1. Section 8.1
Chromosomes

2. Chromosome structure
Chromosomes are visible during cell reproduction only
Chromatin – the same material in between cell divisions (uncoiled)
Each chromosome is made of 2 parts:
A single DNA molecule tightly coiled around
Proteins called histones

3. Each chromosome has: Two identical halves
Each half is called a chromatid
Chromatids are made before cell division
Chromatids are joined close to the center at a spot called the centromere

4. Chromosome structure

5. Types of chromosomes
Sex chromosomes – determine the sex of an organism; may also carry other information
In humans sex chromosomes are X and Y
Normal males have XY; normal females have XX
Autosomes – all the other chromosomes (body chromosomes)
In humans there are 44 autosomes (46 total)

6. Homologous chromosomes
In sexually reproducing organisms, every cell has two copies of each autosome
One copy comes from each parent
These are called homologous chromosomes or homologues
They are the same size and shape, and carry information about the same traits

7. Karyotypes A picture of the chromosomes taken during cell reproduction
All the chromosomes are arranged in homologous pairs
Can be used to detect genetic defects, such as incorrect chromosome number

8. Chromosome number Diploid cells – 2 sets of chromosomes
2 of each autosome + 2 sex chromosomes
Represented with 2n (humans n=23; 2n=46)
Haploid cells – 1 set of chromosomes
Only one autosome of each homologous pair + only 1 sex chromosome
Represented with 1n
Egg and sperm cells are haploid  after fertilization the new cell is diploid
All eggs have one X chromosome; sperm have either X or Y  sperm determine the sex of the child

9. Chromosome number