Molecular Biology: Replication, Transcription, Translation and Mendel

DNA Replication

Replication is the process by which a DNA double helix molecule is synthesized into two identical molecules. The parent molecule serves as a template for the daughter strands.

Eukaryotic Replication

This process has three important properties: it is semiconservative, bidirectional, and multifocal. It occurs in three stages:

• 1. Unwinding: The double helix opens at a point called the ori-C. Proteins involved in this process are collectively called the replisome.
• 2. Synthesis: Two new DNA strands are synthesized by DNA polymerases I, II, and III. The 3'-5' chain is read by DNA polymerase III without issues. The 5'-3' chain cannot be read directly; this is solved by reading small segments called Okazaki fragments (5'-3'), which are joined later. This is known as the lagging strand.
• 3. Correction: DNA polymerase III corrects errors during replication.

Prokaryotic Replication

Prokaryotic replication is also semiconservative and bidirectional, featuring a leading and lagging strand with Okazaki fragments. There are various types of polymerases (α, β, δ, ε, γ), where α and δ are involved in replication and repair. Unlike eukaryotes, prokaryotes lack telomerase activity and possess circular DNA.

Transcription

Transcription (occurring in the eukaryotic nucleus) is the process by which an RNA molecule is synthesized complementary to a DNA strand. This includes mRNA, tRNA, and rRNA. It is selective, single-copy, and repetitive. The enzyme responsible is RNA polymerase.

Stages of Transcription

• Initiation
• Elongation
• Termination
• Maturation

The DNA is transcribed in the 3'-5' direction, resulting in an RNA strand synthesized in the 5'-3' direction. Prokaryotic transcription follows the same phases.

Translation

Translation is the process by which a protein is synthesized from an mRNA template. The mRNA sequence contains the information necessary to determine the amino acid sequence of the protein. Translation is unidirectional (5'-3'), repetitive, selective, and requires an adapter (tRNA). It has four stages: amino acid activation, initiation, elongation, and termination.

Mendel's Laws of Inheritance

1st Law: Law of Uniformity

When crossing homozygous individuals that differ in a single character, all individuals of the first filial generation are identical in both phenotype and genotype.

2nd Law: Law of Segregation

Genetic factors are transmitted from generation to generation; they separate in the parents and join at random in the descendants to define the characteristics of new individuals.

3rd Law: Law of Independent Assortment

When considering two characters simultaneously, genetic factors do not interfere with one another; the factors determining one characteristic are inherited independently of those determining the other.