An Introduction to Genetics
In most organisms, the hereditary
is Deoxyribonucleic Acid (
). In eukaryotes, DNA is found mainly in the
A virus that infects an bacterium such as Escherichia coli is called a bacteriophage, or phage. Here a T phage infects an E. coli cell; the protein coat remains outside the cell, while its DNA genome enters the cell and directs the reproduction of more phages.
Colorized image of human mitotic chromosomes as seen with a scanning electron microscope. A complete complement of visualized chromosomes for an individual is called a karyotype.
This is a human male karyotype, a display of condensed chromosomes arranged in pairs. Each chromosome consists of two sister chromatids joined at a centromere. Humans possess 23 pairs of chromosomes: one member of a pair is derived from each parent. 22 pairs are homologous and contain the same genes; the other 2 are sex-determining chromosomes: females are XX and males are XY.
Somatic cells divide by
, where each daughter cell receives a
set of chromosomes, one set from each parent.
Gametes (sperm and eggs) are produced by
and contain a
(single) set of chromosomes.
According to the
theory of inheritance, traits are inherited by
residing on chromosomes and transmitted through
A gene and may have several versions, called
, that are caused by changes in the DNA called
. The kind of alleles an individual possesses is called its
DNA has a
strands each made up of subunits called
DNA has a double helix structure. Each strand is made of subunits called nucleotides. Each nucleotide is made of a sugar (deoxyribose), a phosphate group, and a nitrogenous base which is either Adenine (A), Guanine (G), Cytosine (C), or Thymine (T). The bases form complementary AT and GC pairs across the helix.
According to the central
is transcribed to
, which is translated into
Central dogma of genetics. Gene expression involves transcription of DNA into mRNA, followed by translation of mRNA on a ribosome into a protein, which is made of amino acid subunits. The translation requires a dictionary, or genetic code.
RNA is similar to DNA except:
it has Uracil (U) in place of Thymine (T). + the sugar in its nucleotides is ribose instead of deoxyribose.
The genetic code consists of
nucleotides present in
. Each triplet encodes for insertion of a specific
acid into a growing
A protein is a sequence of 20 different kinds of amino acids. In this model, the three-dimensional shape, or conformation, of a protein is shown as a ribbon.
The process of producing a
via the central dogma is called gene
; the action of proteins play an important role in determining the
A hemoglobin (the oxygen-carrying molecule in red blood cells) is composed of 2 kinds of protein chains. Two a (alpha) globin and two b (beta) globin chains make up the molecule. A mutation (change in DNA) in the gene for the beta chain produces abnormal hemoglobin molecules in sickle cell anemia.
A single nucleotide change (mutation) in the DNA encoding the beta globin gene (CTC -> CAC) leads to an altered mRNA codon (GAG -> GUG). The resulting change in the amino acid (glu -> val) produces a mutant beta globin, causing sickle cell anemia.
The use of
has allowed us to understand the genetic basis of inheritance and diseases in humans.
Model organisms for genetic study are easy to grow, have a short life cycle, and produce many offspring. Model organisms include the roundworm Caenorhabditis elegans and the zebrafish.
Advances in DNA technology has enabled the science of
to study the entire
A colorized electron micrograph of Haemophilus influenzae, a bacterium that was the first free-living organism to have its genome sequenced, in 1995. This bacterium causes respiratory infections and bacterial meningitis (inflammation of the protective membranes of the brain) in humans.
A basic technique in molecular DNA manipulation is using
enzymes to produce
of desired genes.
Cloning a recombinant gene. A vector, such as a virus or plasmid, and a DNA fragment produced by cutting with a restriction enzyme are joined to produce a recombinant DNA molecule that is transferred to a bacterial cell. The desired recombinant DNA is cloned into many copies by replication of the DNA and by division of the bacterial cell.
Applications of biotechnology include:
Biotechnology has been used to produce genetically modified (GM) crop plants for increased herbicide, insect, and viral resistance, as well as for nutritional enhancement.
therapy and genetic
for the treatment of genetic
Glass plates in a DNA microarray contains thousands of fields to which DNA molecules are attached. DNA from an individual can be tested using to detect mutant copies of genes.
Diagram of the human chromosome set, showing the location of some genes whose mutant forms can cause hereditary diseases. Conditions that can be diagnosed using DNA analysis are indicated by a red dot. 1
Mice from the onc strain were the first genetically altered organisms to be patented in 1988. These mice were genetically engineered to be susceptible to many forms of cancer for studying cancer development and the design of anticancer drugs.
Dolly was cloned in 1996 from a mammary cell taken from a Finnish Dorset sheep by nuclear transfer. Dolly may have been susceptible to premature aging, but she did grow to adulthood and gave birth to a little lamb - Bonnie.
Other applications of genetics include elucidating the evolutionary
between species and solving
Comparison of a protein found in diverse vertebrates.