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Part II
CHROMOSOMES…
-complexes of DNA and protein.
-Chromosomes are the package for
DNA and genes.
-duplicated before the cell divides.
-chromosome number is specific for
each species. A human cell has 46 [or
23 pairs] while a fruit fly has 8 [or 4
pairs].
centromere
sister chromatids
Cells in the body of sexually producing
organisms can be…
SOMATIC CELLS
-body cells
-contain the DIPLOID [2n]
chromosome number.
-produced through MITOSIS.
-examples: muscle cell, nerve
cell, stomach cell.
GAMETES
-sex cells
-contain the HAPLOID [n]
chromosome number.
-produced through MEIOSIS.
-examples: sperm cell and egg
cell
Fertilization restores the
diploid number.
+
Egg is “n” =
haploid
Sperm is “n”
= haploid
ZYGOTE is
2n.
Repeated mitosis
ultimately leads to the
development of multicellular organism.
A picture of all the
chromosomes in a somatic
cell is a karyotype.
A human karyotype
reveals 23 pairs of
chromosomes. Each pair
is called a homologous
pair.
The 23rd pair is easy to
spot [unequal in size].
They are the sex
chromosomes, called X
and Y.
Organism
Organism
Human
Chromosome
#
46
Corn
Chromosome
#
20
Yeast
32
Onion
16
Cow
60
Lily
24
Dog
78
Donkey
62
Chicken
78
Earthworm
36
Toad
36
Pig
38
CELL CYLE
A cell goes
through a
cell cycle,
going
through
stages of
growth
and
division.
Stages of the Cell Cycle
1. Interphase: sometimes called the “resting
phase.” This refers to the stage in the life of a
cell when it is not dividing.
-made up of these sub-stagesa. G1- cells grow in size, cells get ready for DNA
synthesis.
b. S-phase- DNA replication occurs at this
stage.
c. G2- cell continues to grow and make proteins.
2. Mitosis: cell divides to produce two
daughter cells with the same
chromosome number as the parent
cell. This stage takes only from 1-2
hours.
Important points to remember:
a. Chromosomes condense for easy separation [no tangling].
b. Spindle fibers attach to the centromeres, controlling chromosome
movement.
c. Sister chromatids separate during anaphase.
d. Cytokinesis [division of cytoplasm and other cell contents] happen
during telophase.