DNA and RNA: Structure, Replication, and Mendel's Laws

The Double Helix of DNA

Deoxyribonucleic acid (DNA) is a molecule composed of simpler molecules: a pentose sugar called deoxyribose, phosphoric acid, and four nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C).

DNA is formed by two antiparallel strands of DNA. This double chain is held together by hydrogen bonds that are established between their nucleobases. This forms the characteristic double helix structure of DNA.

Its function is to carry the genetic information encoded in a living organism and it has the ability to replicate itself.

DNA Replication

The information contained in DNA encodes all the bases, the basic pattern of inheritance of each species. It contains all the information that needs to be transmitted intact from cell to cell to ensure the continuity of all information.

For DNA replication, DNA polymerases must operate. When DNA replicates, the two chains of the double helix separate, and DNA polymerases begin to join the nucleotides between them and the template chain. When they finish, two identical DNA double helices are obtained, each of which has an initial template strand and a new strand synthesized by polymerases. This follows a semiconservative pattern.

RNA

Ribonucleic acid (RNA) is a chain formed by the joining of nucleotides, consisting of a pentose sugar called ribose, phosphoric acid, and a nitrogenous base: adenine (A), guanine (G), cytosine (C), and uracil (U). We can distinguish different types:

• Nuclear RNA (nRNA): Found in the nucleus, forming a structure called the nucleolus.
• Messenger RNA (mRNA): Responsible for copying DNA.
• Ribosomal RNA (rRNA): Forms structures called ribosomes, which are involved in protein synthesis.
• Transfer RNA (tRNA): The only one with double-stranded fragments, it transports amino acids to the ribosome for protein synthesis.

The role of RNA is to copy genetic information contained in DNA, translate it, and finally decode the information to produce a protein.

Mendel's Laws of Inheritance

Mendel's First Law: Law of Uniformity

When two varieties of purebred individuals (homozygous) for a given character are crossed, all the offspring (heterozygous) of the first generation are uniform. The dominant allele is expressed, while the recessive allele remains hidden.

• Phenotype: The proportion in which the character is expressed.
• Genotype: The proportion of organisms that are homozygous or heterozygous.

Mendel's Second Law: Law of Segregation

Alleles are distributed randomly to gametes, and when they recombine to form new individuals, recessive phenotypes from the parents may reappear. The recessive alleles not expressed in the first generation (F1) have not disappeared but are hidden. They reappear in the second generation (F2) in a constant ratio. One of Mendel's deductions is that organisms are diploid.

When an individual expresses a dominant character, it is not initially known whether it is homozygous or heterozygous. This is determined through a test cross.

Mendel's Third Law: Law of Independent Assortment

Characters are transmitted following the previous laws, regardless of the presence of another character.