Chromosomes

Introduction

Cell's genetic information is stored in the form of DNA molecules. DNA molecules and associated proteins form the chromatin. During cell division, the chromatin forms DNA structures called chromosomes. This chapter looks at the structures and types of chromosomes.

Chromosomes and biological staining

Under the microscope most cells look transparent so they are hard to study unless they have been stained with biological dyes. Today, different biological staining techniques are used to view organelles better. Different dyes are used for different organelles and tissues. Some dyes stain protein structures, some stain lipids, and some stain DNA. These types of dyes are called class specific.

Chromosomes were discovered in the 19th century by Karl Wilhelm von Nageli after he stained the nuclei of some plant cells. During this research, some of the substance of the nucleus became intensely coloured by the biological dye. Because Greek and Latin languages are normally used for new scientific names, this intensely-coloured substance was named chromatin. Chromosomes derived their name from two Greek words: 'chroma' meaning 'colour' and 'soma' meaning 'body'.

Chromosome structure

Each chromosome is made up of DNA molecules tightly coiled many times around proteins called histones. Histones support chromosome structures. The spiral shape of DNA is called a helix. Each strand of the DNA double helix is a linear arrangement of repeating similar units called nucleotides. In its normal state, a cell's chromosomes resemble thread-like structures that form the chromatin of the nucleus. Chromatin is not visible under a microscope. When a cell divides, its DNA takes a more compact form. These tightly-packaged DNA molecules form chromosomes which can be seen under a microscope. Eukaryotic organisms have many chromosomes and prokaryotes usually have just one in the form of circular DNA.

Each chromosome has two 'arms' joined by a centromere. The centromere plays an important role during cell division and also divides each chromosome into a short arm and a long arm. The short arm of the chromosome is called the 'p arm' and the long arm of the chromosome is called the 'q arm'. The location of the centromere on each chromosome gives the chromosome its characteristic shape and can be used in the identification of a gene's location. The specific location of a gene on the chromosome is called the locus. Scientists can tell different chromosomes apart based on the size and length of their arms. See image 1.

Chromosome sets

The number of sets of chromosomes in a cell is called ploidy. All cells can be divided into two groups: somatic cells and gametes. Somatic cells are all body cells. In all somatic cells, chromosomes exist in pairs. These types of cells are called diploid cells. To form the pair, one cell is inherited from the 'male' organism and the other is inherited from the 'female' organism.

Gametes, also called sex cells, are reproductive cells. Male gametes are called sperm and female gametes are called ova, or eggs. Gametes are haploid cells, meaning that they have only a single set of chromosomes. When gametes fuse, they form a diploid cell called a zygote. The zygote later develops into the organism that is made from diploid somatic cells.

Cells with multiple sets of chromosomes are called polyploid. Examples of polyploid organisms are plants like wheat and tobacco.

Different species have different numbers of chromosomes. Most bacteria have just one. The number of sets varies in different species. A higher number of chromosomes does not equate to a higher position on the evolutionary tree. Here are some examples of chromosome numbers of different species. See image 2.

Karyotype

To determine the number of chromosomes of an organism, cells are locked in a stage of cell division called metaphase (see Topic 3, Chapter 2). Cells are then stained and photographed. Their chromosomes are arranged into a karyotype - the chromosomal picture of the cell. A karyotype is often shown as a set of chromosomes arranged in order from largest to smallest. This arrangement helps scientists quickly identify chromosomal alterations that may result in a genetic disorder. See image 3.

Types of human chromosomes

Normal human cells have 46 chromosomes: 44 autosomes and 2 sex chromosomes. Autosomes are chromosomes that just contain genetic information and are not involved in sex determination. Chromosomes that determine an organism's sex are called sex chromosomes. Human sex chromosomes are X and Y. These chromosomes determine a female or male pattern of an organism's development. Human ova have only X chromosomes and human sperm can have X or Y chromosomes. The type of the sperm cell determines the sex of the offspring. Female somatic cells have two X chromosomes and male somatic cells have X and Y chromosomes.

Chromosome mutations

When there are problems during cell reproduction, chromosome mutations occur. When chromosomes exchange parts it is called translocation. When a segment within the same chromosome is reversed, it is called inversion. Some chromosome abnormalities do not cause disease but they may lead to a higher chance of having a child with a chromosomal disorder. An abnormal number of chromosomes is called aneuploidy. Aneuploidy usually is lethal or leads to different genetic disorders. For example, a well known genetic disorder called Down syndrome is caused by an extra copy of chromosome 21.