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Cell Division
Cell Division
All cells are derived from pre-existing cells.
Cell division is the process by which cells produce new cells.
Cell division differs in prokaryotes and eukaryotes.
Some tissues must be repaired often such as the lining of the gut, white blood cells, skin cells
with a short lifespan.
Other cells do not divide at all after birth such as muscle and nerve.
Reasons for Cell Division
o Cell growth
o Repair and replacement of damaged cell parts
o Reproduction of the species
Copying DNA
Since the instructions for making cell parts are encoded in the DNA, each new cell must get a
complete set of DNA molecules.
This requires that the DNA be copied (replicated) before cell division begins.
Chromosome Structure
Plans for making cells are coded in DNA
DNA is a long thin molecule that stores genetic information.
DNA in a human cell is estimated to consist of six billion pairs of nucleotides.
DNA is organized into giant molecules called chromosomes.
Chromosomes are made of protein and a long, single, tightly-coiled DNA molecule visible only
when the cell divides.
When a cell is not dividing the DNA is less visible and is called chromatin.
DNA in eukaryotic cells wraps tightly around proteins called histones to help pack the DNA
during cell division.
Nonhistone proteins help control the activity of specific DNA genes.
Kinetochore proteins bind to centromeres and attach chromosomes to the spindle in mitosis.
Centromeres hold duplicated chromosomes together before they are separated in mitosis.
Telomeres are the ends of chromsomes which are important in cell aging.
When DNA makes copies of itself before cell division, each half of the chromosomes is called a
sister chromatids.
DNA in prokaryotes is one, circular chromosome attached to the inside of the cell membrane.
Chromosome Numbers
Diploid vs. Haploid
o Diploid (2n) – cells with two of each kind of chromosome
o Haploid (n) – cells with one of each kind of chromsome
Human somatic (body) cells have 23 pairs of chromosomes or 46 chromosomes.
The two chromatids of a chromsomes pair are called homologues (have genes for the same trait
at the same location.
Production
Fertilization is the joining of the egg and sperm. It restores the diploid chromosome number in
the zygote (fertilized egg cell).
Sex chromosomes, either X or Y, determine the sex of the organism.
Two X chromosomes, XX, will be female and XY will be male.
All other chromosomes, except X and Y, are called autosomes.
Chromosomes from a cell may be arranged in pairs by size starting with the longest pair and
ending with the sex chromosomes to make a karyotype.
A human karyotype has 22 pairs of autosomes and 1 pair of sex chromosomes (23 total).
Genes
A section of DNA which codes for a protein is called a gene.
Each gene codes for one protein.
Human have approximately 50,000 genes or 2000 per chromosome.
About 95% of the DNA in chromosomes is “junk” that does not code for any proteins.
Cell Cycle
Cells go through phases or a cell cycle during their life before they divide to form new cells.
The cell cycle includes two main parts – interphase and cell division.
Cell division includes mitosis (nuclear division) and cytokinesis(division of the cytoplasm.
Interphase is the longest part of a cell’s life cycle and is called the “resting state” because the cell
isn’t dividing.
Cells grow, develop, and carry on all their normal metabolic functions during interphase.
Cell Division in Prokaryotes
Prokaryotes like bacteria do not have a nucleus.
They divide into two identical new cells by the process of binary
fission.
Binary fission is an asexual method of reproduction.
In binary fission, the chromosomes, attached to the cell membrane,
makes a copy of themselves and the cell grows to about twice its
normal size.
Next, a cell wall forms between the chromosomes and the parent cells
splits into two new identical daughter cells (clones).
Interphase
Interphase consists of three parts –
o G1 – cells mature and increase in size by making more cytoplasm and organelles while
carrying on normal metabolic activities
o S – follows G1; genetic material of the cell (DNA) is replicated (copied)
o G2 – occurs after S phase; the cell makes all the structures needed to divide.
Cell Division in Eukaryotes
Eukaryotes have a nucleus and membrane-bound organelles which must be copied exactly so the
2 new cells formed from division will be exactly alike.
The original parent cell and 2 new daughter cells must have identical chromosomes.
DNA is copied in the S phase of the cell cycle and organelles, found in the cytoplasm, are copied
in the G phases.
Both the nucleus (mitosis) and the cytoplasm (cytokinesis) must be divided during cell division
in eukaryotes.
Stages of Mitosis
Division of the nucleus or mitosis occurs first.
Mitosis is an asexual method of reproduction.
Mitosis consists of 4 stages – Prophase, Metaphase, Anaphase, and Telophase
Prophase
Chromosomes become visible when they condense into
sister chromatids.
Sister chromatids attach to each other by the centromere.
Centrioles in animals cells move to opposite ends of the
cell.
Spindle forms from centriole (animals) or microtubules
(plants).
Kinetochore fibers of spindle attach to centromere.
Polar fibers of spindle extend across cell from pole to pole.
Nuclear membrane dissolves.
Nucleolus disintegrates.
Metaphase
Chromosomes line up in center or equator of the cell
attached to kinetochore fibers of the spindle.
Anatphase
Kinetochore fibers attached to the centromere pull the sister
chromatids apart.
Chromosomes move toward opposite ends of the cell.
Telophase
Nuclear membrane forms at each end of the cell around the
chromosomes.
Nucleolus reforms.
Chromosomes become less tightly coiled and appear as
chromatin again.
Cytokinesis begins
Cytokinesis
Cytoplasm of the cell and its organelles separate into 2 new daughter cells.
In animals, a groove called the cleavage furrow forms pinching the parent cell in two.
In plants, a cell plate forms down the middle of the cell where the new cell wall will be.
Cancer – uncontrolled mitosis
Mitosis must be controlled, otherwise growth will occur without limit (cancer).
Control is by special proteins produced by ongogenes.
Mutations in control proteins can cause cancer.