1. Lecture #13 Honors Biology Ms. Day
Chromosomes
Lecture #13
Honors Biology
Ms. Day

2. Why is Cell Division Important?
Unicellular organisms
Reproduce by cell division  increasing the population.
100 µm
(a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM).
Figure 12.2 A

3. Why Do Multicellular Organisms Depend on Cell Division?
Development & Growth
Repair (ex: tissue renewal)
Maintenance
20 µm
200 µm
(b) Growth and development This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM).
(c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).

4. Cell Division (called Mitosis)
Makes 2 genetically identical daughter cells from 1 parent cell (think= mi “two” sis)
Before cells divide
They duplicate their DNA  so each daughter cell receives an exact copy of the genetic material (DNA)

5. Organization of the Genetic Material
All of a cell’s DNA (genetic information) is called its genome
In prokaryotes
Genome=single, long DNA molecule in a circle
In eukaryotes
Genome = several DNA molecules grouped in clumps (called chromosomes)

6. DNA Molecules DNA can be in 2 forms
Chromatin : DNA is not tightly packed together (loosely coiled; decondensed)
Occurs during interphase
Think = chromatin
Chromosomes : tightly packed together (TIGHTLY coiled; condensed)
Occurs during mitosis
Think = chromosomes

8. How Does DNA Fit into a Small Cell?
Prokaryotic organisms
No nucleus; instead have a nucleiod region
DNA = 1 chromosome

9. How Does DNA Fit into a Small Cell?
Eukaryotic organisms split their DNA into several pieces (called chromosomes)
stored within the cell's nucleus
DNA is coiled around proteins known as histones
Uses nucleosomes to form a “supercoil”

10. DNA + histones  form nucleosomes (help to “super coil DNA)

11. Making Chromosomes **Occurs right before cell division (mitosis)
Steps:
DNA copies itself through DNA Replication
The DNA coils (wraps) around proteins called histone proteins.
Chromosomes are made.

12. Two shapes of chromosomes Occurs right before cell division (mitosis)
Chromosome Structure
Two shapes of chromosomes
Single chromosome (1 copy of DNA); “V” shaped
Double Chromosome (2 copies of DNA); “X” shaped; a.k.a-duplicated chromosome
Occurs right before cell division (mitosis)

13. Chromosomes and Cell Division
In preparation for cell division (mitosis)
DNA is replicated (single  double chromosome)
V shape  X shape

14. Separation of sister chromatids
Each duplicated chromosome has two sister chromatids, which separate during cell division
0.5 µm
Chromosome duplication (including DNA synthesis)
Centromere
Separation of sister chromatids
Sister chromatids
Centromeres
A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule.
Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the DNA molecule.
Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells.
Figure 12.4

15. Double Chromosome Structure
Kinetochore attaches to spindle fibers
or RNA
Sister

16. Draw and Label the Pieces of a Duplicated Chromosome
chromatid
Sister chromatids
kinetochore
Spindle fibers
centromere
Gene band
(Duplicated) chromosome

17. Think: “t” for TWO chromatids
and
“d” for DUPLICATED

18. Chromosomes
Every eukaryotic species has a characteristic, unique # of chromosomes in EACH cell nucleus
Ex: Humans = 46 chromosomes Adler Fern (plant) = over 2000 C’s
# of chromosomes does NOT necessarily equal complexity