1. Chapter 10: Cell Growth & Division
Section 2: The Process of Cell Division

2. Chromosomes
DNA (deoxyribonucleic acid) – a molecule that stores genetic information, which controls the development and functioning of living things
double helix shape (looks like a twisted ladder)
In a eukaryotic cell (cell that has a nucleus), DNA is coiled into chromosomes

3. Chromosome Structure
During cell division, chromosomes are “X” shaped structures made of coiled-up DNA
Chromosomes are intended to organize and compact DNA
DNA is formed into a double helix
Imagine 2 strings twisted around each other.

4. Chromosome Structure
In order for DNA to fit within a cell the double helix is wrapped around protein balls called histones
The DNA and histones are formed into coils, which are formed into supercoils
The supercoils are again coiled to form a chromosome

5. Chromosome Structure The chromosome is shaped like an X
Chromatids
Joined strands of duplicated genetic material
The left and right halves
The center where the two sister chromatids are held together is the centromere
When a cell divides, the chromosome splits at the centromere
The chromatids separate
Each chromatid goes to one of the new cells

7. Chromosome Structure
When cells aren’t dividing, DNA is not coiled into chromosomes
They are uncoiled so that the genetic information may be easily read and used
This uncoiled form of DNA is called chromatin; substance made up of DNA & protein

8. Chromosome Structure
Prokaryotes usually only have 1 circular chromosome
Since they don’t have a real nucleus, the chromosome is often attached to the cell membrane

9. Chromosome Numbers
Different species have different numbers of chromosomes
The number of chromosomes has little or nothing to do with the complexity of the species
Humans have 46 chromosomes
Gorillas and chimpanzees have 48
Dogs have 78

10. Chromosome Numbers
Animals have 2 types of chromosomes
Autosomes contain genetic information for the individual organism
Sex Chromosomes determine the sex of the organism
They may also contain some genetic information
Humans have 2 sex chromosomes and 44 autosomes

11. Chromosome Numbers
Every cell of an organism that is produced by sexual reproduction has 2 copies of each autosome
One autosome came from mom, the other came from dad
The twin chromosomes are called homologous chromosomes, or homologues
Homologous Chromosomes code for the same traits

12. Chromosome Numbers
Homologues are the same size and shape, and they carry the same genes
The genes may not be coded the same!!!
Mom may give you a blue-eyed gene, but dad may give us a green-eyed gene
The way our bodies choose which gene to follow is complicated. We’ll see how it works later.

13. Chromosome Numbers Chromosomes are often seen in a karyotype
A karyotype is a picture of chromosomes
It is taken from a cell that is getting ready to divide
The homologues are paired together
The chromosomes are ordered and numbered so they are easy to examine

14. Karyotype

15. Chromosome Numbers
Normal cells are diploid, meaning they have the 2 sets of chromosomes
They are abbreviated as 2n
In humans, diploid(body or somatic) cells have 46 chromosomes

16. Chromosome Numbers
Reproductive cells/gamates (sperm or egg cells) are haploid, meaning that they have 1 set of chromosomes
They are abbreviated as 1n
This is necessary so that, during reproduction, offspring end up with the correct amount of chromosomes
In humans, haploid(sex) cells have 23 chromosomes

17. Cell Division in Prokaryotes
Prokaryotic cells divide to form offspring through binary fission
The chromosome copies itself, forming 2 rings
The cell grows until it is almost double in size
The cell membrane pinches the cell in half, and 2 new cells are formed
Each of the 2 new cells is identical to the parent

18. Binary Fission

19. Cell Division in Eukaryotes
There are 2 types of cell division in eukaryotes
Mitosis results in new cells identical to the parent cell
This occurs in reproduction of unicellular organisms
This results in growth of multicellular organisms
Meiosis results in sex cells with half the normal number of chromosomes

20. Cell Division in Eukaryotes
The Cell Cycle is a set of events that repeat in the life of a cell
Cell division is a small part of the cycle
The time between divisions is called Interphase
The cell carries out its activities
The cell grows and prepares for division
Cells that seldom divide spend most of their time in this phase (interphase)

21. Cell Division in Eukaryotes
Interphase
G1 Phase: cells grow to mature size; normal activities are also carried out during this stage which is why G1 is the longest phase of interphase.
S Phase(Synthesis): cell DNA is copied/replicated
G2 Phase: cells prepare for division by producing organelles and molecules needed for division
Centrosome: place where spindle fibers are produced
G0 Phase: some cells exit the cell cycle and never divide
This usually happens after G1 Phase. (does not always happen)
Ex. – nerve cells

22. Most of the time is spent in G1

23. Cell Division in Eukaryotes
Mitosis results in new cells identical to the parent cell
There are 4 parts to mitosis:
Prophase
Metaphase
Anaphase
Telophase

24. Cell Division in Eukaryotes
Prophase:
DNA forms into chromosomes
The nucleus breaks down and disappears
Dark spots (centrosomes) form at the poles; organelle that helps assemble the spindle & aids chromatid movement in a dividing cell
Animal cells have additional cylinders called centrioles

25. Cell Division in Eukaryotes
Prophase continued:
The mitotic spindle (made of spindle fibers) forms
Kinetochore fibers attach to the chromosomes’ centromeres (center) at the kinetochore
These are like ropes that will attach to the chromosomes and pull them apart
Polar fibers extend across the dividing cell between the centrosomes (dark spots at the poles)

26. Cell Division in Eukaryotes
Metaphase:
The kinetochore fibers move the all the chromosomes to the cell equator
This is when the chromosomes are easiest to identify, so karyotypes are usually taken during this phase

27. Cell Division in Eukaryotes
Anaphase:
The chromatids of each chromosome separate
The chromosomes begin to move toward the poles
When chromatids separate, they are considered to be individual chromosomes; thick strand made up of condensed DNA

28. Cell Division in Eukaryotes
Telophase:
The chromosomes reach opposite ends of the parent cell
The spindle fibers disappear
Chromosomes uncoil and disappear
A nucleus forms around each set of chromosomes
Cytokinesis
The cell divides through cytokinesis
Separating the organelles and the cytoplasm of the cell
Animal cells form a cleavage furrow which pinches the cytoplasm in two & forms 2 cells; 2 daughter cells
Plant cells form a cell plate between the 2 forming cells

29. The Cell Cycle
Cytokinesis

31. Cell Cycle Sequence G1SG2MitosisCytokinesis
G1, S, & G2 (the first three phases) together make up Interphase

32. Importance of Mitosis
growth
repair
replenish cells that have died