One Metaphase Chromosome

Chromosome structure - A metaphase (i.e.. doubled) chromosome is shown on the left. The doubling occurred during the S-stage of the Cell Cycle.

Each doubled Chromosome consists of two Sister
Chromatids. The DNA sequence of the two chromatids is identical.

The ends of each chromosome (and each chromatid) are called Telomeres. Recent studies indicate telomeric DNA may be involved in determining how long humans live.

The central, constricted parts of chromosomes are called Centromeres. The centromeres divide the chromosomes into two "arms".

The spindle fibers attach to a special protein region on the centromere called theKinetochore.
The Kinetochore is composed of proteins bound to centromeric sequences. The spindle fibers bind to the kinetochores.

Two other words that are used often in genetics are Phenotype and Genotype. The phenotype of an organism is the result of the genes it carries, sometimes thought of as its observable traits, and these traits are produced by the organism's genotype. Genotype is literally the genes an organism has, for any particular trait

Play with the chromosomes below to make sure you understand the difference between a chromosome and a chromatid. Chromatid One source of confusion for Biology students is all the "doubling" that is associated with DNA.

There are two strands of nucleotides in the DNA double helix.

There are two of each "type" of chromosome in a diploid organism.

Each chromosome is doubled during the S-phase of the cell cycle (we will discuss the cell cycle next).


Which chromosomes are homologs and which are sister chromatids?


On to the Cell Cycle