Understanding Genetics: From DNA to Mutations

Posted on May 12, 2024 in Biology

The Genetic Code

The genetic code is a set of three-nucleotide combinations of nucleotides of the mRNA, or codons, each of which corresponds to a specific amino acid of the polypeptide or protein.

Key Characteristics:

  • Universal: This means that almost every organism or living thing uses the same code. The same codons code for the same amino acids in all living things.
  • Degenerated: This means that a given amino acid may be coded for by more than one codon.

Introns and Exons

Introns are non-coding sections of an RNA transcript that are spliced out before the RNA molecule is translated into a protein.

Exons are regions of the genome that end up within an mRNA molecule. Some exons are coding, meaning they contain information for making proteins.

Summary:

  • Introns: Non-coding regions of DNA
  • Exons: Coding regions of DNA

Translation

Translation involves “decoding” a messenger RNA (mRNA) and using its information to build a polypeptide, or chain of amino acids. It occurs in the ribosomes.

Stages of Translation:

  1. Initiation: The ribosome binds to the mRNA.
  2. Elongation: In this cyclical stage, amino acids are brought to the ribosome, according to the nucleotide sequence, by tRNAs and linked together to form a polypeptide chain.
  3. Termination: This happens when a stop codon (UUA, UAG, UGA) appears in the A site. In the last stage, the finished polypeptide and the ribosomal subunits are released.

Transcription

Transcription is the process of synthesizing an RNA molecule using the sequence of a gene as a template. RNA polymerases are the enzymes in charge of transcription.

Steps of Transcription:

  1. Initiation: The RNA polymerase-promoter complex forms.
  2. Elongation: The transcription begins, and the RNA strand gets longer, thanks to the addition of new nucleotides.
  3. Termination: The process of ending transcription is called termination, and it happens once the polymerase transcribes a sequence of DNA that signals the end of the gene.

tRNA

tRNA serves as the physical link between mRNA and the amino acid sequence of proteins.

Four Arms of tRNA:

  1. Amino acid arm: The one that carries amino acids.
  2. DHU arm: The one that binds with the active center of the enzyme aminoacyl tRNA synthetase.
  3. TψC arm: The one that binds to the ribosome during protein synthesis.
  4. Anticodon arm: The one that pairs with the codon of mRNA during protein synthesis.

DNA Duplication

Before cell division, the DNA must be duplicated to ensure that each new cell obtains a complete set of information. By replication, two identical DNA molecules are synthesized from a parent DNA molecule.

Semiconservative Replication:

In this model, the two strands of DNA unwind from each other, and each acts as a template for the synthesis of a new, complementary strand. This results in two DNA molecules, each with one original strand and one new strand.

Bidirectional Replication:

DNA replication occurs in both directions from the origin of replication. As the double helix unwinds, it creates the replication bubble, with each newly separated single strand serving as a template for DNA synthesis. Structures called replication forks are formed and will move in opposite directions as replication proceeds.

Semi-discontinuous Replication:

The enzyme DNA polymerase that catalyzes the synthesis of DNA works in the 5’→3’ direction, and the DNA strands are antiparallel.

Genome

A genome is a complete set of nucleic acid sequences for a living thing, encoded as DNA.

The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within 23 chromosome pairs in cell nuclei and in small DNA molecules found within individual mitochondria. It includes both protein-coding DNA genes and noncoding DNA.

Mutations

Mutations are alterations in genetic material that can be transmitted to the cell’s or the virus’s descendants. They happen randomly and are permanent. Mutations cause genetic variability, cause new information to appear in genes, and as a result, they promote the evolution of species.

Gene Regulation

Gene regulation is the process of controlling which genes in a cell’s DNA are expressed (used to make a functional product such as a protein).

Operon Model:

The operon model drives cellular differentiation and morphogenesis in the embryo, leading to the creation of different cell types that possess different gene expression profiles from the same genome sequence.

Lac-Operon (Enzyme Induction):

This operon drives cellular differentiation and morphogenesis in the embryo, leading to the creation of different cell types that possess different gene expression profiles from the same genome sequence.

His-Operon (Enzyme Repression):

This operon drives cellular differentiation and morphogenesis in the embryo, leading to the creation of different cell types that possess different gene expression profiles from the same genome sequence.

Scale of Mutations

A) Small-Scale Mutations and Point Mutations (Genetic Mutations):

These mutations affect a gene in one or a few nucleotides. Nucleotides are added or lost, so the protein sequence changes. They include:

  • Insertions (Addition Mutations): Insertion of one or more extra nucleotides into the DNA or gene sequence.
  • Deletions: Removing or deleting one or more nucleotides from the DNA or gene sequence.
  • Substitution Mutations: When some bases of a gene sequence are replaced by other bases.

According to Effects:

  • Silent: Different triplet but the same amino acid (and protein) is synthesized. There is no effect on the organism.
  • Nonsense: Early termination of translation (stop). There is no protein (enzyme) synthesis.
  • Missense: A different amino acid is produced. The protein could be different or similar in properties.

B) Chromosomal Mutations: Rearrangements (Chromosomal):

This is the mutation of the chromosomal segments of the DNA strands. It can involve:

  • Deletion: Loss of part of the chromosomes.
  • Duplication: Extra copy of a chromosome.
  • Inversion: Reversal of the direction of part of the chromosome.
  • Translocation: Part of one chromosome attaches to another chromosome.
  • Insertion: Adding a part of a chromosome to another chromosome.

Origin of Mutations

: a) Natural, spontaneous or innate mutations: they occur naturally in the genome. They generally occur due to error during DNA replication, mitosis, meiosis, etc. Mostly due to inherent defects in the process of DNA replication. b) Induced or artificial mutations: they do not occur spontaneously. They are induced through various chemical and physical agents known as mutagens. Mutagens greatly enhance the frequency of mutation. Caused by short wavelength radiation, chemicals…

Impact on living things: Harmful: those that prevent the survival of the living beings (often causing death). They can often be harmful to the individual, as they have caused deficiencies, sometimes lethal. Beneficial: They create more chances of survival; they affect the genetic variability of the population and improve their ability to adapt to the environment (evolution, natural selection). They have a positive side to the species: they give it genetic variability; and they allow it to evolve. In the event of a change in the environment, mutated individuals increase the possibility of adapting to new conditions (natural selection), and the species will survive. Neutral: they have no positive or negative effect.