Key Concepts: Genetic Code, Chromosomes, DNA, Cell Cycle

Genetic Code

• Each codon or triplet codes for a specific amino acid.
• It is degenerate: Amino acids can be coded by more than one codon or triplet (some triplets are synonyms). There are 64 codons for 20 amino acids.
• It is not ambiguous: Each codon encodes only one type of amino acid.
• It is universal: Apart from a few small exceptions.
• There are triplets that determine the beginning of translation ('start codon'): AUG (It codes for the amino acid methionine - Met).
• There are triplets that determine the end of translation ('termination' or 'stop' codons): UAA, UAG, UGA (They do not code for any amino acids).

Chromosomes

They are stick-shaped structures that arise from the condensation and packaging of chromatin during the division phases (mitosis and meiosis). They are made up of DNA and proteins (histones). They have a constricted area (centromere) that divides them into two arms (equal or unequal length).

Chromosome Types

• Simple Chromosomes (Anaphasic): 1 chromatid. 1 centromere that divides it into 2 arms.
• Duplicated or Replicated Chromosomes (Metaphasic): 2 sister chromatids (one is a copy of the other due to replication). 1 centromere that joins both chromatids and divides the chromosome and its chromatids into arms. 2 kinetochores, which are disc-shaped proteins, assemble on the centromere and are essential during cell division. Kinetochores appear in prophase.

Chromosome Morphology

Chromosomes are classified by centromere position:

• Metacentric
• Submetacentric
• Acrocentric
• Telocentric

DNA Replication

The process involves several stages:

1. The DNA unwinds, and the two chains separate.
2. Each strand serves as a template for a new complementary DNA chain (A pairs with T, G pairs with C), synthesized by the enzyme DNA polymerase (also called replicase).
3. Two identical double helices are obtained, each consisting of one new strand and one old strand.

This process is known as semiconservative replication.

Cell Cycle: Interphase

Interphase consists of three main phases:

• G1 Phase: Synthesis of proteins for cell growth. Formation or duplication of cell organelles and cytoplasmic structures.
• S Phase: DNA replication. Synthesis of histones.
• G2 Phase: Transcription and translation of genes encoding proteins necessary for cell division. Duplication of centrioles.