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: A single DNA molecule tightly coiled around Proteins called histones

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

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 parent These are called homologous chromosomes or homologues They 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