1. Chapter 6: Chromosomes and Cell Reproduction
Biology II

2. Cell Division
Cell division occurs at different times in an organism’s life
Growth and development
Repair
Reproduction
Gametes – reproductive cells (egg/sperm)
When a cell divides, DNA is first copied and then distributed so that each cell ends up with a copy of DNA

3. Prokaryotic Cell Division
Prokaryotes have single circular DNA molecule
Reproduce by binary fission
Form of asexual reproduction that produces identical offspring
Occurs in 2 stages
DNA is copied
Cell divides by adding a new cell membrane to a point on membrane between the 2 DNA copies

4. Genes Information encoded in DNA is organized into units called genes
Segment of DNA that codes for a protein or RNA molecule
Single DNA molecule has 1000s of genes linked together
Genes play important role in determining how an organism’s body develops and functions
When genes are being used, the DNA is stretched out so that its information can be used to direct protein production

5. Eukaryotic Cell Division
As eukaryotic cell prepares to divide, chromosomes become visible
DNA and proteins associated with DNA
Before DNA coils, DNA is copied, forming chromatids
2 exact copies of DNA that make up each chromosome
Attached at a point called a centromere
Chromatids separate during cell division and are placed into each new cell, ensuring that each new cell will have same genetic information as the original cell

6. Homologous Chromosomes
Each human somatic (body) cell has 2 copies of 23 chromosomes (total of 46 chromosomes)
Each pair is made up of homologous chromosomes
Similar in size, shape, and genetic content
Each homologue comes from one of the 2 parents
Differ in size, shape, and sets of genes
Each contains 1000s of genes

7. Sets of Chromosomes
All cells in the body, other than gametes, are somatic cells
Said to be diploid (2n), since they contain 2 sets of chromosomes
Symbol “n” is used to represent 1 set of chromosomes (human diploid #: 2n = 46)
Gametes are said to be haploid (n), since they contain only one set of chromosomes
Fusion of 2 haploid gametes, known as fertilization, forms a diploid zygote
Fertilized egg cell

8. Numbers of Chromosomes
Each organism has a characteristic number of chromosomes
Number of chromosomes remain constant within each species

9. Sex Chromosomes
Humans have 22 pairs of autosomes and 1 pair of sex chromosomes
Autosomes – chromosomes that are not directly involved in determine the gender of an individual
Sex chromosomes – contain genes that will determine sex of individual
Referred to as X and Y chromosomes in humans and many other organisms
Sex of offspring is determined by male
XX: Female
XY: Male
In some insects (grasshoppers), females are XX and males are XO (O = absence of chromosome)
In birds, moths, and butterflies, male is XX and female is XO

10. Karyotypes
Presence of normal number of chromosomes is necessary for normal development and function
Abnormalities in chromosome number can be detected by analyzing a karyotype
Photo of the chromosomes in a dividing cell that shows the chromosomes arranged by size

11. Trisomy and Nondisjunction
Trisomy is a condition in which humans have more than 2 copies of a chromosomes
Occurs if one or more chromosomes fail to separate properly, an event known as nondisjunction
Trisomy 21 is known as Down’s syndrome
Characterized by short stature, round face with upper eyelids that cover inner corners of eyes, and varying degrees of mental retardation
More likely in pregnancies of older women because eggs can accumulate increasing amount of damage over time

12. Change in Chromosome Structure
Changes in organism’s chromosome structure are called mutations
Breakage of chromosomes can lead to 4 types of mutations
Deletion mutation – piece of chromosome breaks off completely
Duplication mutation – chromosome fragment attaches to its homologous chromosome
Inversion mutation – chromosome piece reattaches to original chromosome but in reverse orientation
Translocation mutation – chromosome piece reattaches to a nonhomologous chromosome

13. Section 6-2
The Cell Cycle

14. The Cell Cycle
Cell division more complicated in eukaryotes because it involves dividing chromosomes, cytoplasm, and other organelles
Cell cycle: repeating sequence of cellular growth and division during life of an organism
90% of cell’s time is spent in first of 3 phases, collectively called interphase
Cell enters last 2 phases only if it is about to divide

15. Stages of Interphase
First growth phase (G1): cell grows rapidly and carries out routine functions
Cells remain in this phase until they prepare to divide
Some cells (nerve/muscle cells) never divide
Synthesis phase (S): cell’s DNA is copied
At end of this phase, each chromosome consists of 2 chromatids attached at a centromere
Second growth phase (G2): preparations are made for nucleus to divide and microtubules are rearranged in preparation for mitosis

16. Stages of Cell Division
Mitosis: process during cell division in which nucleus of cell is divided into 2 nuclei
Each nucleus ends up with the same number and kinds of chromosomes as original cell
Cytokinesis: process during cell division in which cytoplasm divides
Recall: Mitosis and cytokinesis produce new cells identical to the original cells
Allows organisms to grow, replace damaged tissues, or to reproduce asexually

17. Control of Cell Cycle
Cells know when to divide based on a series of three main checkpoints at which feedback signals from cell can trigger or delay the next phase
Cell growth (G1) checkpoint: makes decision whether cell will divide
Are conditions favorable for division?
Is cell healthy and large enough?
If so, certain proteins will stimulate cell to begin synthesis phase
DNA synthesis (G2) checkpoint: DNA replication is checked by DNA repair enzymes
Mitosis checkpoint: triggers exit from mitosis and signals beginning of G1 phase

18. Cancer
Specific genes contain information to make proteins that regulate cell growth and division
If one of these genes is mutated, protein may not function and regulation of cell growth and division may be disrupted
Cancer: uncontrolled growth of cells
Some mutations cause cancer by overproducing growth-promoting molecules, speeding up cell cycle
Others cause cancer by inactivating control proteins that normally act to slow or stop cell cycle

19. Mitosis and Cytokinesis
Section 6-3
Mitosis and Cytokinesis

20. Mitosis and the Spindle
During mitosis, the chromatids of each chromosome are physically moved to opposite sides of dividing cell
Structures called spindles are involved in moving these chromosomes
Made up of centrioles and individual microtubule fibers

21. Spindle Formation
Organelles that organize the assembly of the spindle are called centrosomes
Found at each of the cell’s poles
In animals, a pair of centrioles is found inside each centrosome
Like spindle fibers, centrioles are made of microtubules
Each spindle fiber consists of an individual microtubule
Each centriole is made of nine triplets of microtubules arranged in a circle
Though plant cells do not have centrioles, they form a spindle almost identical to that of an animal cell

22. Separation of Chromatids
As cell prepares to divide, microtubules in spindle extend out toward opposite poles of cell
Once microtubules attach to centromeres and poles, the 2 chromatids (now called chromosomes) can be separated
As the paired chromatids separate, they move toward opposite poles along paths described by microtubules to which they are attached
Chromatids draw closer to poles of cell as spindle microtubules break down and become shorter
Once breakdown of the spindle is complete, each pole has one complete set of chromosomes

23. Stage 1: Prophase Stage 1 of mitosis is called prophase
During this stage:
Chromosomes coil up, becoming visible
Nuclear envelope dissolves
Spindle forms

24. Stage 2: Metaphase The second stage of mitosis is called metaphase
During this stage:
Chromosomes move to center of cell and line up along equator
Spindle fibers link chromatids of each chromosome to opposite poles

25. Stage 3: Anaphase The third stage of mitosis is called anaphase.
During this stage:
Centromeres divide
Separated chromatids (now called chromosomes) move toward opposite poles as spindle fibers attached to them shorten

26. Stage 4: Telophase The last stage of mitosis is called telophase.
During this stage:
Nuclear envelope forms around chromosomes at each pole
Chromosomes (now at opposite poles) uncoil
Spindle dissolves as spindle fibers break down and disappear

27. Stages of Cell Cycle

28. Cytokinesis As mitosis ends, cytokinesis begins.
Cytoplasm of cell is divided in half
Cell membrane grows to enclose each cell, resulting in formation of separate but genetically identical cells

29. Cytokinesis: Plant vs. Animal Cells
Cytokinesis varies according to cell type:
In animals (lack cell walls), cells are pinched in half by belt of protein threads
In plants (have cell walls), vesicles formed by Golgi apparatus fuse at midline of dividing cell to form a cell plate
A new cell wall then forms on both sides of cell plate, separating the plant cell into 2 genetically identical cells