Chromatin—Chromatid—Chromosome

Relaxed form of genetic material

Chromatids are identical to each other,
called sister chromatids.

These chromatids are held together by a
centromere forming a chromosome

Chromosomes – condensed segments of
protein and DNA, our genes.
Chromosomes are made
of two chromatids.

Chromosome number differs between
species.
 Number of chromosomes each
organism has will be unique.

Examples : Humans 46
 Corn – 20
 Housefly- 12
 Elephants - 56

Humans contain 46 chromosomes or 23
pairs
n – hapliod(23)
2n –diploid(46)


Organism
Pea
Frog
Chicken
Diploid
14
26
?
Haploid
?
?
39

Autosomes – all chromosomes, not
including sex chromosomes

Sex Chromosomes- chromosomes that
determine sex
Example: Humans – 44 autosomes ; 2 sex
chromosomes
 XX Chromosome – Female
 XY Chromosome - Male


Somatic Cells – the body cells, Not sex
cells. Somatic cells contain 2n (diploid)
number of chromosomes

Sex cells – the gametes. Egg or sperm
cells. Sex cells contain n(hapliod)
number of chromosomes
› Gametes – sex cells ex. Egg or sperm
What is asexual reproduction?
 What method of reproduction will
prokaryotes go through? What is this
called?


What is the purpose of cell division/cycle
in eukaryotes?

What must be replicated before cell
division can start?

What is the cell cycle?
1) What are the 3 stages of interphase?
A type of Cell Division that results in 2
identical daughter cells
 Occurs in somatic cells
There are 4 Stages of Mitosis (PMAT)
- Prophase
- Metaphase
- Anaphase
- Telophase

Chromatin condenses to form visible
chromosomes
 Spindle fibers begin to form
 Nuclear Membrane Disappears


The chromosomes
move to the middle
of the cell aligning
along the equator.

Sister chromatids will
separate and move
to opposite ends of
the cell.



Spindle fiber break
apart.
Nuclear membrane
reforms.
Chromosmes
decondense



Last stage of cell cycle
In animal cells a
cleavage furrow will split
the cells into 2 daughter
cells.
In plant cells, a cell wall
or cell plate will form
between the two cells.



After cytokinesis,
there are now 2 cells
present.
The 2 daughter cells
are identical to the
parent cell.
Now both cells will
move into interphase
to repeat cell
division

The daughter cells
will have the same
amount of
chromosomes as the
parent cell!!!
Meiosis is the forming of sex cells
(gametes).
 Gametes are egg and sperm cells.

Meiosis involves Two divisions :
Meiosis I
Meiosis II
It starts off with a 2n cell and results in 4 (1n),
cells.
 4 haploid Cells


The formation of sperm is called
Spermatogenesis

The formation of egg is called oogenesis
Spermatogenesis occurs in the testes of
males
 Oogenesis occurs in the ovaries of
females

In humans the cell will start off with 46
chromosomes and each daughter cell
will have 23 chromosomes.
 Daughter cells will be non-identical to
the parent cell.


Non-identical because crossing over
occurs, creating genetic diversity!!!!
Homologous Pair- pair of chromosomes
with the same length, same centromere
position, carry same genes that control the
same traits BUT possibly different alleles
Crossing Over- process where
chromosomal segments are exchanged

-
Prophase I
Involves homologous chromosome pairs,
tetrad
Crossing over will occur in this phase
Nuclear membrane will disappear
Metaphase I
- Homologous chromosome pairs move to the middle of
the cell


-
Anaphase I
Homologous chromosomes will separate and move
to opposite ends of the cell.
-
Telophase I
Nuclear Membrane Reforms
Chromosomes Decondense

Cytokinesis – Cell splits

-

Recall before division the cell must
replicate its DNA in the S phase of
Interphase.

However, after Meiosis I the 2 cells will
move directly into Meiosis II.

-
Prophase II
Nuclear Membrane disappears
Chromosomes decondense
Metaphase II
- Chromosomes align in middle of cell

Anaphase II
- Chromatids separate
- Move to opposite ends of the cell

Telophase II
- Nuclear membrane reforms, chromosomes
decondense
 Cytokinesis
Cell will split – Now 4 , non identical cells
results
