Whether in school biology class, a discussion of medical problems, a police investigation, a protest against GMO foods or science fiction, everybody has heard of chromosomes, genes and DNA. Understanding what they are is another matter. A lot of people get confused about this and make stupid mistakes. They’re all related to how living things store hereditary information and pass it on to future generations.
DNA is an acronym for deoxyribonucleic acid, the chemical that encodes the hereditary information in all living things except a few viruses, which use the similar ribonucleic acid (RNA). DNA has the famous double helix structure discovered by Watson and Crick. Each strand consists of alternating phosphate groups and pentose (deoxyribose) sugars. Each sugar has one of four bases attached to it. The bases are adenine (A), cytosine (C), guanine (G) and thymine (T). The sequence of these bases is the genetic information. In the double helix, the opposite bases are joined by hydrogen bonds (which are weaker than the normal bonds between atoms in molecules), with A always pairing with T and C with G. This means that when the two strands are separated, as they are during replication, a copy of the other strand can be made using this pairing rule. This doesn’t just happen by magic, it’s a complicated process involving enzymes. This means that DNA can be replicated allowing reproduction, the defining characteristic of life. Normally a cell replicates its DNA and then divides into two cells, with identical DNA in each. This process is called mitosis.
Organisms are divided into domains (this excludes viruses and some other things): Bacteria, Archaea and Eukaryota. Bacteria and Archaea are all single celled microbes, or simple multicelluar organisms (it’s debatable where to draw the line between a colony of single celled organisms and a multicellular organism). Both have chromosomes that float free in the cytoplasm with no membrane-bound nucleus. They also often have smaller bits of DNA called plasmids. Bacteria can have one or more chromosomes, which can be circular (a loop with no ends) or linear (with two ends) or both (if there’s more than one). The plasmids may also be circular or linear. See Chromosomes in Bacteria. Archaea are poorly understood but seem to have circular chromosomes and plasmids. Plasmids can be transferred between cells, not necessarily of the same species.
Eukaryota includes animals, plants, fungi and a few other groups. Some are single celled and microscopic but this domain includes most multicellular organisms. They have a number of chromosomes contained in the nucleus as well as circular DNA in their mitochondria (parts of the cell responsible for using oxygen to produce energy) and plastids (in plants). Eukaryotes can also have plasmids. The chromosomes in the nucleus are bound to proteins called histones and comprise most of the organism’s genetic material.
The modern definition of “gene” as described by Helen Pearson in Nature in 2006 is “a locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions, and or other functional sequence regions”. Much genetic material seems to have no function but differences in this can be used to determine biological relationships. Many genes get transcribed as RNA in a process similar to replication called transcription. Some types of RNA, called rRNA, are used to make ribosomes (the parts of the cell that make proteins). Transfer RNAs (tRNAs) combine with amino acids. Other types of RNA, called messenger RNA (mRNA) are used to make proteins. Eukaryotes process mRNA by adding bits to the beginning and end and removing sections called introns before exporting it from the nucleus. Ribosomes latch onto mRNA and move along it, adding amino acids to the growing protein as they do so. Each amino acid is coded for by a group of three bases called a codon. Each tRNA has a group of three bases called an anti-codon, which is the complimentary sequence to the codon, so tRNA acts as a translator between the bases and the amino acids. The correlation between codons and amino acids is called the genetic code. Like the amino acids, the genetic code also contains codons to start and end making the protein. The codon to start the protein is the same as the codon for the amino acid methionine, so this is always the first amino acid added, but it is often removed subsequently.
There are also genes that regulate the actions of others – certain proteins bind to them to promote or inhibit another gene from being expressed. For example, if a certain animo acid is plentiful, the organism switches off the gene for making it. This gets particularly complicated in multicellular eukaryotes as they have many genes that are only used in certain cells.
Different versions of a gene are called alleles. For example the gene for ABO blood types has three alleles: A, B and O.
Most organisms can reproduce asexually at least some of the time. This creates a genetically identical organism known as a clone. “Clone” can also refer to a group of genetically identical organisms.
Many eukaryotes have two sets of chromosomes (diploid) and reproduce sexually at least sometimes. During sexual reproduction, the two pairs of chromosomes pair off and parts are exchanged between the corresponding chromosomes in each pair. This is called “crossing over”. Genes that are near each other on the same chromosome tend to get inherited together and this can be used to determine the position of genes on the chromosomes. Sometimes genes are grouped closely into supergenes which rarely get separated by crossing over. Then the cell divides into four, with one set of chromosomes in each. This process is called meiosis. In some plants, such as ferns and mosses, these cells (which are describes as haploid because they have only one set of chromosomes) develop into spores that then grow into individuals that produce gametes (sex cells). This sort of life cycle is called “alternation of generation”. In other organisms, the haploid cells simply develop into gametes. Two gametes fuse to form a zygote with two sets of chromosomes, which becomes a new diploid individual with a new combination of alleles.
The mitochondria and plastids are only inherited from the female parent (the one that produced the egg or seed), so any genes on them show female line inheritance.
Different eukaryotes have differing numbers of chromosomes but it tends to be fairly consistent within a species (although some plant species have different numbers of sets). All normal humans have 46 and most people who don’t have a fairly obvious abnormality (e.g. Down Syndrome, Turner Syndrome or Kleinfelter Syndrome) but men with an extra Y chromosome are fairly normal.
In many animals, the sexes have differences in their chromosomes. In almost all mammals, including humans, males have an X chromosome and a small Y chromosome while females have two X chromosomes. In birds the males have two W chromosomes while the females have a W and a Z chromosome. Flies work like mammals. Male butterflies have two X chromosomes while the females have only one. In Hymenoptera (ants, bees and wasps), the males are haploid and develop from unfertilized eggs while the females are diploid.
The characteristics of an organism tend to be the result of multiple genes and environmental influences and it can be hard to determine which genes are responsible and to what extent. Diploid eukaryotes can have different alleles for the same gene. In this case one (which is referred to as “dominant”) can over-ride the other (“recessive”). This is the case if you have a allele for blood type A and another for blood type O, you will have blood type A. It’s also possible for the two alleles to produce a combined effect (partial dominance). This is what happens if you have an allele for blood type A and another for type B, you will have blood type AB. Dominant characteristics run in families in an obvious way while recessive characteristics tend to affect siblings but don’t show a parent-to-child pattern (unless two organisms with it produce offspring in which case all the offspring will have the recessive characteristic). There are also genes that are carried on the sex chromosomes, mitochondria or plastids which follow the male or female line or cause characteristics that are more likely in one sex than the other (e.g. haemophilia and color blindness). Some genes only affect one sex but are not carried on a sex chromosome.
DNA has been sequenced from many organisms, including humans and some extinct species. The complete DNA sequence of an organism is called a genome. DNA is increasingly being used to determine evolutionary relationships between species and populations. It was recently found that all non-African humans have some Neanderthal DNA and some Asians have DNA from a mysterious extinct species called Denisovians. DNA is often used in criminal investigations and paternity cases to determine who biological material came from and the biological relationships between people and sometimes other organisms.
Since time immemorial, humans have been genetically manipulating organisms by cross breeding and selective breeding. It’s now possible for scientists to manipulate genes in more specific ways. This can produce organisms called GMOs (acronym for “genetically modified organism”) with genes from unrelated species or even artificial genes (a protein can be designed with specific characteristics and then the genetic code can be used to generate an allele that will code for that protein). Changing the genes in an organism that’s already alive is harder but there are ways to do it (e.g. replacing natural tissue with genetically modified tissue or using a genetically modified virus to introduce new genes into cells). It’s theoretically possible to genetically modify organisms using nanotechnology. This is somewhat controversial because it may produce very dangerous organisms (very lethal viruses or ecosystem destroying plants are more likely than re-created Tyrannosaurus) and, certainly with humans, it leads to issues of eugenics, prejudice and fascism. It could raise awkward problems like whether a genetically modified human (e.g. a recreation of a neanderthal), or a non-human with human characteristics (e.g. a chimp with human intelligence) should be legally considered human. A certain seed company has thrown gasoline on this fire with its aggressive legal actions, such as suing farmers who’ve apparently had their fields contaminated with GMO seeds.
DNA, genes and chromosomes have always been part of life on Earth, although we’ve only recently come to understand them and still don’t fully. There are bound to be increasing advancements (e.g. curing previously incurable diseases and producing more efficient crop plants) and controversies, if not disasters, relating to them.
