7 March 2005
Lecture 24
Reading: Chapter 8, Chapter 9, Chapter 10
VI. Genes
A. Protein synthesis (The Central Dogma)
B. Genomes
All the DNA in the cells of an organism are its "genome". In recent years, sophisticated chemical methods for determining the nucleotide sequence of large pieces of DNA have been combined with computing technology to discover and begin analyzing the genomes of organisms. A handful of bacteria were sequenced first and the earliest of these were completed nearly 10 years ago. Bacterial genomes are very small compared to those of eukaryote organisms and so they were relatively easy to sequence. More recently, the genomes of eukaryote organisms have been sequenced. These are so large that international consortia of large sequencing facilities are required to sequence them and the effort takes years. The human genome is the most famous to have been sequenced but the list of sequenced organisms is steadily growing.
Genome sequences are a new body of information about living things. Using them to understand living things is a branch of biology called "genomics". Nobody really knows what genomics is. We are making it up as we go along. Much of what is learned from genomes requires sophisticated computer analysis and the branch of biology called"bioinformatics" has arisen with genomics as a result.
New concepts in biology that have been generated by genomics include the following:
- Genome size is highly variable between different organisms. Both the total amount of DNA and the fraction of it that codes for proteins (genes) can be very different in organisms that are otherwise similar. Most surprising in this regard is that organisms we consider to be very complex, e.g. humans, have fewer genes than organisms that are traditionally considered simple, e.g. corn plants.- Much of the DNA in eukaryote organisms does not appear to have any function. It does not code for proteins nor do anything else that we can understand. At present, this DNA is referred to as "junk DNA". In the case of humans, about 0.1% of the total DNA in our cells codes for proteins. The rest appears to be "junk", also known as "non-coding DNA".
- The non-coding parts of DNA can be different, even between related individuals of the same species. That is how DNA evidence can be used to link a DNA sample to a single individual human. The coding parts of the DNA, however, can be similar even between very different organisms. Genes that code for the parts of ribosomes, for example, are very similar between humans and cells of baker's yeast.
The table below allows comparisons of genome size and gene number between a group of fully sequenced organisms.
Human
C. Gene regulation
Nearly all your cells have all the genes of the human genome. In a given cell, however, only a subset of these genes are active. In fact, different cell types, e.g. muscles cells or nerve cells, result from the activation of different sets of genes. Thus the regulation of genes controls the identity of cells. In addition to this, cells often change gene activity in response to their surroundings, e.g. the presence of a hormone in the external environment may cause cells to make new proteins or change their function in other ways. These changes are often the results of changes in the set of genes that is active.
Genes are regulated in several ways. Mostly, they are regulated by control of transcription but there are also some types of regulation that act after transcription.