Lecture 6: Cell Division
Reasons for Cell Division
- Cell division is required for:
- a) growth
- b) repair & replacement of damaged parts
- c) reproduction of the species
In Cell Division Copies of the
DNA Must Be Sent to Both New Cells
- Since the instructions for making cell parts are encoded
in the DNA, each new cell must get a complete set of the DNA
molecules
- This requires that the DNA be copied (replicated, duplicated)
before cell division
- Speculate on what will happen if DNA
is unevenly distributed in cell division? Discuss
your answer.
Genetic Blueprints for Cells Are
Organized Into Chromosomes
- The plans for making cells are coded in DNA
- DNA is organized into giant molecules called chromosomes
- Each chromosome is a single DNA molecule containing many
genes
- Each gene gives the directions for making 1 protein
- In humans each chromosome has approximately 2000 genes
- Chromosomes have distinct parts
- Centromeres:
- Hold duplicated chromosomes together before they are separated
in mitosis
- Kinetochore proteins bind to centromere and attach chromosome
to spindle in mitosis
- Telomeres: ends of chromosomes: important in cell aging
- DNA in chromosomes is associated with proteins
- Proteins strengthen DNA fiber
- Package chromosomes when they condense
- Control activity of genes
- Humans body cells have 23 pairs of chromosomes (46 total)
- Diploid = pair of each chromosome = 46 total
- The members of a chromosome pair are called homologues
- One of each pair came from mother, the other from father
- Human reproductive cells (sperms & eggs) have 23 single
chromosomes
- Haploid = single copy of each chromosome = 23 total
- One of each pair came from the father and the other came
from the mother
- If a species has 90 chromosomes in
its body cells, how many chromosomes will its sperms and eggs
have? How many chromosomes will the zygote (fertilized egg) have?
Check your answers.
Genetic Instructions are Organized
Into Genes
- A section of DNA which codes for a protein is called a gene
- We have approximately 50,000 genes (approx. 2000 per chromosome)
- Most of DNA in chromosome (~95%) is "junk" DNA-
function not known
Before a Cell Can Divide it Must
Duplicate its Chromosomes
- To make a new cell the old cell must duplicate all its parts
- Duplication takes place in interphase
- DNA (chromosomes) duplicated in the S subphase
- Entire chromosome is duplicated at the same time
- The duplicated chromosome remains attached to the original
chromosome by its centromere
- The original chromosome and its duplicated partner are called
sister chromatids
- In duplication the DNA strands separate ("unzip")
- DNA is a double helix (spiral) with the 2 strands held together
by hydrogen bonds
- In replication the 2 strands come apart and each acts as
a template (pattern) to form a new strand
- The coming apart ("unzipping") is made possible
because the strands are held together by hydrogen bonds
Chromosomes Must Be Tightly
Packaged for Division
- DNA must be tightly packaged for division- otherwise it would
tangle
- DNA is wound up on histones and other proteins
- Strands become 10,000 times shorter and much thicker (called
condensation)
- They become visible in microscope
- Condensation occurs in prophase
Mitosis is Used for Growth and
Repair
- Object of mitosis is to produce 2 identical cells (same number
of chromosomes)
- DNA duplicates and there is a single division, giving each
cell 23 pairs of chromosomes
- Some tissues must be repaired often: lining of gut, white
blood cells, skin- cell lifespan is only a few days
- Other cells do not divide at all after birth: nerve and muscle
- Red blood cells intermediate- lifespan is ~ 120 days
- General scheme of mitosis:
- DNA duplicates -> 2 sister chromatids
- Chromosomes attach to spindle and separate
- Used for growth, repair and reproduction (in single-cell
organisms)
- Makes 2 identical cells (each has the original number of
chromosomes)
- 2D = DNA content of diploid cell; 4D = amount after duplication
Meiosis is Used for Sexual Reproduction
- Object of meiosis is to reduce the number of chromosomes
to single copy of each (23 total)
- Used for making gametes: sperm and eggs (haploid)
- When a sperm fertilizes an egg to form a zygote the diploid
number of chromosomes is restored (23 + 23 = 46)
- In meiosis cells divide twice after a single DNA duplication
- General scheme of meiosis:
- DNA duplicates -> 2 sister chromatids
- Chromosomes attach to spindle & separate
- Two divisions
- First separates homologues
- Second separates sister chromatids
- Used for sexual reproduction
(makes sperm & eggs)
- Makes 4 haploid cells (each has half the number of chromosomes)
- 2D = amount of DNA in diploid cell; 4D = amount after duplication;
1D = amount of DNA in haploid cells (sperm & eggs)
- Be able to compare mitosis & meiosis.
Do the different mechanisms make sense in terms of the different
goals & uses? Discuss
your answer.
In Mitosis (and Meiosis) Chromosomes
are Separated by the Spindle Apparatus
- Spindle is formed of microtubule fibers between the 2 centrosomes
(see below)
- Chromosomes attach to spindle at centromeres
- Separation of chromosomes requires energy (ATP)
The Cell Division Cycle Has Five
Stages:
- Interphase:
- Longest phase: essentially the cell must duplicate all its
parts
- DNA replicates
- Proteins synthesized
- Centrioles duplicated
- Replicated chromosomes (sister chromatids) remain attached
by centromeres until anaphase
- Longest phase
- Prophase:
- Chromosomes condense, become visible
- Centrosomes move to opposite ends of cell
- Spindle forms
- Nuclear membrane dissolves
- Metaphase:
- Chromosomes attach to spindle fibers at their centromeres
- Line up in center of spindle apparatus
- Anaphase:
- Centromeres split, freeing the sister chromatids
- Chromosomes move toward centrosomes to opposite ends of cell
- Telophase:
- Cell cleaves to form 2 cells (cytokinesis)
- Nuclear membrane reforms
- The last 4 phases are called mitosis
- Mitosis = Prophase + Metaphase + Anaphase + Telophase
- (PMAT)
Cancer is Uncontrolled Mitosis
- Mitosis must be controlled, otherwise growth will occur without
limit (cancer)
- Control is by special proteins produced by oncogenes
- Mutations in control proteins can cause cancer
Summary of Mitosis:
|
|
Interphase: Duplication
of DNA, organelles, proteins. Nucleolus present in nucleus. Chromosomes
not visible. |
Early Prophase: Chromosomes
condense, become visible. Centrosome divides. Spindle starts
to form between the centrosomes. |
|
|
Late Prophase: Spindle
formed, with centro- somes at opposite poles. Nuclear membrane
dissolves. Chromosomes start to attach to spindle at their centromeres. |
Metaphase: Chromosomes
line up on spindle in center of cell. |
|
|
Anaphase: Centromeres
split apart. Then chromosomes move to towards centrosomes at
opposite poles of cell. |
Telophase/Cytokinesis:
Nuclear membrane reforms. Cell pinches into 2 cells in animals.
In plants a cell plate separates the 2 new cells. |
More Information
San Diego State University has a good
animated
tutorial of the cell cycle and mitosis.
You can control the speed of the movie by increasing or decreasing
the frames per second.
The University of Arizona has good tutorials
on both mitosis
and meiosis.
Test your knowledge of mitosis by identifying
dividing
onion root tip cells.
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