Chromosomes, genes and DNA fingerprinting
Chromosomes are thread-like bodies in the nucleus, each one DNA molecule with proteins. Genes are segments of that DNA. Humans have 46 chromosomes: 2n = 46.
Features of chromosomes
- Every cell nucleus has a fixed number of chromosomes. Mature red blood cells of mammals have no nucleus, so they have none.
- Chromosomes come in pairs: the father gives one of each pair and the mother gives the other. The full paired set is the diploid number, 2n.
- They can be seen clearly only during cell division. In a resting cell they form a tangled network called chromatin.
- Each chromosome consists of a single DNA molecule along with some proteins.
- Before a cell divides, the DNA copies itself, so each chromosome has two identical halves called chromatids, joined at a point called the centromere.
- A bacterium has only one chromosome (one DNA molecule). With no proper nucleus, it lies in a region of the cytoplasm called the nucleoid.
Human chromosomes
Each human body cell has 46 chromosomes, so 2n = 46. A sperm or an egg (a gamete) has half this number, 23, called the haploid number (n). Chromosomes can be photographed in a dividing cell and arranged in 23 pairs. A matching pair, one from each parent and carrying the same genes, is a homologous pair. 22 pairs are autosomes, the same in men and women. The 23rd pair is the sex chromosomes: XX in females and XY in males. The X chromosome carries many genes, some needed for survival. The Y chromosome carries genes for maleness, such as the testes determining factor.
DNA, gene and chromosome
Think of a chromosome as a long necklace (the DNA) and the genes as beads at fixed places on it.
DNA fingerprinting
Every cell of a person grows from one fertilised egg, so the DNA is the same in every cell. A child's DNA also resembles the parents' DNA, because it is inherited from them. Like a fingerprint, each person's DNA pattern is unique. So a single hair or drop of blood left at a crime scene can be tested and matched against a suspect's DNA. This test is called DNA fingerprinting. Because each person's DNA pattern is unique and the same in every cell, it is a very reliable test of identity.
Did you know: a man-made gene
Dr Har Gobind Khorana, born in undivided India, shared the Nobel Prize in 1968 for helping to work out the genetic code. In 1970 his team made the first artificial gene in a laboratory, a landmark in molecular biology. Your NIOS lesson says he won the Nobel Prize in 1970 for the artificial gene; in fact the prize came in 1968, before that work.
Police and courts in India use DNA fingerprinting to solve crimes and to settle cases about who a child's parents are. A single hair root or a drop of blood is enough for the test.
1 marks: “How many chromosomes are present in a human sperm?” 23, the haploid number (n).
2 marks: “A photograph of a dividing human cell shows its chromosomes arranged in 23 pairs. How is the last pair different from the other 22, and how does it differ between a man and a woman?” The other 22 pairs are autosomes, alike in men and women. The last pair is the sex chromosomes: XX in a woman and XY in a man.
2 marks: “A drop of blood is found at a crime scene. Explain how DNA fingerprinting can show whether it came from a suspect.” DNA is the same in every cell of a person and unique to that person. DNA from the blood is compared with the suspect's DNA; a match shows it is the suspect's.
46 in the body, 23 in the gamete: 22 pairs of autosomes plus XX or XY.