Genetics, revised edition. Karen Vipond
1.11).
Figure 1.11 Peroxisomes
CHROMOSOMES
Each of the trillions of cells in the body, with the exception of red blood cells, has a nucleus. Within each nucleus are structures called chromosomes. Chromosomes are not usually visible under a light microscope, but when a cell is about to divide, the chromosomes become denser and can be viewed at this stage.
Chromosome structure
A chromosome is composed of DNA and proteins and includes structures that enable it to replicate and remain intact (see Figure 1.12). During cell division, chromosomes have a constriction point termed a centromere. The centromere divides each chromosome into two sections or ‘arms’. The long arm is referred to as the q arm and the short arm as the p arm (p for petite).
The location of the centromere gives the chromosome its characteristic shape and can be used to describe the location of specific genes.
Figure 1.12 A chromosome
Telomeres
Telomeres are distinctive structures found on the end of each arm of the chromosome. They are made up of the same short sequence of DNA, which is replicated about three thousand times. The function of the telomeres appears to be twofold.
1. They protect the chromosome by ‘capping’ off the ends to prevent them from sticking or joining onto other chromosomes.
2. Due to the way that chromosomes are replicated, the ends of the chromosomes are not copied. Telomeres shorten during every cell replication, but the loss of DNA within the telomeres protects against loss of essential DNA within the chromosome itself.
Chromosome numbers
Chromosomes exist in pairs. Although not actually joined together, each pair has a characteristic length. The human cell nucleus has 23 pairs of chromosomes; in other words, 46 individual chromosomes. One chromosome from each pair is inherited from the father and one from the mother. Twenty-three individual chromosomes are inherited from each parent. The total number of chromosomes in each cell is called the diploid number (diploid 46) while the number of pairs is called the haploid number (haploid 23).
Of the 23 pairs of chromosomes, 22 pairs are termed autosomes and do not differ between the sexes. For ease of identification, these autosomes are numbered from 1 to 22. The chromosomes are numbered according to length, with chromosome number 1 being the longest and chromosome 22 being the shortest. The remaining two chromosomes are known as the sex chromosomes. These two chromosomes are not numbered but are known as the X chromosome and the Y chromosome. The Y chromosome determines maleness. A female will have two X chromosomes while a male will have one X and one Y chromosome.
Karyotype
The chromosome complement within the nucleus is called a karyotype. Charts called karyographs (see Figure 1.13) display chromosomes in pairs in size order. The 22 paired autosome chromosomes are displayed first, ranging from number 1 to 22 (largest to the smallest). The sex chromosomes, X and Y (male) or X and X (female) are always placed at the end of the chart. Karyographs can be a useful clinical tool to help confirm diagnosis through the identification of chromosomal aberrations, abnormalities or anomalies.
Figure 1.13 A karyograph
The centromere
Another physical characteristic of the chromosome, the centromere, also helps identification, as the position of the centromere varies in different chromosomes (see Figure 1.14).
Figure 1.14 Centromere positions
| ACTIVITY 1.1 |
What are the haploid and diploid numbers of chromosomes in humans?
CHROMOSOMAL INHERITANCE
The human cell has two sets of chromosomes, one set inherited from each parent. The complete genetic makeup within the cell is termed the genome. The total number of chromosomes within the cell has to be kept constant from one generation to the next. Each individual has a total of 46 chromosomes in each cell nucleus, 23 of which are inherited from their mother and 23 from their father.
For normal cell division two daughter cells are formed, both of which have the full 46 chromosome complement. This type of cell division is called mitosis and results in new cells that are genetically identical to the parent cell. Mitosis is cell division that is used by the body for growth and repair. Meiosis, on the other hand, is a type of cell division that produces new cells with only half the chromosomal complement (a total of 23 chromosomes). These 23 chromosomes are half the set of the original cell. Meiosis only occurs in the germ line cells, i.e. the ova in women and the sperm in men. If fertilisation occurs, the resulting offspring will inherit 23 chromosomes from the mother and 23 chromosomes from the father, resulting in a full 46 chromosomal complement. Meiotic division prevents the doubling of chromosomal numbers from one generation to the next.
Mitosis
Mitosis occurs rapidly during growth and tissue repair. It is a well-controlled process and consists of two major steps – the division of the nucleus followed by the division of the cytoplasm. Although mitosis is a continuous process it can be described as a series of four stages followed by a resting period where there is no cellular division (Table 1.1).
Table 1.1 The stages of mitosis and interphase
| Stages | Events |
| Chromosomes get shorter and fatter by coiling themselves. They now become visible under a light microscope. Each chromosome has two strands (two copies of the original chromosome) that are held together by the centromere. Strands of protein called spindle fibres appear. | |
| Chromosomes line up together and the spindle fibres become attached to each side of the centromere. | |
| The spindle fibres contract, pulling the two copies of each chromosome to opposite areas within the nucleus. | |
| The two new sets of chromosomes form two new nuclei. The chromosomes revert to being long and thin. The cytoplasm then divides to form two new cells. | |
| Normal cellular function. The cells make copies of their chromosomes ready for the cycle to start again. |
With mitosis each daughter cell is an exact copy of the previous cell. All cells receive identical chromosomal material.
The cycle of events during mitosis usually lasts several hours. The mitotic division of the chromosomal material during prophase, metaphase, anaphase and telophase takes a relatively short period of time and the resting phase