CELL DIVISION
Chromosomes



Made of DNA
Carries cell genetic
information
23 pair
 Differs
for different
organisms
 fruit fly: 8
 Carrot: 18
chromosomes






Before a cell can divide it must duplicate DNA
Each chromosome consists of identical sister
chromatids
Since they are identical, where will each one go?
One goes to each new cell
Each pair of chromatids are attached at centromere
Usually located near the middle of chromosome
Cell cycle





Series of events that cells
undergo as they grow
and divide
During cell cycle the cell:
Grows
Prepares for division
Divides to form 2
daughter cells, each of
which will re-enter into
cell cycle
Cell cycle

Interphase: “in-between” period of growth
 Time

1.
2.
3.
spent Between divisions
Cycle consists of 3 phases:
G1
S
G2
Cell cycle: G1 (growth)


Most of the growing
Synthesize proteins
Cell cycle: S phase



Chromosomes get
replicated
Synthesis of DNA
When completed, cell
completes remainder
of cycle
Cell cycle: G2




Shortest of phases
Cells prepare for
division
Organelles reproduced
Molecules needed for
replication created
Mitosis


1.
2.
3.
4.
Mitosis: process in cell
division where nucleus
divides
Divided into 4 stages
Prophase
Metaphase
Anaphase
Telophase
prophase

1.
2.
3.
4.
5.
First and longest phase
Chromosomes become visible
Centrioles separate and move to poles
Centrioles help organize mitotic spindle (spindle
apparatus): fanlike microtubule structure that helps
separate chromosomes
Chromosomes attached to fibers in spindle at the
centromere
Nuclear envelope breaks down
metaphase
1.
Chromosomes line up in middle of cell
anaphase
1.
Each sister chromatid separates and becomes an
individual chromosome
 Centromeres
split
 Chromosomes continue to move until they are in
opposite poles
telophase
1.
2.
3.

Chromosomes uncondense
Spindle breaks apart
Nuclear envelope reforms
Essentially, the opposite of prophase
cytokinesis


Separation of cytoplasm and its contents
In animal cells, membrane is squeezed in the middle
until 2 cells are formed
 Cleavage


furrow
In plant cells, a cell plate will form
This will eventually become the cell wall
Uncontrolled cell growth




Cancer
Cancerous cells do not respond to signals that
regulate growth of most cells
Divide uncontrollably  tumor
Cells can break lose and travel to other parts of
body
 Metastizing


Causes can be genetic or environmental
Effects depend on affected tissue
Chromosome number review







Humans have how many chromosomes?
23 pairs of homologous chromosomes
We get half from mom and the other half from dad
23 + 23 =46
Diploid cell: contains 2 sets of chromosomes
Represented as 2n (n=number of chromosomes of
organism)
Diploid cells contain 2 complete sets of genes
Chromosome number







Haploid cells: contain only a single set of genes
Represented as n (n= number of chromosomes)
Diploid number for humans? 2n=?...
2n=46
Haploid number in humans? N
If 2n=46, then n=
n=23
Haploid cells







Haploid cells are found in sexually reproducing
organisms
Each parent offers on set of their own DNA (n)
Combination of gametes (sex cells) n + n = 2n
In humans:
Sperm (n) has 23 chromosomes
Egg (n) has 23 chromosomes
They combine to form a diploid zygote (2n)
Section 10-3 Meiosis


Meiosis: process of reduction division in which the
number of chromosomes per cell is cut in half
through the separation of homologous chromosomes
in a diploid cell.
***Very similar to mitosis only with an extra set of
phases
Meiosis

Composed of two distinct divisions
 Meiosis
I
 Meiosis II


Begins with a single diploid cell
Ends with 4 haploid cells
Homologous chromosomes

Homologous chromosomes: pairs of
corresponding chromosomes from each parent
 Ex.
# 12 from mom matched up with # 12 from dad
Meiosis 1:




Prior: chromosomes replicated
Meiosis I: division of Homologous chromosomes
Results in 2 haploid daughter cells
Prophase I, Metaphase I, Anaphase I, Telophase I
and cytokinesis
Prophase I:



Homologous chromosomes
pair up (one from each
parent) to form a tetrad
(structure formed when
homologues come together)
Crossing over occurs: results
in the exchange of genetic
information between
homologous chromosomes
Produces new combinations
of genes
Crossing over



Occurs during Prophase I
Overlapping arms of
homologues have same
information and can
swap info at any point
along their length
*****increases genetic
variation*****
Metaphase I


Spindle fibers attach to
centromere
Tetrads (pair of
homologous
chromosomes) line up in
middle of cell
independent of each
other
Anaphase I

Spindle Fibers pull
homologous chromosomes
toward opposite ends of the
cell
Telophase I and
Cytokinesis



Nuclear membranes
reform
Cell separates to
become two cells
*Cells produced have
different genetic
composition than the
parent cell
Meiosis II



Meiosis II: paired chromatids divide
Each cell now has 2 chromatids
Almost exactly like mitosis!!
Prophase II



Nothing really happens
Chromosomes are
already condensed and
compacted
Nuclear envelope goes
away
Metaphase II

Chromosomes line up
similar to mitosis
Anaphase II

Paired chromatids
separated
Telophase II and
cytokinesis

Four haploid cells have
been created
overview
2n
Meiosis I
n
Meiosis II
n
Gamete formation



Gamete: cell designed
for sexual reproduction
Example: sperm and
egg
Males make four
sperm while females
make one