1. Chromatin and Chromosomes
AP Biology
Ms. Day

2. RECALL… DNA & RNA = nucleic acids Central Dogma of Biology
DNA (has genes)  RNA  PROTEIN
Proteins give you traits
Gene = a section of DNA that codes for a protein or RNA

3. Gene Expression: 2 parts
Involves DNA, RNA and the Central Dogma of Biology
DNA  RNA  PROTEIN
DNA  RNA called transcription
Going from DNA nucleotides  RNA nucleotides
RNA  protein called translation
Going from RNA nucleotides  amino acids

4. 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)

5. Forms of DNA DNA (in nucleus of eukaryotes) can be in 2 forms
Chromatin : DNA is not tightly packed together (loosely coiled)
Occurs during interphase (most of cell’s life…90%)
Chromosomes : tightly packed together (TIGHTLY coiled/condensed)
Occurs during mitosis (cell division)

6. How Does DNA Fit into a Small Cell?
Eukaryotic chromosomes
stored within the cell's nucleus
Prokaryotic chromosomes
stored within the nucleoid region
DNA is coiled around proteins called histones
Group of 8 histones = nucleosome
helps form a “supercoil”

7. Chromatin includes
2 parts:
DNA
Histones

8. “LINKER” DNA

9. Nucleosomes Like beads on a string

10. Tails have positive charge; DNA has negative charge
Histone Tails + -
Tails have positive charge; DNA has negative charge

11. Histone Modification
Adding acetyl group (CH3COOH) to tail (acetylation)
Neutralizes charges
Making DNA less tightly coiled  more linker DNA
Increasing transcription
“opens” up DNA

12. DNA Modification Adding a methyl group to the DNA (methylation)
Maintains the positive charge
Makes DNA more coiled
Reduces transcription

13. Gene is “active”
Switched on
Open chromatin
Transcription can happen
Unmethylated cytosines (white circles)
Acetyl groups ARE on histones
Gene is “inactive”
No transcription
Methylated cytosines (red circles)
No acetyl groups on histones

14. 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.

15. 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)

16. Chromosomes and Cell Division
To prepare for cell division (mitosis)
DNA is replicated

17. 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

18. Double Chromosome Structure
Kinetochore attaches to spindle fibers
Sister

19. Another view…

20. Chromosomes
Every eukaryotic species has a characteristic, unique # of chromosomes in EACH cell nucleus
Ex: Humans have 46 chromosomes
# of chromosomes does NOT necessarily equal complexity

21. Different Types of Cells
There are 2 types of cells in the human body
Somatic cells: all body cells
2. Gametes: reproductive cells (sperm and egg)

22. Different cell types  different cell divisions
Eukaryotic cell division consists of
1. Mitosis
Occurs in somatic (body) cells
2. meiosis
Occurs in GERM LINE CELL (a special somatic cell in gonads)
CREATES gamete cells

23. Chromosome Number Chromosomes come in PAIRS Gametes (sperm/egg)
have 1 copy of each chromosome
called haploid
Somatic (body)
have 2 copies of each
chromosomes
called diploid
Chromosomes come in PAIRS
All pairs of chromosomes differ in size, shapes, and set of genes.

24. Haploid vs. Diploid
Haploid= when a cell has 1 copy of each chromosome.
Expressed as n = 23
Diploid= when a cell has 2 copy of each chromosome. (a.k.a TOTAL # of C’s in organisms)
Expressed as 2n = 46 n
Haploid sperm cell
Haploid egg cell n
Fertilization 2n
1st diploid Zygote cell of an organism

25. Set of Chromosomes
Each chromosome set contain 2 “homologues” or “twins”
Chromosomes have a “twin” or matching pair
Homologous Chromosome Pair = 2 chromosomes that are similar in shape, size, and genes.
Each homologue in a pair comes from each parent!

26. SIMILAR:
1. Size
2. Shape
3. Genes they carry
DIFFERENT
specific gene info (order of ATCG’s) can differ

27. Sets of Chromosomes in Human Cells
In humans
Each somatic cell has 46 chromosomes
Chromosomes come in pairs or sets
One set of chromosomes comes from each parent
Human somatic cells have 23 PAIRS of chromosomes
All pairs of chromosomes differ in size, shapes, and set of genes.

28. Chromosomes Determine Your Gender
Humans have 23 pairs of chromosomes
22 pairs are autosomes (chromosomes that are NOT involved in making gender)
Also called homologous chromosome pairs
1 pair are sex chromosomes (NOT HOMOLOGOUS; different genes)
The sex chromosomes are “X” and “Y”
“XX” is a girl
“XY” is a boy X
Egg cells ALWAYS have “X” chromosome Y X Y

29. Chromosomal Analysis: A karyotype
Pair of homologous
chromosomes
Centromere
Sister
chromatids
an ordered, visual representation of ALL chromosomes in a human cell
Arranged according to homologous pairs

30. Karyotypes (pictures of chromosomes)
Used to study chromosomes & look for LARGE mutations (ex: chromosomal translocations, deletions, extra chromosomes)
To make a karyotype
1. scientists take a picture of a cell’s chromosomes during metaphase and STAIN the chromosomes
2. Cut out the chromsomes and arranged them according to size and gene band pattern
3. They # the chromosome pairs
**Use #1 through 23
4. #23 = ALWAYS sex
chromosome pair
(not homologous-NOT similar in genes it holds)