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

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.

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

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

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

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)

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

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

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

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