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This text was drafted by award-winning MMW TAs Tara Carter and Beth Peterson, (Anthropology) in the summer of 2008, supported by the MMW-based research funds left in his account on the death of Prof. Donald F. Tuzin, long an instructor and supporter of MMW. The text was substantially modified by Professor David K. Jordan in summer, 2011 to fit it to the needs of MMW-11.
This text may be freely reproduced for non-commercial educational purposes.
Most college students are veterans of a high school biology class that included one or more units on genetics. This essay briefly reviews some basic principles of cell biology, genes, DNA, cell division, and sources of genetic variation in populations.
This is, of course, an important part of understanding how evolution works, but the broad outline —the vision— of natural selection operating on biological variation is older than the details of genetics, as we have seen. So this essay is consigned to an appendix that you can refer to as needed or desired. Although a broad understanding of it is assumed in the other essays, the details are not.
Human beings are made up of hundreds of billions (maybe even trillions) of cells, the basic unit of all life. Life on earth began about 3.5 million years ago with the first cells, called prokaryotic cells. These are single-celled organisms like bacteria and blue-green algae. The kind of cells that make us up are eukaryotic cells. These are complex cells containing different structures: cytoplasm, mitochondria, ribosomes, nucleus, endoplasmic reticulum, DNA, and RNA.
There are two types of eukaryotic cells in the human body: (1) somatic cells and (2) gametes. Somatic cells are the cells of the body and its tissues such as skeletal cells, muscle cells, neurons, and blood cells. Gametes, or sex cells, are involved in sexual reproduction.
Animals have two kinds of gametes: ova (singular: ovum) or egg cells, produced in the female ovaries, and sperm cells produced in the male testes. In sexual reproduction, the two gametes meet to form a zygote (fertilized egg) which has the potential to form a new organism.
Inside the nucleus of a cell are two important molecules: DNA (deoxyribonucleic acid), and RNA (ribonucleic acid). DNA and RNA contain the genetic information that controls cell function, including directing protein synthesis. Proteins in our bodies are essential for virtually all body functions and include hemoglobin, enzymes, hormones, and the proteins that regulate the function of DNA.
A DNA molecule is composed of two chains of smaller molecules (called nucleotides), which are lined up next to each other and twisted around, forming a ladder-like structure referred to as a double-helix. There are 4 different types of nucleotides —adenine, thymine, cytosine, and guanine— and they always come in pairs. Adenine is always paired with thymine (RNA uses uracil instead of thymine, but we do not need to worry about that) and cytosine is always paired with guanine.
When cells multiply they make copies of themselves, and one of the first steps in cell division is DNA replication. This ensures that each new cell has the same DNA so that it can function properly.
A gene is a sequence of DNA at a particular locus (or location) that codes for a specific protein, RNA, or nucleotide —the functional substances at the root of life. Genes and their actions are extremely complex and, even today, little understood. Some genes, called regulatory genes, control the function and expression of other genes by turning them on and off.
All somatic cells contain all of the genetic information of an organism, but in any one cell, only certain genes are active. For example, all of the cells in your stomach contain exactly the same genetic information as all of the cells in your brain. But stomach cells do not produce neurotransmitters and brain cells do not produce digestive enzymes. The function of cells is determined during embryonic development (i.e., from fertilization through the 8th week of pregnancy) by regulatory genes that turn some genes off and some genes on, making stomach cells produce digestive enzymes and brain cells produce neurotransmitters.
There are genes that code for blood type, hair color, earlobe shape, height, and just about any other physical characteristic you can think of. Human beings vary greatly in appearance and physical endowment, meaning that the genes that code for particular characteristics must differ from one another. These variants are called alleles, about which more below.
There are 2-3 meters (6-9 feet) of DNA in the nucleus of each of our somatic cells and this DNA is organized into discrete structures called chromosomes.
Chromosomes are structures composed of DNA and proteins that are only visible under a microscope during certain phases of cell division. Humans have 46 chromosomes in 23 homologous pairs (23 from our mothers and 23 from our fathers). Homologous chromosomes carry genes that govern the same traits. Of those chromosomes, 44 (22 pairs) are autosomes, meaning they carry the genetic information for all physical characteristics.
An exception is sex, which is determined by the 2 sex chromosomes. In mammals, females have 2 “X” chromosomes and males have 1 “X” and 1 “Y” chromosome. The ovum from the mother always contributes an X chromosome while the sperm from the father contributes either an X or a Y.
Cells with all 46 chromosomes are called diploid. Cells with only 23 chromosomes, one from each pair, are called haploid.
Other species have different numbers of chromosomes: chimpanzees, gorillas, and potatoes have 48, dogs and chickens have 78, cows have 60, and the Adder’s tongue fern has a whopping 1,440!
Alleles are alternative forms of the same gene at the same locus on the same chromosome. For example, the ABO blood type —from the familiar list of types A, B, AB, and O— is governed by three alleles —A, B, and O— that are found on the 9th chromosome. Since there are two copies of every chromosome in every cell, if an individual has one A allele and one B allele he or she will have AB blood. Two As or two Bs will produce Type A or Type B blood. And two Os will produce Type O. But A and B alleles are each dominant over O, so a person with an O allele plus an A or a B will have Type A or Type B blood. In contrast, alleles for A and B blood type are co-dominant with respect to each other, meaning that individuals with one copy of each will have AB blood. Both dominance and co-dominance are common relationships among alleles.
Some traits, like blood type, are governed by the expression of alleles at only one locus, i.e., at only one location on each of the paired chromosomes. The associated traits are called Mendelian traits after the famous monk Gregor Mendel who was the first person to discover principles of inheritance. Other traits are governed by the action of more than one gene; these are called polygenic traits.
There is a special kind of DNA found in cells that is worth a little extra attention. Mitochondria are the power-houses of our cells, responsible for energy, to drive cellular functions. Mitochondria have their own DNA, called mitochondrial DNA (abbreviated mtDNA), which does not mix with nuclear DNA.
(It is likely that this is a result of the fact that the mitochondria in our cells today were once independent organisms, prokaryotic cells, that formed a symbiotic —i.e., mutually beneficial— relationship with eukaryotic cells and over time evolved to become a part of those cells.)
Now we come to a fact of great importance to evolutionary biologists: It turns out that all of the mitochondria we have in our bodies come from our mothers. Sperm cells are very small and basically carry as little genetic material as possible so that they can swim faster, increasing the likelihood that they will fertilize an egg. Egg cells, on the other hand, are very large and contain many different structures that facilitate zygotic development, including mitochondria. Since all of our mitochondria come from our mothers, mtDNA can be used to trace descent without the complications that sexual reproduction and recombination (to be discussed below) produce. The importance of mitochondrial DNA for studies of human evolution is discussed in Essay 10.
There are two processes of cell division: mitosis and meiosis. Mitosis is the division of somatic cells and occurs during growth and the repair and replacement of tissues. The result of mitosis is two identical daughter cells that are genetically identical to the original cell. Meiosis is the production of gametes and, for present purposes, is deserving of a little more attention.
The first step in mitosis is DNA replication. Specialized proteins break apart the bonds holding the 2 nucleotide strands of a DNA molecule together. Then the two nucleotide strains match up with corresponding unattached nucleotides floating around in the nucleus. The end result is two newly formed strands of DNA. Each new strand is joined to one of the original strands of DNA, or chromosomes. Once the DNA have replicated, the doubled pairs of chromosomes split from one another, migrate to different sides of the cell. The cell, now with two complete sets of paired DNA (23 pairs, 46 individual chromosomes), divides in two. There are now 2 identical cells, each with full complement of chromosomes.
Meiosis, on the other hand, is characterized by two divisions that result in four daughter cells, each of which contains only 23 chromosomes (1 member of each pair of chromosomes). Similar to mitosis, the first step in meiosis is DNA replication. Inside the nucleus of the diploid cell, the DNA replicates, resulting in a cell with two identical copies of each pair (46 chromosomes times two —two copies of each pair!). These paired chromosomes then line up next to one another and swap genetic material. This process is called recombination —the exchange of genetic material between homologous chromosomes during meiosis. Recombination is an extremely important component of the production of sex cells.
After recombination occurs, the homologous pairs separate from each other into new cells, but they are still double-stranded. In other words, there are now two daughter cells and each contains two copies of only one member of each original pair (23 exactly duplicated, non-partnered chromosomes).
In the second meiotic division the two double-stranded chromosomes split and the strands move to different sides of the cell as the cell divides. The whole process has created a total of 4 new haploid cells, that is, cells each of which has half the usual number of chromosomes. Some of these cells may mature to be functional gametes that pass genetic information to the next generation.
The process of meiosis, which takes place in all sexually reproducing organisms, increases genetic variation in a population because it produces offspring that are not genetically identical to the parents —offspring are a combination of the genetic material of both sexes in the parental generation. Thus, sexual reproduction produces variation upon which natural selection can act. We recall that variation in a population is one of the necessary conditions for natural selection. Some individuals, because of the expression of their genes (their phenotypes ) are more likely to produce offspring that survive to reproduce, and this variation in individual fitness is what allows species to adapt to changes in selection pressures.