The branch of biology that studies genes, heredity, and variation in living things. The functional units of heredity are genes, which are composed of DNA and located on chromosomes. These instructions dictate the structure, function, and regulation of the body's cells, which are composed of proteins.
Key Concepts in Genetics
- DNA is a molecule that conveys genetic information for the growth, function, and reproduction of living creatures. It is composed of the nucleotides adenine (A), guanine (G), cytosine (C), and thymine (T) (T).
- Chromosomes are thread-like structures found in the nucleus of cells that are composed of DNA and proteins. The 46 chromosomes in humans are organized in 23 pairs. The sex of a person is determined by one of these pairs: XX for girls and XY for males.
- Genes are DNA segments that carry the instructions for producing proteins. They determine the qualities or characteristics of an organism, such as eye color or blood type.
- Alleles are different versions of a gene that determine differences in a characteristic. For instance, there are several alleles for eye color, including brown and blue.
- Certain alleles are dominant, which means they will be expressed even if only one copy is present (heterozygous condition). In contrast, recessive alleles are only expressed when two copies are present (homozygous condition).
- Phenotype and genotype: The genotype is an individual's genetic makeup, whereas the phenotype is the genotype's observable expression, such as physical or biochemical qualities.
Patterns of Inheritance
Genetics enables us to comprehend how qualities are transmitted from one generation to the next. Examples of common inheritance patterns include:
- Mendelian Inheritance: This system of inheritance, based on Gregor Mendel's ideas, involves dominant and recessive alleles and considers features determined by a single gene. Eye color and certain hereditary illnesses, such as cystic fibrosis and sickle cell anemia, are examples.
With this type of inheritance, numerous genes influence a single trait, resulting in a variety of phenotypes. Height, skin color, and body weight are examples. These qualities are determined by genes placed on sex-specific chromosomes (X or Y). Hemophilia and color blindness are examples. Genetic disorders are diseases or ailments resulting from mutations in a gene or genes. They may be inherited from parents or arise spontaneously as a result of mistakes in DNA replication. These are examples of prevalent genetic disorders: The disease known as cystic fibrosis
- Sickle cell anemia
- Down syndrome
- Huntington's illness
- Duchenne muscle disease
Genetic Counseling and Testing
Genetic testing is the process of evaluating a person's DNA to detect specific genes or mutations that may cause genetic illnesses. These data can be utilized for diagnostic, prognostic, and carrier testing. Genetic counseling assists people and families in comprehending the ramifications of genetic testing and making well-informed decisions on their health or reproductive possibilities.
Significant improvements have been made in industries such as health, agriculture, and biotechnology due to the study of genetics. Some important genetics applications include: By introducing, altering, or manipulating genes within an individual's cells, "'gene therapy"' tries to treat or prevent genetic illnesses. Gene therapy has demonstrated promise in the treatment of illnesses such as severe combined immunodeficiency (SCID), certain forms of inherited blindness, and certain types of cancer.
- Pharmacogenomics is the study of how a person's genetic composition effects their response to medications. This information can be utilized to generate personalized medicine, in which healthcare providers customize pharmacological treatment programs based on a patient's genetic profile, thereby maximizing drug efficacy and minimizing unwanted effects.
- Genetic engineering techniques can be used to alter the genetic makeup of creatures such as plants, animals, and microbes. GMOs have a variety of uses, including agriculture (e.g., generating crops with enhanced insect or environmental stress resistance) and biotechnology (e.g., producing insulin or other therapeutic proteins in bacteria).
- Gene Editing: Technologies such as CRISPR-Cas9 enable the exact change of the DNA of an organism. These techniques have the potential to be utilized in the treatment of genetic illnesses, the improvement of crop yield, and even the eradication of certain diseases transmitted by insects.
As genetics advances, it creates numerous ethical, legal, and social concerns that must be addressed. Among the primary considerations are:
- Privacy and Confidentiality: It is essential to protect the privacy and confidentiality of genetic information, as its abuse could lead to discrimination or stigmatization.
- People should be provided with accurate and understandable information regarding genetic testing, its potential advantages and hazards, and the implications of the results so that they can make educated decisions.
- Gene Editing and Designer Babies: The use of gene editing technology to modify human embryos raises ethical concerns around the possibility of generating "designer babies" with specific characteristics, such as intelligence or physical appearance. This may result in unforeseen outcomes and socioeconomic inequality.
The introduction of GMOs into the environment raises concerns over their possible influence on ecosystems, biodiversity, and human health.
- As genetics continues to evolve, it is crucial for society to engage in open dialogue and adopt appropriate standards and laws to guarantee that the advantages of genetic advances are achieved while addressing possible hazards and ethical challenges.
- Population genetics and evolutionary processes
- Population genetics is a subject of genetics that investigates the distribution and evolution of genetic diversity within populations. This research enables us to comprehend the evolutionary processes of mutation, natural selection, genetic drift, and gene flow.
- Mutations are alterations in the DNA sequence of an organism that can contribute new genetic variety into a population. Depending on their influence on an organism's fitness, mutations may be neutral, beneficial, or destructive.
- Natural selection is the process through which animals with specific heritable features have a greater chance of surviving and reproducing, hence increasing the frequency of those traits in the population. This process can result in populations adapting to their surroundings.
- Genetic drift is the random variation in allele frequencies within a population resulting from random events such as the survival and reproduction of individuals. Genetic drift can result in the loss of genetic diversity and the fixation of certain alleles in a population over time.
- Gene flow is the transfer of genetic material between populations as a result of the migration of individuals or the transfer of gametes. This method can provide new genetic variety into a population to combat genetic drift.
Genetics of Conservation
- Conservation genetics is the application of genetic principles to the preservation and administration of biodiversity. This field seeks to comprehend the genetic basis of species adaptations, evaluate the genetic diversity and population structure of endangered species, and devise methods for preserving and restoring genetic diversity in wild populations.
- The following are some applications of conservation genetics:
- Identifying unique populations or subspecies requiring distinct conservation management.
- Estimating inbreeding levels and the risk of genetic erosion to determine the genetic health of populations.
- Assessing the efficacy of conservation activities, like as habitat restoration and captive breeding programs, in preserving or restoring genetic diversity.
- Informing the development of conservation measures, including the selection of individuals for translocation or reintroduction initiatives.
- Epigenetics is the study of heritable changes in gene expression or cellular phenotype that do not entail DNA sequence changes. By regulating the DNA's accessibility to the transcription machinery, epigenetic changes such as DNA methylation and histone modification can affect gene expression.
- Environmental exposures, food, and stress can influence epigenetic alterations, which can occasionally be passed down from generation to generation. Epigenetics research has revealed the intricacy of gene regulation and the interaction between genetic and environmental variables in determining the phenotype of an organism.
- Understanding epigenetic mechanisms has enormous ramifications for numerous sectors, including as health, agriculture, and ecology. For instance, epigenetic alterations may have a role in the development of some diseases, such as cancer, and may be amenable to therapeutic intervention.
DNA carries an organism's genetic instructions – the blueprints for their physical, biochemical and behavioural characteristics. These are all inherited, but are also subject to environmental influences. It was discovered by James Watson and Francis Crick using data that Rosalind ??? provided. DNA is a double-stranded macromolecule and its backbone contains alternating phosphate and sugar (deoxyribose) residues linked to one of four bases – adenine (A), thymine (T), cytosine (C) and guanine (G). The structure is in a double helix – a twisted ladder that have base-pairs as the rungs. The two strand are antiparallel, running in opposite directions with 5' (phosphate) and 3' (sugar) ends. These two strands are held together through complementary base-pairing by H-bonding. The base-pairing rule is A-T and C-G.
From the human genome project, it was determined that the human genome contained 3×109 base-pairs of DNA. This DNA is divided up into discrete packages called chromosomes. About 30,000 genes are scattered along these chromosomes, separated by DNA of unknown function (intergenic DNA). Genes only represent about 2% of all DNA.
A chromosome contains a single molecule of DNA, consisting of chromatin (DNA and protein [histones] packaged to form a coiled structure. Each somatic cell contains 23 pairs of chromosomes in the nucleus, 22 autosomal pairs and 1 pair of sex chromosomes (XY for males and XX for females). Chromosomes are elongated during interphase and are of varying lengths.
Chromosomes replicated prior to mitosis, containing two chromatid (although still a single chromosome). Chromosomes are condensed during metaphase and can be ordered according to length and position of centromere as a karyotype.
Some chromosomes are 'gene-rich' such as chromosomes 1 and 2 while others are 'gene-poor' such as 13 and 21. Mitochondria also contain a circular chromosome coding for a limited number of genes. Mitochondrial DNA is maternally inherited.
Before DNA can be replicated, it needs to unwind (denature). This allows the base sequences on each strand to be copied. It is the complementarity between the two strands that allows a faithful copy to be made of the entire chromosome.
Genes are the DNA sequences that contain all the information required to form specific peptides or RNA molecules. It is basically the information required for body cells to form. This information can include structural as well as regulatory elements. The can code for peptides (proteins) via messenger RNAs (mRNA), directly for ribosomal RNAs (rRNA) or directly for transfer RNAs (tRNA). Usually, there are many copies of genes that code for rRNAs and tRNAs, and both are involved in protein synthesis (translation). As cells grow, divide and differentiate only a small, select number of genes are expressed to form protein at any one time. Altogether, about 20,000 to 30,000 genes work at different times.
Different cells make different proteins according to the cell structure, function, development and growth. Gene expression is under tight control. Each gene is localised at a specific point (locus) on the chromosome.
Within DNA, DNA replication, DNA repair and genetic recombination occurs. This leads to RNA synthesis (transcription) which then leads to protein synthesis (translation). Codons, which are base triplets, are read to determine the type of amino acid that is used to form the protein. There are also stages in between post-transcriptional processing and post-translational processing. This translation happens at the cytoplasm.
In a sequence of DNA, 5'-3' is called the coding (sense) strand while 3'-5' is called the noncoding (anti-sense) strand. RNA polymerase synthesises an RNA copy of the sense strand, using the anti-sense strand as a template. Thus the mRNA is a copy of the coding strand and is complementary to the non-coding strand.
Transcription starts when transcription factors (proteins) and RNA polymerase (enzyme) bind to promoter. Both exons and introns are transcribed. Many RNA molecules can be transcribed from the same gene during that period of gene expression.
During post-transcriptional processing, the precursor RNA is modified at both ends and introns are removed to prduce the final mRNA. The mRNA then leaves the nucleus for translation in the cytoplasm. (Alternative splicing?)
The sequence of bases along the mRNA is read in groups of three called codons. Each codon specifies a particular amino acid and ultimately the sequence of the amino acids along a peptide. There are 64 (43) possible codon combinations with 61 specifying amino acids and 3 specifying stop signals. There are only 20 amino acids so some amino acids are specified by more than one codon.
In translation, the mRNAs are used as a template on ribosomes. The amino acids are carried to the ribosomes and put into place by specific tRNAs, which match the amino acids with the right codon. There are different reading frames and thus depending on where the tRNA starts reading from, the protein sequence may be very different from the original. However, each mRNA has the same starting codon, which is AUG for methionine. This establishes the reading frame of the mRNA so that the codons are read correctly.
Many peptide chains are synthesised from a single mRNA molecule. Peptides usually undergoes further processing, including removal of initial methionine.
In the regulation of gene transcription, the chromatin is more open because the actively transcribed genes need to be accessed by transcription factors while the tightly packed for inactive genes. Methylation (addition of methyl group) of cytosine residues in promoter contributes to the silencing of genes. Different gene promoters bind to different transcription factors present in a cell which may be cell-specific. So, gene expression is influenced by the conformation of chromatin, the methylation of DNA and the availability of transcription factors.
Prenatal testing helps families to make informed choices during pregnancies and provides choices. Screening only gives risk values while genetic diagnostic testing is more definitive, however, it requires obtaining a sample of fetal cells or tissue for analysis.
Maternal serum screening by looking at chemical in the blood and ultrasonography/nuchal translucency are examples of prenatal screening and are non-invasive procedures but give risk values only.
Chorionic Villus Sampling and Amniocentesis are invasive testing techniques with about a 1% and 0.5% risk of miscarriage respectively. These are examples of diagnostic testing.Other diagnostic testing procedures include umbilical vein sampling, fetal cells/DNA in maternal blood, pre-fertilisation and pre-implantation (PGD), but are much less common due to the higher risks involved.
The main techniques used to look for mutations are polymerase chain reaction (PCR), Restriction fragment polymorphisms (RFLPs) and DNA sequencing.
In vivo gene therapy is an exciting and emerging area of medical science that involves the direct delivery of genetic material into a patient's cells while they are still in the body. This is in contrast to ex vivo gene therapy, where cells are extracted from the patient, treated with gene therapy outside the body, and then reintroduced into the patient. The objective of in vivo gene therapy is to treat or prevent diseases by modifying specific genes in targeted cells, tissues, or organs.
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Contributors: Prab R. Tumpati, MD