Section 8.1

Chromosomes

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:

1. A single DNA molecule tightly coiled around

2. Proteins called histones

Each chromosome has:

Two identical halvesEach half is called a chromatidChromatids are made before cell divisionChromatids are joined close to the

center at a spot called the centromere

Chromosome structure

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)

Homologous chromosomes

In sexually reproducing organisms, every cell has two copies of each autosome

One copy comes from each parentThese are called homologous

chromosomes or homologuesThey are the same size and shape, and

carry information about the same traits

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

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

Chromosome number