The spiral, ladder-shaped molecule of DNA is a familiar sight. The sides are alternating phosphate and sugar molecules while the rungs consist of four amino acids known as nitrogenous base pairs. The practically limitless sequencing of these base pairs not only results in the variety of plants and animals that exist in the world, it allows for variation within each species.
Chromosomes and Genes
Long threads of DNA containing millions of base pairs are tightly coiled to form chromosomes. Humans have twenty three pairs and we receive a complete set of genetic material from our parents at conception.
Chromosomes are located in the nucleus of a cell and every type of cell in the body, including skin, bone, and hair cells, contains a complete set of an individual’s DNA.
Eye color, hair color, and blood type are examples of inherited traits that are expressed when a series of base pairs called a gene provides instructions for cell growth.
Genes vary in length from hundreds of base pairs to hundreds of thousands. Since our DNA is from both of our parents, every genetic trait has two possible expressions depending on which gene is dominant and which is recessive. Whether an allele is dominant or recessive depends on the genetic trait being expressed. Numerous genetic traits require the activity of more than one allele, so the allele that codes for a protein that helps express that particular genetic trait would be dominant and the one that is less effective or could interfere with protein synthesis might be recessive.
Genes are encoded with instructions for the expression of individual traits within a species. They vary in length from hundreds of base pairs to hundreds of thousands and they’re found in the nucleus of a cell tightly bundled up into chromosomes.
DNA transcription is the process of “decoding” a gene by copying the sequence of base pairs onto a strand of RNA that can be transported around the body to build and repair cells.
Every type of body cell contains a complete set of an individual’s DNA. As these cells grow and divide, the genetic material in the nucleus of the newly formed cell has to be a precise reproduction of that in the parent cell.
- During mitosis, strands of DNA are “unzipped” and used as a template to copy the sequence of base pairs. Mitosis results in two new cells with 23 identical pairs of chromosomes.
- Meiosis occurs in the sperm and egg cells of the reproductive system. New cells are formed with only half of each parent’s DNA. When sperm and egg cells combine at conception, the resulting offspring has genetic traits from both parents within it’s own unique set of DNA.
DNA Up Close
DNA and RNA are nucleic acids, a type of macro-molecule as vital to the existence of life as proteins, carbohydrates and lipids.
A single unit of DNA is a “nucleotide” consisting of a phosphate and sugar molecule bonded together with one nitrogenous base, either A, T, G, or C. These repeating nucleotides form long strands that are not only connected at a complimentary base pair, A + T or C + G, the two strands are lined up in opposite directions.
The sugar is a molecule of deoxyribose made up of hydrogen, oxygen, and five carbon atoms. At the deoxyribose molecule’s fifth carbon atom, it’s bonded to a molecule of phosphate, and scientists refer to this as five-prime alignment. On the opposing side, the phosphate and ribose molecules are aligned in the opposite direction and the “upside down” ribose molecule has it’s third atom of carbon in the same position as the fifth on the other strand. This strand is said to have a three-prime alignment and it’s bonded to it’s own atoms of hydrogen and oxygen.
A section of DNA can only be replicated by adding nucleotides to a three-prime end.
If we receive a complete set of DNA from each parent, what causes a particular gene to be expressed when every genetic trait has two possible outcomes?
Variation Within Species
For centuries, agriculturalists have known how to use selective breeding techniques to amplify the expression of traits in livestock and crops. Gregor Mendel’s experiments with flower sprouts in the 1850’s sought to understand and eventually predict the way genes are passed from parent to offspring by observing the variability in the way genetic traits are expressed.
Mendel concluded that we have two genes for every genetic trait, that these separate “alleles” are inherited from each parent, and that one allele could be dominant and the other recessive. Since numerous genetic traits require the workings of more than one allele for expression, the dominant allele might be the one that codes for a protein that corresponds with the other ones needed to express that particular trait, while the allele that is less effective or would interfere with the expression of a trait might be recessive.
Dominant vs Recessive Genes
Mendel experimented with the way particular traits were inherited in vegetable plants. He found that when a homozygous dominant trait was crossed with a homozygous recessive trait, smooth vs wrinkled seeds for instance, the first generation of offspring was heterozygous dominant while the second generation displayed dominance in a 3:1 ratio. Mendel concluded that the two alleles for the same genetic trait in the parent’s DNA are separated when gametes form, and the resulting offspring receives one allele for each characteristic from each parent (law of segregation).
When he experimented with crossing two separate homozygous traits at one time, a dihybrid cross, the dominance ratio was 9:3:3:1, demonstrating that the gene that made proteins for wrinkled seeds didn’t affect the gene that coded for the trait of seed color. In other words, the different genetic traits were inherited independent of each other (law of independent assortment).