1. CHROMOSOMES
AND
KARYOTYPING

2. DNA double helix
DNA and histones
Chromatin
Supercoiled DNA
Chromosomes
Chromosomes are long continuous threads of DNA which consist of numerous genes that code for specific proteins or functions.
Chromosomes are formed by the condensing of DNA before cell replication (mitosis)
DNA twists into a helix.
DNA wraps around proteins called histones, forming chromatin.
As you get closer to mitosis, chromatin is wrapped around proteins into small thick rods.
During the S phase, the DNA duplicates forming x- shaped chromosomes.

3. One half of a duplicated chromosome is a chromatid.
Sister chromatids are held together at the centromere.
Telomeres protect DNA and do not include genes.
Are made of repeating nucleotides that do not code for genes.
Every time DNA is replicated a short section of nucleotides is lost from the tips, so they help to prevent the loss of genes.
chromatid
telomere
centromere
Condensed, duplicated chromosome

4. Gametes (sperm and eggs) have 23 chromosomes.
Human body cells have 23 pairs of chromosomes, 46 chromosomes altogether.
This is the diploid number, 2n
Each pair is referred to as a homologous chromosome.
Homologous pairs of chromosomes have the same length, general appearance, and have copies of the same genes.
For each homologous pair, one chromosome comes from each parent.
Although they have the same genes, the homologous chromosomes are NOT identical.
Gametes (sperm and eggs) have 23 chromosomes.
This is the monoploid/ haploid number, 1n

5. Chromosome pairs 1-22 are autosomes.
Autosomes are chromosomes that contain genes for characteristics not related to the sex of an organism.
The 23rd pair are the sex chromosomes, X and Y.
Determine gender in mammals.
X,Y= male
X,X= female
These chromosomes are not homologous.
X is larger and contains some genes that are not related to sex.
Y is smaller and only contains genes for male traits.

6. Karyotypes
A karyotype is a picture of an organism’s chromosomes that allows us to study the difference of shape, structure and size of each chromosome
During mitosis, a picture is taken, then the chromosomes are sorted into identical pairs and arranged from biggest to smallest

7. Karyotypes can be used to see abnormalities in chromosomes.
Sometimes when gametes are being made, the chromosomes fail to separate correctly (called nondisjunction), causing changes in chromosome numbers.
Monosomy- when gamete has only 1 copy of the affected chromosome
Trisomy- when the other gamete has 3 copies of one chromosome
Karyotypes can be used to see abnormalities in chromosomes.
Common Disorders:
Down Syndrome – trisomy 21
Turner Syndrome – monosomy X
Klinefelter’s Syndrome – XXY chromosomes (trisomy)
Edwards Syndrome- trisomy 18