Ribosomes are organelles that provide the sites for protein synthesis (Figure 1.7). Ribosomes are attached to the endoplasmic reticulum as well as freely floating in the cytoplasm.
Figure 1.7 Ribosomes
Golgi body
The Golgi body is a structure that packages the molecules produced by the endoplasmic reticulum ready for transport out of the cell (Figure 1.8).
Figure 1.8 Golgi body
Mitochondria
Mitochondria are organelles that convert energy gained from food into a form that the cell can use (Figure 1.9). Adenosine triphosphate (ATP) is the main source of energy used by the cell. These organelles have their own genetic material and can make copies of themselves.
Figure 1.9 Mitochondria
Lysosomes
Lysosomes are organelles that break down bacteria and other foreign bodies, as well as recycling worn out cell components (Figure 1.10).
Figure 1.10 Lysosomes
Peroxisomes
Peroxisomes are responsible for the detoxification of foreign compounds and the oxidation of fatty acids (Figure 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
