Lecture 2 Structure of the Chromosome
Tuesday, 23 July 2013
Most species have a characteristic number of chromosomes, haploid number (n) is the number
of chromatin in a single set while diploid is the total number (2n).
Most fungi and algae only have 1 set of chromosomes (n) and are haploid while most animals
and plants are diploid (2n) and have homologous chromosomes.
Haploid number can range from n=1 in an ant to n=728 in a fern.
There is no relationship between chromosome number and organism complexity.
The greater the number of chromosomes the greater the genetic variation from independent
assortment in gametes. However too many chromosomes causes complicated assortment
resulting in no viable gametes.
Variation in chromosome number:
Organisms can produce 2 gametes where n is haploid and 2n is diploid.
The number of chromosomes is not auto related to the genome size or complexity. Genome
size can also be variable . Genome size is th amount of DNA in the nucleus.
Metocentric: centromere in the centre of the chromosome, 1:1 ratio of arm length.
Acrocentric: Centromere at one end of the chromosome, typically 3:1 ratio.
Chromosomes of a single genome may differ in size.
Humans have 46 chromosomes, 22 pairs of autosomes and a pair of sex chromosomes. The
Karotype is the morphology of the chromosomes at mitotic metaphase.
Chromosome morphology: the centromere is the primary constriction of the chromosome and
is the most important feature. Centromere positioning is either, metacentric, acrocentric or
There is no visible structure to indicate their presence, defined by the distinct DNA sequence
of tandemly repeated arrays of DNA sequence. In humans it is TTAGGG.
Can be veiwed by fluorescent hybridisation.
Position of Nucleolar organiser: