DNA, Genes, and Chromosomes: Understanding Heredity

Posted on Jan 8, 2025 in Biology

Before the Nineteenth Century: Inheritance by Mixing

Sex cells combine hereditary material and mix as do colors. According to this theory:

  • White Animal + Black Animal = All descendants are mid-gray.

Mid-Nineteenth Century

Gregor Mendel (1821-1884)

  • Modern genetics.
  • Inherited characteristics are transmitted by individual factors that are dispatched each generation.
  • Not-ruined compote that was identified as hereditary material.

Mendel’s Experiments

Mendel’s experiments with peas involved smooth peas and wrinkled peas.

  • Parental generation: Smooth pea (50%) + Wrinkled peas (50%)
  • First generation: Smooth pea (100%)
  • Second generation: Smooth pea (75%) + Wrinkled peas (25%)

Introduces the concepts of dominant character and recessive character.

The Double Helix: DNA

In all living things, genetic information is transmitted from generation to generation through DNA: deoxyribonucleic acid.

  • DNA is composed of 4 different nucleotides that are repeated and combined many times.
  • Nucleotide:
    • Phosphate group
    • Deoxyribose (sugar in the form of a ring) equal for all nucleotides
    • Nitrogenous base
  • The different nucleotides differ in the nucleobase. There are 4 different bases:
    • A: Adenine
    • C: Cytosine
    • T: Thymine
    • G: Guanine

1953 – James Watson and Francis Crick

DNA takes the form of a double helix: there are 2 strands of nucleotides linked together. The nucleotides are joined by weak links called hydrogen bonds. Nucleotides always join, forming the following pairs: A-T and G-C.

Replication of DNA

Genetic information:

  • Is transmitted to all cells of the organism.
  • Regulates the physical, physiological, and functional characteristics of an organism.
  • To transmit the full DNA with precision.

Replication: Process of making exact copies of all the genetic information in the daughter cells.

Chromosomes

Our DNA contains 6,400 million nucleotides, which is about 2 meters long.

  • Chromosome: A single package of genetic information; a DNA molecule rolled about itself.
  • Each species has a number of chromosomes:
    • Humans: 23 pairs
    • Ceratitis: 4 pairs
    • Horses: 32 pairs
    • Dogs: 38 pairs
  • Homologous chromosomes: They are those that carry the same type of genetic information. Of each pair of homologous chromosomes, one comes from the father and one from the mother.
  • Humans have 22 pairs of homologous chromosomes and 1 pair of sex chromosomes.

Genes

  • The basic unit of genetic information.
  • A gene is each fragment of DNA that contains information to synthesize a specific protein.
  • Each gene is composed of a specific sequence of nucleotides. It is here where the genetic information resides.
  • Genotype: The genetic constitution with respect to a specific trait.
  • Phenotype: It is the manifestation of the genotype, the attributes that can occur in an organism due to genetic information.

Proteins

  • Proteins are essential molecules for living organisms.
  • They are formed by the union of nonlinear elements, the amino acids.
  • There are 20 different amino acids, leading to many possible combinations and many different proteins.
  • The type of amino acids and their order or sequence determines the shape of proteins and their physicochemical and functional properties.

Functions of Proteins

  • Protect, transport, and give consistency to substances like hemoglobin.
  • Defense mechanisms (immunoglobulin).
  • Produce movement (actin, myosin).
  • Packaging hereditary materials (histones).
  • Receive signals (cellular receptors).
  • Promote biochemical reactions (enzymes).

The Genetic Code

  • The relationship between a nucleotide sequence within a gene and the corresponding amino acid sequence of a protein.
  • It is a universal code: the same for bacteria, viruses, humans, and peas.
  • 3 nucleotides → 1 amino acid
  • Many amino acids → 1 protein
  • The order of nucleotides in a gene determines the order of amino acids in a protein.
  • The genetic code is degenerate.
  • If all possible ways of combining the 4 different nucleotides (A, C, T, G) are grouped 3 by 3, 64 possible triplets are obtained. But there are only 20 different amino acids. This means that there are different combinations of nucleotides that give rise to a single amino acid.

Ways to name amino acids:

  • Full name
  • Three-letter code
  • One-letter code