Chromosomes

Inheriting Traits

We inherit many of our physical characteristics or traits from our parents.This is known as heredity – the passing of traits from one generation to the next.In addition to our physical characteristics we also inherit many other traits that may not be as evident, but may be of particular importance.In scientific terminology, a trait is a particular characteristic or feature of an organism.

Why are traits inheritedChromosomes contain the hereditary (genetic) information in living cells.All living cells and viruses contain genetic information in chromosomes.Each unique sequence of DNA (gene) carries a particular instruction for a cell. Genes vary in size from about 100 to 2.5million base pairs. The length of the sequence of DNA and the precise order of the base pairs in a gene are the critical factors that determine what the gene product (usually a protein) will be like and what it will do in a cell.

Eukaryotes Prokaryotes Viruses

Location of Chromosomes

In the nucleus In the cytoplasm Core

Structure of Chromosomes

Double stranded molecules of DNA, with attached protein molecules.

Single, circular chromosome composed of DNA, with very few or no attached proteins.

Contain either DNA or RNA (not both), with no attached proteins (naked nucleic acids).

Number of Chromosomes

Varies from species to species. Humans have 46.

One. One to many.

Reproduction Nuclear DNA is replicated prior to cell division and the chromosomes distributed evenly to daughter cells.

Single circular chromosome is replicated prior to cell division. Each daughter cell receives one copy of this chromosome.

Reproduce only in host cells. They force host cells to make copies of their DNA or RNA.

Extra-chromosomal DNA

Mitochondria contain DNA.

Small circular DNA molecules known as plasmids.

Not applicable.

Eukaryotic Chromosomes

In eukaryotic cells, dark-staining thread-like structures called chromosomes are located in the nucleus.Chromosomes consist of DNA and therefore carry genes. Chromosomes constantly change their appearance during the life of a cell and come in various shapes and sizes.

Packaging DNA into Chromosomes

In humans the average DNA molecule is about 6.5x107 base pairs in length.The nucleus of a human cells is just 6m in diameter, yet it contains 1.8m of DNA.This can only be achieved because DNA in eukaryotes is tightly packaged into chromosomes.

DNA is coiled around small proteins (histones). Where the DNA is wrapped around a core of histone proteins it forms a particle about 10nm in diameter called a nucleosome.The nucleosomes give the DNA strand the appearance of a string of beads, and this arrangement of DNA wrapped around histones serves to package the DNA efficiently and protected from enzymatic degradation.When a eukaryotic cell is preparing to divide, chromosomes become very condensed and are visible under a light microscope.

Packaging of DNA into Chromosomes

Genes and ChromosomesEach DNA molecule contains many genes.Each gene has a particular position, called a locus, on a specific chromosome.Different forms of the same gene found in members of the same species are called alleles and will have the same locus on the same chromosome. Alleles are the basis of heritable variation.The genes on each DNA molecule are separated by regions called spacer DNA.Spacer regions include DNA that does not encode a protein product, and may function in spacing genes apart so that enzymes or other molecules can interact easily with them.

ChromosomesChromosomes in dividing cells consist of two chromatids. Each chromatid contains an identical copy of the DNA molecule.

Chromosomes in non-dividing cells chromosomes exist as a single-armed structures (each is the equivalent of one of the chromatids in a metaphase chromosome prepared for cell division).

Homologous chromosomesThe same genes are found at the same locations (loci) on the two chromosomes in a pair. This is why they are referred to as homologous chromosomes or homologues.The two chromatids of a chromosome in a somatic cell contain identical DNA molecules. While there can only be a maximum of two alleles (one from each parent), there may be two copies of each.

KaryotypesThe karyotype of an organism is the full set of chromosomes of a cell or individual.Karyotyping involves formally arranging the chromosomes in relation to their length and the position of the centromere.Karyotyping allows us to identify changes in chromosome number and structure. Changes of this nature may be associated with genetic abnormalities.

Classifying chromosomesChromosomes are identified by length, banding and position of the centromere.Banding occurs when eukaryotic chromosomes are stained with a special dye during metaphase to create a characteristic pattern of light and dark bands called G bands.The bands reflect the regional differences in the amounts of A and T versus G and C.Chromosomes that the centromere centrally positioned, giving arms of equal length, are metacentric.Submetacentric chromosomes have the centromere towards one end, resulting in arms of unequal length.When the centromere is very close to one end, the chromosome is acrocentric.Human chromosomes have been assigned to seven groups (A-G) according to their length and the position of the centromere.

The groups of autosomes in humans

Group Autosome Number

Position of centromere

Autosome length

A 1-5 metacentric long

B 4-5 submetacentric long

C 6-12 metacentric, submetacentric

medium

D 13-15 acrocentric medium

E 16-18 metacentric short

F 19-20 metacentric shorter

G 21-22 acrocentric shortest

Human Karyotype

In humans (and some other organisms), sex chromosomes are distinguished from the remaining chromosomes (autosomes).Human somatic cells have a diploid chromosome number of 2N=46 consisting of 22 pairs of autosomes and one set of sex chromosomes.Human gametes have only one set of chromosomes and are referred to as being haploid.

Human female karyotype

When things go wrong…

Extra X Chromsome – Klinefelter SyndromeMale, sterile, often some mental retardation, with female secondary sex traits (e.g. breast enlargement)

Missing X Chromosome – Turner SyndromeFemale, short stature, no sexual maturation, webbing of skin between neck and shoulder

Three copies of chromosome 13 – Patau SyndromeMale or female, small skull, mentally retarded, harelip, cleft palate, usually have heart defects, seldom survive more than four months after birth

Three copies of chromosome 21 – Down SyndromeMale or female, some mental retardation, characteristic palm prints and facial features, usually sterile.