Читать книгу Understanding Anatomy and Physiology in Nursing - John Knight - Страница 15

Find a rubber elastic band and two pens. Loop the elastic band around both pens and progressively wind up the elastic tighter and tighter.

What do you notice is happening to the elastic?

There are some possible answers to all activities at the end of the chapter, unless otherwise indicated.

Now that you understand how chromosomes become visible during cell division, we can examine how they can be used to screen for diseases. Nucleated human cells usually have 23 pairs of chromosomes, giving a total of 46, which is referred to as the diploid number (the normal expected number). The only major exceptions to this rule are the sperm and egg cells (ova) which by necessity must have half the diploid number of chromosomes. Half the diploid number in humans is 23 and this is referred to as the haploid number. Having haploid sperm and ova ensures that during fertilisation the diploid number of 46 is restored and the number of chromosomes remains constant down the generations (Chapter 13). Photographs of human chromosomes can be taken during cell division and placed into their ordered pairs according to size; these photographs are called karyographs and reveal the individual’s chromosomal make-up, which is referred to as their karyotype.

Figure 1.3 Chromosomes and karyotypes

The first 22 pairs of human chromosomes are referred to as autosomes and these appear structurally the same in both males and females (Figure 1.3). The final 23rd pair determines the physical gender of the individual and for this reason these are referred to as the sex chromosomes. Females usually have two XX chromosomes (XX) and males usually have an X and a Y chromosome (XY). However, as we will see in Chapter 13, there are frequently variations in the patterns of sex chromosomes and for this reason not all females are XX and not all males are XY. The sex chromosomes only determine the physical gender of the individual and it is recognised that gender identity can be very fluid; frequently the gender that someone feels aligned to may not necessarily reflect their inherited sex chromosomes.

Examining an individual’s chromosomes is most frequently carried out before they are born. During pregnancy foetal cells may be collected by procedures such as amniocentesis or chorionic villus sampling. During amniocentesis the amniotic fluid that surrounds the developing foetus is collected; this will contain cells that have become detached from the foetus as it moves. Since the foetus is continually growing, a large number of the cells harvested will be dividing and therefore have chromosomes visible. The process of karyotyping that follows commonly reveals chromosomal abnormalities such as Down’s syndrome and Turner’s syndrome (Chapter 14).