1. Chromosomes

2. Chromosome Structure
When a cell is not dividing the DNA is loose, long, and skinny and is called chromatin.
During cell division the DNA in a cell’s nucleus is coiled around proteins to form structures called chromosomes.

3. Chromosome Structure
histones

4. Chromosome Structure in Prokaryotes
Prokaryotic organisms like bacteria contain only one chromosome.
This chromosome is always circular.

5. Chromosome Numbers
Each species has a characteristic number of chromosomes.
Humans for example have 46 chromosomes (23 pairs).

6. Sex Chromosomes vs. Autosomes
There are 2 kinds of chromosomes:
Sex chromosomes
Sex chromosomes - the chromosomes that determine the sex of the organism.
In humans, the sex chromosomes are the X and Y chromosomes.
Humans only have 2 sex chromosomes (1 pair) out of the 46 total chromosomes (23 pairs) we have.
Females = XX
Males = XY
Autosomes
Autosomes - All other chromosomes that are not sex chromosomes.
Humans usually have 44 autosomes (22 pairs).

7. Homologous Chromosomes
Every cell has 2 copies of each autosome.
One copy comes from each parent.
These pairs are referred to as homologous chromosomes or homologues. (homologue means “same size”)

8. Boy or Girl??? Boy Karyotypes
A karyotype is a picture of the chromosomes in a cell.
You can see that in this karyotype there are 22 pairs of autosomes and 1 pair of sex chromosomes.
Homologous Pairs
Boy or Girl???
Boy

9. The Y Chromosome Decides
Boy or Girl?
The Y Chromosome Decides
Y - Chromosome
X - Chromosome

10. Most Cells are Diploid
In humans, all cells except sex cells are called diploid because there are two copies of each chromosome.
The abbreviation for diploid is 2n (= 2 sets of chromosomes in homologous pairs)

11. Haploid Cells Sex cells - sperm and egg cells
All sex cells are called haploid because they have 1 copy of each chromosome; Chromosomes are not in homologous pairs.
The abbreviation for haploid is 1n (= 1 set of chromosomes not
in homologous pairs)

12. Fertilization Results In A Diploid Zygote
In humans, all sex cells are haploid cells.
When sperm (1n) and egg (1n) combine, they create a fertilized egg called a zygote which is a diploid (2n) cell (and will eventually grow into a baby through cell division!)

13. Fertilization Results In A Diploid Zygote
Egg 1n
Haploid
nucleus
Sperm 1n
Haploid
nucleus

14. Fertilization Results In A Diploid Zygote
Egg 1n
Haploid
nucleus
Sperm 1n
Haploid
nucleus

15. Fertilization Results In A Diploid Zygote
Egg 1n
Haploid
nucleus
Haploid
nucleus
Sperm 1n

16. Fertilization Results In A Diploid Zygote
Egg 1n
Haploid
nucleus
Haploid
nucleus
Sperm 1n

17. Zygote 2n
Zygote 2n