds of proteins made by the cell]

def. DNA (Deoxyribonucleic Acid): Double stranded, helical nucleic acid molecule capable of replicating and determining the inherited structure of a protein.


• Normal Human Chromosome Number: 46

-- Humans have 46 chromosomes in all body cells:
-- The genes are located on these 46 chromosomes
-- Other organisms have different normal chromosome numbers

Chimpanzee 48
Dog 78
Cat 38
Corn 20


Review question: What is the number of chromosomes in human cells?




8.3 THE CELL CYCLE


• All cells go through a basic life cycle.

-- Four (4) stages:

1. G1 (gap-phase 1) growth in volume
2.
S (synthesis) DNA replication (DNA duplicated)
3.
G2 (gap-phase 2) synthesis of proteins

G1 , S, and G2 is also called INTERPHASE (Growth Phase).

4.
M (cell division stage)
i. Karyokinesis -- MITOSIS (nuclear division)

-- Four (4) phases:
– prophase
– metaphase
– anaphase
– telophase

ii. Cytokinesis -- cytoplasmic division

Each daughter cell receives a complete set of chromosomes


Question: What is a COMPLETE SET of Chromosomes?
(What is the number of chromosomes in human cells?)


Mitosis



8.4 MITOSIS (Karyokinesis)


1. Stages in mitosis (Karyokinesis)

-- the Human chromosome number 46 is used in this example

i. PROPHASE

a. Individual 46 DUPLICATED chromosomes becomes visible. (condense)
b. Nuclear membrane gradually disintegrates
c. Nucleoli disappears
d. Centrioles move to the poles of the cell*. *
there are no centrioles in plant cells
e. Formation of spindle fibers
(microtubules extending from pole to pole)

- chromosomes can move freely in the cytoplasm

------------------------------- (some scientists like to discuss an intermediate called prometaphase)

ii. METAPHASE

a.. The 46 DUPLICATED chromosomes line up at the equatorial plane
b. The chromosomes become attached to the spindle
(at the kinetochores of the centromere)
------------------------------

iii. ANAPHASE

a. Each 46 DUPLICATED chromosome split at the centromere.
b. The two chromatids separate, along the spindle fibers (mechanism unknown), to the polar regions of the cell.

- Both polar regions now have identical 46 UNDUPLICATED chromosomes
------------------------------

iv. TELOPHASE

a. Individual 46 UNDUPLICATED chromosomes decreasingly visible (decondense)
b. Nuclear membrane forms -- daughter nuclei are formed.
c. Nucleoli disappear.
d. Spindle fibers disappears.




• Mitosis maintains the human chromosome number 46 by distributing chromosomes equally to both cells.


• After Mitosis: Each daughter cell enters INTERPHASE

G1 -- grow
S -- replicate the DNA -- make the 46 chromosomes DUPLICATED
G2 -- synthesize proteins needed for division

‡ enter a new round of mitosis and cytokinesis.



2. Plant and animal cell division differences
Animals: cytokinesis by -- cleavage furrow
Plants: cytokinesis by --
cell plate

Also:
Animals: centrioles present
Plants: centrioles not present (still produce spindle fibers)





8.5 CELL DIFFERENTIATION



def. Cell differentiation: The process in which cells in multicellular organisms activate and / or suppress genes and become specialized in composition, structure, and function.

• All trillion cells in an individual have the same genes on the chromosomes

-- However, not all our cells in the body are the same

• Some cells only activate certain genes

-- result: specialized cells.

- nerve cells
- muscle cells
- bone cells
- skin cells


Question: How many chromosomes are there in a nerve cell? In a muscle cell? Etc.


8.6 ABNORMAL CELL DIVISION
(CANCER)


def. Cancer: a malignant tumor (the term is often used for large group of diseases with uncontrolled growth and spread of abnormal cells).


If cells start to divide in an uncontrolled fashion, a group of cells may form a tumor

i. benign tumor -- cell mass that does not fragment and spread
Such a cell mass can still cause trouble if it interferes with normal body function.

ii.
malignant tumor (= Cancer) -- cell mass that moves from the original site (metastasis). Such a cell mass will spread and establish new colonies in other parts of the body.


• Cancer ia a malignant tumor, the term is often used for large group of diseases with uncontrolled growth and spread of abnormal cells.




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