DNA, Genes and Nucleosides

• Deoxyribonucleic acid (DNA) is the molecule carrying genetic information for the development and functioning of an organism.
• DNA consists of two linked strands that wind around each other, forming a double helix.
• Genes are segments of DNA that contain code for specific functional RNA molecules or proteins functioning in cells.
• Chromosomes are structures within cells containing a person's genes.

Nucleosides

• A nucleoside has a pentose sugar linked to a nitrogenous base or glycosylamines.
• Nucleosides are nucleotides, minus the phosphate group.
• Nucleosides can be divided into purine and pyrimidine types based on the nitrogen base present.
• Nucleosides are integral to nucleotides and serve as their precursors.
• A phosphate group attaching to a nucleoside forms a nucleotide, building the backbone of DNA.
• Nucleoside molecules act as signaling molecules.
• Altered nitrogen bases or minor bases in nucleosides regulate or protect genetic information.
• Nucleoside analogs treat malignancies, tumors, and viral infections through purine and pyrimidine base modifications.
• Cytarabine (cytosine arabinoside) treats acute myeloid leukemia, it was the first drug approved by the US FDA.
• Some nucleoside analogs treat Human Immunodeficiency Virus (HIV), for example Lamivudine.

Gene Structure and Function

• A gene is a basic inheritance unit on a specific chromosome region called the locus.
• Genes are responsible for expressing observable traits (phenotype) through gene expression into proteins.
• Eukaryotic cell genes live in the nucleus, while prokaryotic cell genes exist on a single chromosome suspended in cytoplasm.
• Genes exist inside mitochondria and chloroplasts of eukaryotic cells, primarily in plants.
• A chromosome has hundreds to thousands of genes
• Normal human cells have 23 pairs of chromosomes, totaling 46.
• A trait is a gene-determined characteristic, often influenced by multiple genes.
• Gene mutations cause some traits that are either inherited or result from a new gene mutation.
• Genes are encoded within long DNA strands, which consist of nucleotide monomers
• Nucleotide monomers are comprised of a pentose sugar (Deoxyribose), a phosphate group, and either one of the four nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C), or Guanine (G).
• DNA molecules have a double helix structure, with two DNA strands running antiparallel and bonded by hydrogen bonds.
• Adenine forms double hydrogen bonds with thymine. Cytosine forms triple hydrogen bonds with guanine.
• Genes contain ATCG nucleotide codes, including coding and non-coding sequences.
• Introns are repeats; untranslated regions help in gene expression regulation.

Gene Inheritance and Chromosomes

• Genes come from parents, with one copy inherited from each.
• Copied genes divide and copy themselves until the body is full with its amount
• The human body contains around 20,000 to 25,000 genes.
• Chromosomes, located in the nucleus of cells, consist of DNA and protein.
• Proteins called histones allow chromosomes to pack up small enough to fit in cell`s nucleus.
• Chromosomes carry instructions that make individuals unique.
• Humans have 23 pairs, for a total of 46 chromosomes.
• 22 are autosomes, involved in functions not related to se× determination.
• One pair is se× chromosomes (X and Y), with one from each parent.
• Errors in cell division and replication can lead to an extra chromosome (trisomy, like Down syndrome with 47 chromosomes) or one less chromosome (monosomy, like Turner syndrome without chromosome pair member).

Types of Genes

• Complementary genes are an interaction of two dominant non inter-allelic genes.
• Both must be present for a specific phenotype to develop.
• Each has its own effect, when these genes come together, they interact
• Together, they create a new interaction that is not possible on their own
• Supplementary genes consist of two or more genes.
• when present have effects that are fundamentally different from their individual effects.
• Genes differ from complementary genes and require each other to produce a phenotype.
• Supplementary genes have one dominant gene that can express itself independently, while the second only expresses when paired with the first
• A common example used for this gene type is the mating of two mice, one black and one albino.
• Expression of coat color is neither black nor white but a new brown color
• Duplicate genes occur when two genes, either dominant or recessive, express themselves the same way.
• Polymeric genes, known as additive genes, have an additive or compounding effect on each other, without a pairing.
• Se×-linked genes resides on the X or Y se× chromosomes.
• They determine se× and how traits get inherited.
• Males (XY) with only one X chromosome are more likely to show recessive traits from the X chromosome.
• Color blindness is an example when a colorblind mother (XX) and non-colorblind father (XY) have a colorblind son.

Genetic Mutations

• Genes can deviate from their original form (mutate), leading to variations between individuals.
• The SIRT6 gene plays a role in controlling aging by fixing DNA damage.
• Mutations in SIRT6 are found in centenarians, increasing their ability to repair DNA and delay aging.

Cancer and Genetic Factors

• Cancer defined as the body's cells growing uncontrollably and spreading
• Cancer results when cells grow old or become damaged, they die, (Apoptosis: a programmed cell death that occurs in multicellular.
• Normal human cells grow and multiply (through a process called cell division) to form new cells as the body needs them
• Tumors can be either cancerous or non cancerous (benign).
• Cancer cells grow without growth signals, unlike normal cells and also ignore signals to stop dividing or to die.
• Blood vessels grow towards tumors to supply them with oxygen and nutrients.
• Cancer cells hide from the immune system.

Cancer Genes

• Cancer cells change by duplicating or deleting chromosome parts and using different nutrients.
• Genetic changes alter cell growth and spread resulting in genetic changes that can lead to cancer
• DNA changes mainly affect the genes.
• Mutations in important genes disrupt cell instructions, leading to uncontrolled growth and lack of repair, thus developing into cancer.
• Genes playing a pivotal role in cancer are Oncogenes, Tumor Suppressor Genes, and DNA Repair Genes.
• Oncogenes evolve from proto-oncogenes promoting cell growth.
• Mutations or too many copies can activate oncogenes at the wrong point
• This results in uncontrolled cell growth and potential cancer.
• Gene variants or mutations cause the oncogenes to activate all the time
• Epigenetic changes attach chemical groups to DNA or RNA, leading to the oncogene's activation.
• Chromosome rearrangements change a DNA sequence and may turns proto-oncogenes on
• Gene duplication - when some cells have extra copies of a gene, which might lead to them making too much of a certain protein
• Tumor suppressor genes slow down cell division or trigger apoptosis.
• Loss of function leads to uncontrolled cell growth and likely cancer
• Mutation causes tumor suppressor genes to stop working.
• TP53 is a tumor suppressor gene and inherited changes can also lead to Li-Fraumeni syndrome.
• DNA repair genes fix DNA mistakes, but when dysfunctional, they allow mistakes to build up and cause out-of-control cell growth.
• Mutations can be inherited, and these increase the likelihood of developing some forms of cancer.
• BRCA1 and BRCA2 genes are examples of DNA repair genes.