Understanding Genetics and Heredity: From Genes to Genomes
Sons Inherit the Characteristics of Parents
Living organisms are composed of atoms and molecules. They can make copies of themselves, relaying information about their structure and function.
Living Beings Evolve
Copies of living organisms are almost identical. This subtle difference is key to their diversity, allowing adaptation to different environments. This is the basis for the evolution of species. Natural selection allows the survival of the fittest and gradually determines their appearance.
Mendel’s Laws of Inheritance
Mendel’s Conclusion: Heredity (Genes)
Mendel showed that the units of heredity (genes) determining characteristics do not mix, meaning they maintain their individuality throughout generations. Furthermore, these characteristics are transmitted independently of each other. For each characteristic, there are two versions of the gene, one from the father and one from the mother. If one version is expressed, we say that the gene controlling that characteristic is dominant over the other.
The gene is a unit of hereditary information, controlling a certain character.
Where are the Genes?
The Cell
The cell is the fundamental unit of living organisms. Within a cell, we can distinguish:
- The Cell Membrane: Controls the exchange of substances with the outside.
- The Nucleus: Contains DNA, which directs protein synthesis and the mechanism of heredity.
- The Cytoplasm: Contains various organelles: ribosomes, mitochondria, lysosomes, etc.
Chromatin and Chromosomes
During cell division, called mitosis, chromatin condenses into filaments called chromosomes.
What are Genes Made of and How are They Copied?
The discovery of Frederick Griffith showed that DNA was the molecule that stores genetic information. That is, genes are made of DNA.
The DNA molecule consists of two chains of chemical compounds called nucleotides. Each nucleotide is composed of:
- A Nitrogenous Base: There are four bases (adenine, thymine, guanine, and cytosine).
- A Sugar (pentose)
- Phosphoric Acid
DNA: The Double Helix
Watson and Crick proposed a double helix model for DNA, supported by experimental evidence. The selective pairing of adenine (A) with thymine (T) and guanine (G) with cytosine (C) suggested a potential mechanism for the replication of genetic material.
Duplication of DNA
Replication is achieved by the selective pairing of A-T and G-C bases, which serve as a template to replicate the genetic material. This is the key process for gene duplication, transmitting the genetic message from parent to offspring.
What are Genes?
The genetic code is the set of instructions used to make proteins from the sequence of nucleotides in DNA. This code determines that each group of three nucleotides encodes an amino acid. Genes store hereditary information to produce proteins, which carry out the vast majority of functions in living things.
Central Dogma of Molecular Biology
RNA carries genetic information from DNA to ribosomes. The central dogma of molecular biology established the flow of hereditary information, which is stored in DNA and used to synthesize proteins.
Protein synthesis occurs as follows:
- In the nucleus, DNA transcribes the encoded message into messenger RNA (mRNA).
- In eukaryotic cells, the mRNA leaves the nucleus and reaches the cytoplasm, where it attaches to a ribosome.
- Transfer RNA (tRNA) selects a specific amino acid for each three-nucleotide sequence of mRNA, forming a chain of amino acids (polypeptide).
The Human Genome
The genome of an organism is the set of all its genetic information. It consists of genes (2%) and non-coding DNA (98%), including introns and junk DNA.
Within DNA, we can distinguish:
- Exons and Introns: Exons are the portions of DNA within a gene that encode proteins. Introns are the portions that are not used in protein synthesis.
- Junk DNA: Does not belong to any gene. Most of the genome is actually junk DNA.
Genomics and Complex Organisms
Organisms with more genes/nucleotides are not necessarily more complex than those with fewer. Genomics is the study of genomes. Proteomics studies all the proteins encoded by the genome.
Genetics of Development
Developmental genetics has deciphered the rules governing the development of organisms. Development involves cell proliferation and differentiation.
- Proliferation: Requires cell division and replication of its genome.
- Differentiation: Requires the regulation of gene expression, expressing specific genes for each tissue.
Epigenetics
Epigenetics studies how individual characteristics are not solely determined by the DNA nucleotide sequence.
Tools to Manipulate DNA
- To Cut: Restriction enzymes cut DNA at specific sequences.
- To Paste: DNA ligase joins DNA fragments.
- To Copy: Plasmids are small, circular DNA molecules inside bacteria.
- Transformation: A method to introduce plasmids into bacteria.
Biotechnology and Transgenic Organisms
Transgenic organisms are genetically modified organisms that carry a foreign gene (transgene). Examples include:
- Bacteria that degrade oil slicks
- Bacteria that produce biodegradable plastic
- Plants with resistance to insect pests, like corn