Ross & Wilson Anatomy and Physiology in Health and Illness (197 page)

Introduction to genetics

Chromosomes, genes and DNA
428

Chromosomes
428

Genes
428

DNA
428

Mutation
429

Protein synthesis
430

Messenger ribonucleic acid (mRNA)
430

Cell division
431

Mitosis
432

Meiosis
432

The genetic basis of inheritance
433

Autosomal inheritance
433

Sex-linked inheritance
434

Genetic basis of disease
435

Cancer
435

Inherited disease
435

ANIMATIONS

17.1
Karyotype
428

17.2
Sex determination
428

17.3
Complemetary base pairing of DNA
429

17.4
Transcription
431

17.5
Translation
431

17.6
Crossing over
432

17.7
Hereditary traits
433

17.8
Punnett square
433

17.9
Sex-linked traits
434

All living organisms, including human beings, need to reproduce, so that at the end of their life span they have produced at least one, probably many, other individuals to replace themselves. This ensures the continuation of their species. Babies inherit from their parents a copy of all information needed to develop into a functioning member of their species; this information is carried as deoxyribonucleic acid (DNA), mainly within the cell nucleus. DNA is organised into functional units called
genes
, which themselves are part of much bigger structures, the
chromosomes
. The name given to all the genetic material in a cell is the
genome
. Genetics is the study of genes, and advancing knowledge in this area has a profound effect on many aspects of daily life, e.g. for genetic counselling in families carrying inherited diseases and the production of human insulin from genetically engineered micro-organisms.

The Human Genome project, an international collaboration, was initiated in 1990. Its aim was to identify and sequence every gene on every chromosome, and to be able to draw a map of each chromosome, showing the position of all its genes. It has yielded a great deal of important information regarding human genetic disorders.

Chromosomes, genes and DNA

Learning outcomes

After studying this section you should be able to:

explain the structural relationship between chromosomes, genes and DNA

describe the molecular structure of DNA

explain the terms autosomes and sex chromosomes

define the terms genome, haploid, diploid and karyotype.

Chromosomes

Nearly every body cell contains, within its nucleus, an identical copy of the entire complement of the individual’s genetic material. Two important exceptions are red blood cells (which have no nucleus) and the gametes or sex cells. In a resting cell, the
chromatin
(genetic material, see
Fig. 3.8, p. 29
) is diffuse and hard to see under the microscope, but when the cell prepares to divide, it is collected into highly visible, compact, sausage-shaped structures called
chromosomes
. Each chromosome is one of a pair, one inherited from the mother and one from the father, so the human cell has 46 chromosomes that can be arranged as 23 pairs. A cell with 23 pairs of chromosomes is termed
diploid
. Gametes (spermatozoa and ova) with only half of the normal complement, i.e. 23 chromosomes instead of 46, are described as
haploid
. Chromosomes belonging to the same pair are called
homologous
chromosomes. The complete set of chromosomes from a cell is its
karyotype
(
Fig. 17.1
).

Figure 17.1 
Chromosomal complement (karyotype) of a normal human male, showing 22 pairs of autosomes and sex chromosomes (XY, pair 23).

Each pair of chromosomes is numbered, the largest pair being no. 1. The first 22 pairs are collectively known as
autosomes
, and the chromosomes of each pair are identical. The chromosomes of pair 23 are called the
sex chromosomes
(
Fig. 17.2
) because they determine the individual’s gender. Unlike autosomes, these two chromosomes are not necessarily identical; the Y chromosome is much shorter than the X and is carried only by males. A child inheriting two X chromosomes (XX), one from each parent, is female, and a child inheriting an X from his mother and a Y from his father (XY) is male.