The Cellular Model
So having provided an introduction of some of the basic terminology, we might now extend the description to include an overview of the current cellular model. However, it should be recognised that the full scope of molecular biology, genetics and epigenetics now associated with evolution is far too wide and complex to be addressed in any detail and therefore this discussion will only act as another introduction to the metabolism and biochemistry of cells - see ‘The Structure of Life’ for some further outline details.
So, by way of a starting point for this discussion, it will be simply stated that there are two basic types of cells, i.e. prokaryotic and eukaryotic, where the former is restricted to very primitive lifeforms, e.g. bacteria, which is not the focus of this discussion. As such, the evolution of eukaryotic cells forms the basis of almost all plant and animal life on planet Earth, including humans. While there are many structural details associated with eukaryotic cells, from an evolutionary perspective, the central nucleus in which DNA is stored is the primary structure of interest. In this respect, DNA is considered to be the repository of all information required to build cellular proteins, which is protected by a membrane that surrounds the nucleus and partitions the DNA blueprint from the protein synthesis machinery within the main body of the cell. However, we might briefly characterise the role of the cell in the following way:
- All known living things are made up of one or more cells.
- All living cells develop from pre-existing cells.
- Cells are the fundamental unit of structure and function.
- Cells contains a nucleus that contain chromosomes that contain DNA.
We might infer from the last bullet that even the cell nucleus is not without its own structural complexity, in fact, we might assume that this structure to be the source of all the evolutionary complexity to be discussed. For while DNA provides a mechanism by which heredity information is transmitted, it exists in the structural form of chromosomes, the number of which can vary within each species. Equally, while it might be stated that ‘all living cells develop from pre-existing cells’ there are two distinct processes, e.g. mitosis and meiosis, in which cells divide and multiply. The former, mitosis, is a form of eukaryotic cell division that produces two essentially identical daughter cells with the same genetic information as the parent cell. The latter mechanism, meiosis, defines the sexual reproduction process of primary interest in terms of evolution. Again, it might be useful to summarise some of the basic cellular mechanisms required to support meiosis as a frame of reference:
- Human DNA is decoded into some 3 billion base pairs .
- Base pairs are a construct of adenine/thymine plus guanine/cytosine.
- The human DNA sequence is broken into 46 chromosomes grouped into 23 pairs.
- The alignment of the first 22 pairs represents DNA from each parent.
- The final pairing of the X and Y chromosomes relate to sexual evolution.
- Females have a XX chromosome pairing inherited from female/male parents, i.e. Xf.Xm.
- Males have a XY chromosome pairing inherited from female/male, i.e. XfYm.
- DNA sequences within chromosomes are identified in terms of 21,000 genes.
As sons can only inherit their Y chromosome from their father, it is implicit that sons can only get their X chromosome from their mother. As such, the Y chromosome is a paternal inheritance along the male evolutionary line. The Y chromosome is the smallest unit containing only 50 genes, while the X chromosome contains about 800 genes. Most of the other chromosome pairs (1-22) contain between 200-2000 genes. In this respect, evolution proceeds by the offspring having a mix of genetic DNA information from both male-female parents, where the sex of the offspring is determined by the XX or XY chromosome permutations as outlined above.
Note: All the genes on the other 22 chromosomes associated with either female (X) or male (Y) chromosomes broadly have the same function, although mutations exist. However, these paired variant genes, or alleles , are often described in terms of their dominance, although this characteristic will be outlined later in the discussion.
In order to better understand the reproductive process, two distinct types of cells need to be introduced. i.e. somatic and germinal. The division of somatic cells is linked to mitosis, while the germ-line cells associated with sexual reproduction are linked to meiosis:
- Somatic Cells: Are used to construct all the tissue and organs within the body.
- Germinal Cells: Are the egg or sperm cells of the ovary or testis also referred to a gametes.
However, the key structural difference between these two types of cell is that somatic (diploid) cells have 46 chromosomes contained within the cell nucleus, which are inherited from both male-female parents, while germinal (haploid) cells only have a half-set of 23 chromosomes.
So how does sexual reproduction via meiosis work?
Again, the full details of all the processes associated with meiosis is way beyond the scope of this discussion, such that only a general outline of some of the key issues will be highlighted. In this context, meiosis is a process by which sexually reproducing organisms, e.g. human, generate gametes, otherwise known as sex cells in the form of eggs and sperm. In this respect, humans like all other mammals use meiosis to serve a number of important functions, e.g. the promotion of genetic diversity and the conditions for subsequent reproductive success. As indicated, one of the primary function of meiosis is the reduction of the number of chromosomes from 46, i.e. 23 pairs, to just one set of 23 chromosomes. However, while both males and females use meiosis to produce their gametes or germinal cells, there are some key differences in the overall process that might simply be highlighted. While the process starts with ‘primordial germinal cells’ being separated from the body’s normal somatic cells, it ends via a complex, multi-stage process in the production of a germinal cell, i.e. sperm or egg, that contains just 23 chromosomes of DNA information. As indicated, there are number of key differences in the formation of male-female germinal cells, which will not be detailed but might be highlighted by general examples. For instance, human males start to produce sperm at the onset of puberty, while human females start to produce eggs while still in the womb, although this process is then stalled until the females reaches puberty.
Note: Unlike sperm, which can be produced daily throughout the lifetime of a male, a human female is born with about 1-2 million potential eggs, although only about 400,000 might survive to puberty. However, we might estimated that only 480 of these eggs will ever be released during her reproductive years, i.e. one egg per monthly cycle over a potential period of 40 years. It might also be highlighted that a woman’s eggs originated from her mother while her mother was still inside her grandmother’s womb at around the 20th week of gestation.
Of course, the structure of the germinal cells in male and females have to meet very different ‘design’ requirements, e.g. sperm only has to be mobile in order to reach and fertilize the egg. As a consequence, sperm cells have very few cellular organelles and essentially only represent the 23 required chromosome of DNA information needed to combine with the 23 chromosomes within the female egg. In stark contrast, the human female egg cell is estimated to be over 100,000 times larger than the sperm cell because it has to contain all the necessary sub-structures and environmental support required by cell division after sperm fertilization has occurred. When the sperm and egg gametes combine, the composite cell structure is called a zygote, which continues to develop based on the DNA information in the two sets of 23 chromosomes, i.e. 46 in total, contained in all subsequent cell division taking place within the zygote by mitosis.