Genetics, DNA, and the Human Genome: A Comprehensive Overview
1. Chromosomes
Genes reside within the cell nucleus, on structures called chromosomes. Every species has a characteristic number of chromosomes in each cell. Humans possess 46 chromosomes. This number remains constant due to the halving of chromosomes during sex cell formation. Human sperm and eggs each contain 23 chromosomes, which combine during fertilization to form a zygote with 23 pairs. Gametes (sex cells) are haploid (n=23), while somatic cells (body cells) are diploid (2n=46). Of the 46 chromosomes, 44 are autosomes, and two are sex chromosomes (X and Y).
A karyotype is an ordered display of an individual’s chromosomes. Chromosomes vary in size and gene content. Human chromosomes are arranged by size. We inherit one copy of each chromosome from each parent. An abnormal number of chromosomes can lead to diseases or deformities, and can also contribute to tumor formation.
2. Genes
Genes are DNA segments that encode instructions for protein synthesis. The same gene can manifest differently depending on environmental factors (Phenotype = Genotype + Environment). Gene expression allows individuals to adapt to their environment. For example, sun exposure triggers melanin production, darkening the skin.
All body cells contain the same genes, but their expression varies depending on cell type (e.g., heart, liver, muscle). DNA molecules carry genetic information. Watson and Crick, building on Rosalind Franklin’s work with X-ray diffraction, discovered the double helix structure of DNA in 1953. Franklin’s contribution was crucial, but she was not recognized with the Nobel Prize.
3. DNA
DNA consists of two intertwined strands forming a double helix. A human cell’s DNA, spread across 23 chromosome pairs, measures two meters and contains about 6 billion base pairs. Each strand comprises phosphate groups, sugars, and nitrogenous bases (adenine, guanine, cytosine, and thymine). Adenine pairs with thymine, and cytosine pairs with guanine.
The sequence of these bases encodes biological information. DNA replication involves separating the two strands, each serving as a template for creating a new complementary strand. A gene is a DNA fragment containing instructions for making a protein. RNA carries these instructions from the nucleus to the protein synthesis machinery.
Genetic differences reflect evolutionary distance. Humans and chimpanzees share 98.4% of their genes.
4. The Genetic Code
The genetic code links nitrogenous bases to amino acids. Each three-base sequence (codon) codes for a specific amino acid. Important proteins include antibodies, hemoglobin, insulin, and keratin.
Mutations are changes in the DNA base sequence. Their effects range from negligible to lethal, and they are also a source of new traits.
5. Genetic Engineering
Genetic engineering involves transferring genes between species. Restriction enzymes, produced by bacteria, cut DNA at specific sites, enabling gene isolation. The first recombinant DNA was created in the 1970s by inserting an amphibian gene into a bacterium. This marked the birth of genetic engineering, initially met with a moratorium due to safety concerns.
Plasmids, extrachromosomal DNA in bacteria, often carry antibiotic resistance genes. Genetic engineering has produced oil-degrading bacteria, insecticide-producing plants, and larger rabbits. Research is ongoing to develop biofuel-producing plants and bacteria with synthetic DNA, taking the first steps toward artificial life.
6. Transgenic Organisms
Transgenic or genetically modified organisms (GMOs) contain genes from other species. Plasmids serve as vectors to carry these genes. The use of transgenic plants in food raises safety concerns. This issue has scientific, economic, political, and social dimensions, requiring societal decisions.
Transgenic animals are created by integrating genes into early embryos. Genetically modified sheep, cows, pigs, and chickens exist. The primary goal is to produce medically useful substances. For example, goats have been modified to produce spider silk protein in their milk.
7. Gene Therapy
Gene therapy utilizes retroviruses, which can integrate their genes into host chromosomes. Genetic testing for hereditary diseases is becoming increasingly important, especially for prenatal diagnosis. Concerns exist about potential discrimination based on genetic profiles, such as in employment or health insurance.
While human DNA is largely the same, small variations make each individual unique. DNA analysis is used in paternity testing, criminal investigations, and historical research. For example, DNA analysis confirmed the identity of the Romanov family remains.
Gene therapy research began with non-pathogenic retroviruses carrying human genes. The”bubble bo” disease, caused by a single gene defect, was an early target. Addressing diseases involving multiple genes, like cancer, is more complex. Gene doping, a new form of sports doping, is also a concern.
8. The Human Genome Project
a) Overview
The Human Genome Project, completed in 2000, mapped the entire human genome. The project, initiated in 1990, was accelerated by competition between a public consortium and Celera Genomics. Humans have about 30,000 genes. A single gene can produce multiple proteins. 95% of the human genome was initially considered”junk DNA” but it now appears to play a regulatory role. While 99.9% of human genes are identical, the remaining 0.1% accounts for individual differences. Pharmacogenetics aims to develop personalized drugs based on individual genetic profiles.
b) Genetic Fingerprinting
In 1985, Alec Jeffreys developed a genetic fingerprinting method. Minisatellites, short repeating DNA sequences, vary in number between individuals. Analyzing these variations creates a unique”barcod” for identification, used in paternity tests, criminal investigations, and other applications.
c) Stem Cells and Cloning
Fertilization produces a zygote, which develops into a blastocyst containing embryonic stem cells. The blastocyst implants in the uterus, forming the placenta and fetus. Stem cells are unspecialized cells capable of differentiating into various cell types. They are crucial for growth and tissue repair. There are three types: totipotent, pluripotent, and multipotent. Stem cell potential decreases with development. Stem cell research holds promise for treating diseases and regenerating tissues.
Dolly the sheep, cloned in 1997, was the first mammal cloned using nuclear transfer. Human cloning was achieved in 2001. Therapeutic cloning aims to produce stem cells for tissue regeneration without rejection. Cloning efficiency is currently low, and ethical concerns exist regarding the use of embryos.
d) Assisted Reproduction
Assisted reproduction techniques help couples with fertility problems. In vitro fertilization (IVF) involves fertilizing an egg outside the womb. The first IVF baby was born in 1978. Ethical questions arise regarding embryo selection and the storage of frozen embryos.
9. Nonlinear Systems
Nonlinear systems, like the climate, involve interconnected factors. For example, increased CO2 leads to warming, which increases evaporation, further enhancing warming. However, cloud formation can cause cooling. Human activities contribute to net atmospheric warming.
10. The Atmosphere
The Earth’s atmosphere consists of the troposphere, stratosphere, and ionosphere. The troposphere contains most of the atmospheric gases and is where weather occurs. The stratosphere contains the ozone layer, which protects us from UV radiation. The ionosphere reflects radio waves.
11. The Water Cycle
The water cycle involves evaporation, transpiration, condensation, and precipitation. Water evaporates from the surface, condenses into clouds, and returns to Earth as precipitation. Plants contribute to the cycle through transpiration.
12. Air
Air is composed mainly of nitrogen and oxygen. Human activities release CO2, contributing to the greenhouse effect. Other pollutants include sulfur oxides, nitrogen oxides, methane, carbon monoxide, and particulate matter. CFCs damage the ozone layer. The ozone hole, discovered in 1977, has been slowly recovering in recent years.