1. Chromosomes
Biology
DNA is a long thin molecule that stores genetic information. It consists of 6 billion pairs of nucleotides.

2. Chromosome Structure
During cell division, the DNA in a eukaryotic cell’s nucleus is coiled into very compact structures called chromosomes.
Rod shaped structures made of DNA and protein
DNA coils to make chromosomes
They wrap around histone proteins
Histone proteins help maintain the shape of the chromosome and aid in the tight packing of DNA
Nonhistone proteins are involved in controlling the activity of specific regions of the DNA.

3. Chromosome Structure, cont.
When they undergo cell division, they are visible as darkened structures inside the nuclear membrane
Each chromosome consists of two identical halves
Each half is called a chromatid
Chromatids form as the DNA makes a copy of itself before cell division
Once the cell divides, each of the two new cells will receive one chromatid from each chromosome

4. Chromosome Structure, cont.
The centromere is the constricted area of each chromatid
It holds the two chromatids together until they separate during cell division
They help in the movement of chromosomes during cell division
In between cell divisions, DNA is not tightly coiled into chromosomes. Regions of DNA uncoil in between cell division so they can be read and so the information can be used to direct the activities of the cell.
The less tightly coiled DNA-protein complex is called chromatin

5. Chromosome Structure, cont.
Chromosomes are simpler in prokaryotes than in eukaryotes
Prokaryotes have one chromosome attached to the cell membrane
They have a circular DNA
DNA must be compact to fit in the cell

6. Chromosome Numbers
Each species has a characteristic number of chromosomes in each cell
Sex chromosomes and autosomes
Human and animal chromosomes are categorized as either sex chromosomes or autosomes
Sex chromosomes determine the sex of an organism
They carry genes for other characteristics
In humans, sex chromosomes are either X or Y
Normal females have two X chromosomes (XX)
Normal males have one X and one Y (XY)

7. Chromosome Numbers
All other chromosomes in an organism are called autosomes
Two of the 46 human chromosomes are sex chromosomes while the remaining 44 chromosomes are autosomes
Each cell of an organism produced by sexual reproduction has two copies of each autosome
One from each parent

8. Chromosome Numbers
Two copies of each autosome are called homologous chromosome or homologues
They are the same size and shape and carry genes for the same traits
Karyotypes display homologous chromosomes in a photomicrograph of the chromosomes in a dividing cell found in a normal human
The 46 chromosomes exist as 22 homologous pairs of autosomes and two sex chromosomes

9. Diploid and Haploid Cells
Diploid—cells that have two sets of chromosomes for each homologous pair
This includes two sex chromosomes in animals
All normal human cells, except reproductive cells (sperm cells and egg cells) are diploid
Denoted by 2n
In humans, a diploid number, 2n, is 46, that is 22 pairs of homologous chromosomes and 2 sex chromosomes

10. Chromosome Numbers
Haploid—cells that contain only one set of chromosomes such as human sperm and egg cells
These cells contain half the number of chromosomes that are present in diploid cells
In humans, a haploid number, 1n, has one chromosome of each homologous pair and only one sex chromosome
Sperm cells are 1n
Egg cells are 1n
Both combine to form 2n (diploid)when a new organism is formed
If each were diploid from the start, each new cell would have too many chromosomes and would not be functional