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Cell Division Unit
Chapter 10 Section 2
Chapter 11 Section 4
Process of Making a Chromosome
Chromosome
Nucleosome
DNA
double
helix
Coils
Supercoils
Histones
Chromatin – DNA tightly coiled around proteins called Histones
Nucleosome – beadlike structure made of chromatin
Supercoils – nucleosomes packed together
GoChromosome
to
Section:
– tightly packed supercoils
Chromosomes

Made of DNA and Histones (protein)

Function :
- passes genetic info. from 1 generation to the next

Genes- located on chromosomes
- information about traits are located here

Traits – characteristics passed on by your parents
Chromosomes Numbers

Every organism has a specific # of
chromosomes
 Humans

=
46 chromosomes
The # of chromosomes DOES NOT indicate
the COMPLEXITY of an organism
Example – a goldfish has 124 chromosomes
Structure of Chromosomes

Chromosomes are replicated (copied) before
cell division (when a new cells are
developed)

This makes them visible because they
contain TWICE the amount of DNA
Structure of Chromosomes

Each chromosome consists of:
* CHROMATIDS – identical parts
- called “sister chromatids”
* CENTROMERE – point at which each
chromatid are attached to each other
chromatid
centromere
Structure of Chromosomes (cont.)
the cell divides into 2 new cells –
the “sister chromatids” separate and
each one goes into a new cell
 When
 Each
chromosome consists a pair
of chromatids (2)
 Humans = 46 chromosomes
= 92 chromatids
Cell Cycle
The series of events that cells go through as
they grow and divide.
 During this cycle the cell:
1. grows
2. prepares for division
3. divides to form 2 “daughter”
(identical) cells
Then each daughter cell prepares to
begin the cycle again

Chromosome Quiz
1.
2.
3.
4.
5.
Name the 2 substances that make up
chromosomes. (2 points)
How many chromosomes do humans
have? (1 point)
What is the function of chromosomes?
(1 point)
At what point are chromosomes visible in
the nucleus? Why? (2 points)
Draw and label a chromosome. (4 points)
Events of the Cell Cycle
Cell Cycle
includes
G1 phase
Go to
Section:
Interphase
M phase
(Mitosis)
is divided into
is divided into
S phase
G2 phase
Prophase
Metaphase
Anaphase
Telophase
Interphase

Longest phase of the cell cycle

Known as the “in-between” period of growth

Divided into 3 Phases: G1, S, and G2
G1 (Gap 1) Phase

Cells do most of their growing during this
phase

Cells increase in size

Manufacturing of new proteins & organelles
S (Synthesis) Phase

Chromosomes are replicated

Synthesis of DNA molecules & Histones occur
G2 (Gap 2) Phase

Shortest of the phases

Organelles & molecules needed for cell
division are produced

When this phase is complete, cells are ready
for the M Phase (mitosis) and begin the
process of cell division
Mitosis

The 1st stage of cell division in eukaryotic cells

Depending on type of cell, this phase can last
from a few minutes to several days.

Divided into 4 phases
Prophase





The 1st & longest phase of
Mitosis
(50 – 60%of whole process)
Chromatin condense to form
chromosomes
Centrioles separate and move
to opposite sides of the nucleus
structures called SPINDLES
begin to form – they help to
separate chromosomes
Nucleolus disappears & nuclear
envelope breaks down
Metaphase

Often last only a few
minutes

Chromosomes line up
across the center of
the cell

Each chromosome is
connected to a spindle
fiber at its centromere
Anaphase

The spindle fibers pull
apart the sister chromatids

Each chromatid becomes
its own chromosome
and move towards the
centrioles at opposite ends
of the cell

The phase ends when the
chromosomes reach the
centrioles & stop moving
Telophase

Chromosomes lose their
distinct shapes

Nuclear envelope re-forms
around each group of
chromosomes

Spindle fibers disappear & a
nucleolus becomes visible in
each new nucleus

Mitosis is complete, but cell
division is not
Cytokinesis

Cytoplasm pinches in half

Each daughter cell has an
identical set of chromosomes
from the parent cell

Usually occurs at the same
time as telophase

Cell division is not complete
until cytokinesis is complete
Overview of Mitosis
Cell Cycle Quiz 5-3-07 – 16 points
1.
2.
3.
4.
5.
Name the 3 phases of Interphase in order.
State the events that occur in each phase.
(4 points)
What organelles are responsible for cell
division? (1 points)
What is the longest phase in MITOSIS?
(1 point)
What event ends cell division? (1 points)
What is the final result of Mitosis? (1 point)
6. Label each picture with the correct phase of
mitosis (8 points)
A
B
D
C
Assignment

Complete the following questions in your
notebook:

chapter assessment p. 257
#’s 3-7, 15-21, 28-34
Homologous Chromosomes


Chromosomes that
are in all sexually
reproducing
organisms
Occur in pairs
2 identical chromosomes
= homologous
chromosomes


same size and shape.
are different than all
other chromosomes
Diploid Cells

Diploid Cells – contain both chromosomes of
a homologous pair
Human cell = 23 pairs of homologous
chromosomes
2 x 23 = 46
Human Chromosome # = 46
Haploid Cells

Haploid Cells – contain only 1 chromosome
of a homologous pair
23 pairs of homologous chromosomes
1 x 23 = 23
**all egg & sperm cells = 23 chromosomes
ALSO KNOWN AS GAMETES
(egg)23 + (sperm) 23 = 46 chromosomes – human baby

Zygote – a new cell formed by a sperm & egg cell
- has the normal number of chromosomes
MEIOSIS

The process of reducing the number of
chromosomes per cell in half

Separating homologous chromosomes in
a diploid cell

Making a diploid (46) into a haploid (23)
Process of Meiosis

Involves 2 distinct divisions:
Meiosis I
Meiosis II

Begin with 1 diploid cell (46)

End with 4 haploid cells (23)
Meiosis I

Looks like MITOSIS

But in Meiosis I, each chromosome is paired
with it corresponding HOMOLOGOUS
CHROMOSOME

This forms a TETRAD – 4 chromatids
( 2 from each chromosome)
Crossing Over
TETRAD
•When homologous chromosomes pair up to make
tetrads – they exchange portions of their chromatids
•This is called CROSSING OVER
•This results in the exchange of genes between the
homologous chromosomes
•This produces new combinations of genes
RESULTS OF MEIOSIS I
The chromosomes go through the same
phases as Mitosis
 Produces 2 new cells with 46 chromosomes
each
BUT ………………..
1. the new “daughter” cells DO NOT have
identical sets of chromosomes like the parent

2. are different from each
- because of crossing over & the separation
of the homologous chromosomes
Meiosis II

The 2 cells from Meiosis I enter this
2nd division

Unlike Meiosis I – the 2 cells …..
do not replicate the chromosomes

The cells go through each phase just like in
Meiosis I
Results of Meiosis II

4 daughter cells that are haploid cells

Have only half of the # of chromosomes

All are different from each other

All are different from its parent cell
Gamete Formation
The making of reproductive cells
 MALE GAMETE = SPERM CELLS


Through meiosis – males produce 4 new
sperm cells

FEMALE GAMETE = EGG CELLS (OVUM)

Through meiosis – females produce only 1
egg cell and 3 polar bodies (incomplete)
Gregor Mendel

Read handout and section 11-1
Probability & Punnett Squares

When Mendel realized when he crossed the
pea plant to make hybrids he always had the
same result :
75 % - dominant (3/4)
25% - recessive (1/4)

He realized that the Principle of Probability
could be used to explain the results of genetic
crosses.
Probability
The likelihood that a particular event will
occur
 Example – flipping a coin
 What is the probability that the coin will land
on heads?
 What is the probability that the coin will land
on tails?
 This relates to genetics because of the way
alleles segregate is completely random, like
a coin flip.

Punnett Square





Used to predict & compare
the genetic variations that
will result from a cross.
Shows the results of
Mendel’s experiment
The letters inside &
outside represent alleles
Outside = parents
Inside = possible offspring
T
t
T
TT
Tt
t
Tt
tt
Punnett Squares (cont.)

Dominant allele – represented by
CAPITAL LETTERS (T)

Recessive allele – represented by lower
case letters (t)
Homozygous alleles
2 identical alleles
 Know as the “True Breed”


Can be:
1. TT – homozygous dominant
2. tt – homozygous recessive
Heterozygous Alleles
2 different alleles
 Known as “hybrids”


Can be only:
1. Tt
Phenotype

Physical characteristics

What you see
Genotype
Genetic make up
 What is located on DNA
 Tt , TT , or tt
 Tt & TT = tall
- each has the same phenotype but
have different genotypes

Punnett Square Activity

Create Punnett Square for the following crosses:
1. MM x mm
6. Bb x bb
2. Pp x Pp
7. FF x Ff
3. AA x AA
4. hh x HH
5. rr x rr
For each one write the results below each one
Mendel & Punnett Square Quiz – 20 points
1. Define Genetics.
2. Define Heredity.
3. Define Gene.
4. Define Allele.
5. State the Principle Of Dominance.
6. The dominant allele is represented by _____.
7. The recessive allele is represented by ___.
8. Create a Punnett Square for each of thr
following. Make sure you state all
information for each one:
Gg x GG AA x aa Rr x Rr Bb x bb