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CH. 8 – CELL REPRODUCTION
8.1 CHROMOSOMES
CHROMOSOMES…
 are
DNA and the proteins that the DNA coils
around
 have 2 identical parts called chromatids or
sister chromatids


1/2 a chromosome = 1 chromatid
Halves are attached at the centromere region by
a kinetochore protein
 So
every chromosome is a long strand of
DNA coiled up around proteins and
attached to its copy
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2.
4.
3.
1.
6.
5.
7.
CHROMOSOMES…

exist only during cell division
Every piece of DNA must be copied before the cell
divides
 Chromosomes form as DNA copies itself before cell
division.

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WHAT DOES THE DNA LOOK LIKE IN A
NON-DIVIDING CELL?

If the cell is not dividing, DNA and proteins exist
in the nucleus as chromatin.


chromatin - thin, stringy DNA; not coiled
Why must the DNA be long and drawn out in a
non-dividing cell?

proteins must be able to move along DNA and
read/decode info used to direct activities
EACH CELL HAS 6’ OF DNA IN THE
NUCLEUS. HOW DOES IT FIT?
Supercoiling!
packaging of chromosomes with proteins
 coils within coils within coils, etc.


DNA is measured in
nanometers (nm) which
are 1 billionth of a
meter or 1x 10-9 meters.
How small is that?
If 1 nanometer = 1
second, then 1 meter =
1 billion seconds or 31.7
years!!!!
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TWO CATEGORIES OF PROTEINS
ASSOCIATED WITH DNA:

Histone proteins
 are
used for shaping and tightly
packing the DNA

Non-histone proteins
 are
not involved in the packing
involved in controlling activity
of specific regions of DNA
 are
DIFFERENCES IN PROKARYOTIC AND
EUKARYOTIC DNA

Eukaryotes
1. multiple chromosomes
2. linear or rod-shaped
3. found in nucleus

Prokaryotes
1. single chromosome
2. circular
3. found in a nucleoid
region
CHROMOSOME NUMBERS
 Each
species has a certain number of
chromosomes
 Is the number of chromosomes related to the
complexity of the species?

NO! CHROMOSOME # HAS NOTHING TO DO
WITH COMPLEXITY!

Exp. Humans have 46 chromosomes, dogs have 78
See p. 152 for various species

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THERE ARE TWO TYPES OF
CHROMOSOMES FOUND IN THE
NUCLEUS OF A CELL:
Sex Chromosomes - chromosomes that have
genes on them used to determine the sex of the
organism.
In humans, they are either X or Y – the names
have nothing to do with their shape!
 Autosomes - all other chromosomes in the
nucleus

HOMOLOGOUS CHROMOSOMES
 Every
organism produced by sexual
reproduction receives two copies of each
chromosome, one set comes from mom the
other from dad
 The two similar chromosomes are called
homologues, homologous
chromosomes, or a homologous pair
 homologous chromosomes are the same
size and have the same centromere region
and banding pattern
KARYOTYPE

What are they?

A photomicrograph of chromosomes in a dividing cell



Made from nucleated blood cells
Sample is chemically treated to stimulate mitosis, then
arrested in metaphase.
How are they made?
 Chromosomes are photographed, cut out, and
arranged into pairs by size and shape beginning
with the longest pair and working toward the
smallest; the sex chromosomes are last!
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DIPLOID NUMBER VS. HAPLOID
(THESE WORDS
CELL)

REFER TO CHROMOSOME NUMBER IN THE
Diploid: represented by 2n
Organisms that reproduce sexually have two sets of
chromosomes in every somatic cell – one set from
each parent
 the given chromosome number is the 2n number
 sex chromosomes are part of this number; they are
not counted any differently!


Haploid: represent by n

→
number of chromosomes in a gamete (sex cell
egg, sperm); the haploid number must be half the
diploid
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
In humans:

46 chromosomes; this means 2n = 46
23 total pairs
 22 pairs (or 44) are autosomes
 1 pair (or 2) are sex chromosomes
 n = 23; humans only have one set of chromosomes in every
sex cell; 22 autosomes + 1 sex chromosome

EXAMPLES
Organism
2n
n
Humans
46
23
# of
homologous
pairs
23
Dogs
78
39
39
Lettuce
18
Gorillas
48
9
Earthworms
18
Cats
16
7