Genetics and DNA

Genetics

• The field of biology that involves the study of how genetic information is passed from one generation of organisms or cells to the next

DNA (Deoxyribonucleic Acid)

• The hereditary material in humans and almost all other organisms
• "De" means to take away
• "Oxy" refers to oxygen
• "Ribo" is a type of sugar
• "Nucleic" refers to the nucleus
• "Acid" is an acid-like molecule with a lot of hydrogen molecules
• DNA has deoxyribose as a sugar group, while RNA has ribose

Composition of DNA

• Consists of two long, antiparallel strands that form a spiral shape known as the double helix
• The individual units of each strand are called nucleotides
• Nucleotides are composed of a phosphate group, a sugar group, and a base
• Four bases, also known as nucleotides include: adenine (A), guanine (G), thymine (T), and cytosine (C)
• In DNA, A is paired with T, and G is paired with C
• These A-T and G-C pairings are called complementary base pairs
• The bases protrude inward at regular intervals

DNA Mutations

• Random changes in the sequence of bases in DNA
• Sometimes disrupts the A-T and C-G pairing
• Can be caused by various factors like radiation (UV rays, X-rays, nuclear radiation), mutagens (chemicals inhaled while smoking), or mistakes during DNA replication
• UV radiation may cause bonding between adjacent bases on the same strand, preventing normal bonding between complementary bases and causing a bulge in the DNA double helix
• If this damage is not repaired, it leads to mutation
• Harmful mutations can cause disorders/diseases like cancer
• Mutations that occur in gametes (sex cells) may lead to heritable diseases
• Not all mutations lead to disorders/diseases
• Beneficial mutations in DNA accumulate over time, driving evolution

Genome

• The complete set of nucleic acid sequences
• For eukaryotic organisms, is the sum of the sequences for all of its chromosomes and a small amount of mitochondrial DNA
• Includes approximately 20,000 protein coding genes, accounting for 1.5% of the genome.
• Contains around 44,500 RNA-only coding regions
• Over 98% of the human genome is non-coding, including telomeres, centromeres, pseudogenes, and introns
• The Human Genome Project, an ongoing effort as of July 2023, has sequenced the 22 autosomes and the X and Y chromosomes

Chromosomes

• DNA is packaged into thread-like structures
• When uncoiled, the chromatin form can be read and replicated
• During mitosis/meiosis, the chromatin condenses into visible chromosomes with the DNA tightly coiled around proteins called histones
• The location of the centromere on each chromosome gives the chromosome its characteristic shape and can be used to describe the location of specific genes

Histones

• Proteins providing structural support for a chromosome
• DNA wraps around histone proteins to give the chromosome a more compact shape
• Histones regulate genes as to whether they are turned on or off
• Histones near a gene can have a particular marker, signaling the cell whether the gene is "open for business"

Epigenetics

• Behaviors and environment can cause changes that affect the way genes work
• Epigenetic changes are reversible and do not change the sequence of DNA bases, but they can change how the body reads a DNA sequence
• Histone modification is an example of epigenetics

Genes and Alleles

• Genes: Sections of DNA that contain genetic information for the inheritance of specific traits (e.g., skin tone, hair color)
• Homologous chromosomes: Carry genes for a specific trait at the same location, but can carry different traits of that gene
• Alleles: Different forms of the same gene which account for the difference in specific traits

Replicating DNA

• The double helix unwinds
• Each strand serves as a template for a new strand
• Semiconservative replication results in each new DNA molecule conserving half of the original