This article discusses genetic diversity and cell division, the makings of a unique human.
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Alyson Bexfield, PhD, BSc, is senior research assistant; Yamni Nigam, PhD, MSc, BSc, is lecturer; both at School of Health Science, Swansea University.
The cell cycle
As discussed in part 1, human development starts at fertilisation with the formation of a zygote. To progress through all the stages from zygote to adult human, cells must divide (replicate). In so doing, they pass on genetic instructions to new (daughter) cells that, in turn, grow and divide. This is known as the cell cycle and has two major periods:
- Interphase: where cells spend most of their time, during which they grow, carry out their function and replicate their DNA;
- Cell division: in this shorter phase cells divide to form two new daughter cells.
Interphase: DNA replication
Before a cell can divide, it must duplicate its DNA. This usually exists as two polynucleotide strands, coiled into a helix. Each strand is a chain of four types of nucleotide - adenine (A), guanine (G), cytosine (C) and thymine (T). The specific pairing of these nucleotides (A+T, G+C) allows both strands to be used to construct new DNA molecules (Marieb, 2000). Each new molecule contains one original and one new strand, a process known as semi-conservative replication (Fig 1).
During interphase, DNA forms chromatin, a network of unstructured threads throughout the nucleus (Fig 2), which allows easy access to the DNA for copying of genes for protein synthesis. When a cell begins to divide, chromatin condenses into more compact, manageable structures - the chromosomes, which are formed from two identical copies of DNA (called sister chromatids), joined by a centromere.
There are two types of cell division:
Mitosis: The most common type of cell division, this occurs for growth and repair. It produces two daughter cells that are genetically identical to the parent cell. Mitosis has four main stages - prophase, metaphase, anaphase and telophase (Fig 2).
During mitosis, the chromosomes line up along the equator of the cell attached to spindle fibres by their centromeres (metaphase). The spindle fibres contract, dividing the centromeres in half and pulling the chromatids to opposite ends of the cell (anaphase). The cell then divides to produce two identical daughter cells.
Many factors influence mitosis, including some hormones and peptides, available nutrients, space in which to grow and a cell’s degree of specialisation. Generally, the more specialised a cell, the less likely it is to reproduce. For example, neurons (nerve cells) cannot replicate, so damaged cells cannot be replaced (Brooker, 1997). In contrast, epithelial cells lining the gut may divide every day (Montague et al, 2005). Cancers are a result of uncontrolled mitosis that causes malignant cell growth.
Meiosis: It is vital that sex cells (gametes) that come together to form a new zygote after sexual reproduction have the correct number of chromosomes and the necessary genetic information to ensure growth and development. This requires the complex process of meiosis, which occurs in the testes in males to produce sperm cells, and in the ovaries in females to produce egg cells. Unlike mitosis, it produces non-identical daughter cells with half the amount of DNA (haploid) compared with the parent cell (diploid). To halve the number of chromosomes, cells undergo two divisions, meiosis I and meiosis II (Fig 3).
Meiosis I has four stages. In prophase I, homologous chromosomes pair up and ‘crossing over’ occurs, during which sections of one chromatid may break off and reconnect to the other, swapping DNA (this contributes to genetic variation in the haploid daughter cells). During meiosis I, homologous pairs of chromosomes line up along the equator of the cell attached to the spindle fibres (metaphase I). Chromosome pairs can each line up in one of two orientations - there are 8,388,608 possible combinations of chromosomes for the final gamete. When the spindle fibres start to contract in anaphase I, whole chromosomes are pulled to opposite poles and the cell undergoes cytokinesis to form two daughter cells, each containing 23 chromosomes.
Meiosis II starts with half the amount of chromosomes. At the end of meiosis II, the parent cell has produced four non-identical daughter cells, each with one chromatid of 23 chromosomes. In males, all four cells become sperm cells and in females, one gives rise to an egg (Montague et al, 2005).
The crossing over of chromatids and the independent assortment of chromosomes during meiosis contribute to genetic variation in individuals.
Genes and inheritance
Since chromosomes exist in pairs (except the X and Y chromosomes), genes are also present in pairs. The two chromosomes in a pair (homologous chromosomes) each carry a gene for the same characteristic in the same place (locus). For example, let’s say a gene determining whether individuals can roll their tongue sits on each chromosome 5. There are two kinds of these genes - one of which enables individuals to roll their tongue (R) and one that does not (r). Different variants of genes are known as alleles, and define the characteristic in different ways. For tongue rolling, the following combinations of genes are possible - RR, rr, Rr and rR - so there are three possible genotypes - RR, Rr or rr - that determine whether or not individuals can roll their tongue. This is known as the phenotype.
Some alleles for genes exhibit dominance over others. In the case of tongue rolling, R is dominant over r (which is recessive) so, even if individuals inherit only one R allele, they are able to roll their tongue. Those who cannot roll their tongues have the double recessive allele combination rr (Jones and Jones, 1997).
Some alleles show co-dominance, such as the genes controlling blood group inheritance (Montague et al, 2005). The alleles for group A and group B are both dominant over that for group O but are not dominant over each other. Thus, individuals acquiring the allele for group A from one parent and group B from the other will have blood group AB.
Brooker, C. (1997) Human Structure and Function: Nursing Applications in Clinical Practice. New York, NY: Elsevier.
Jones, M., Jones, G. (1997) Advanced Biology. Cambridge: Cambridge University Press.
Marieb, E.N. (2000) Essentials of Human Anatomy and Physiology. San Francisco, CA: Benjamin Cummings.
Montague, S.E. et al (2005) Physiology for Nursing Practice (3rd ed). New York, NY: Elsevier.