Gene

What Exactly Are Genes?

Imagine genes as the building blocks of life, much like LEGO pieces that come together to form a complex structure. In biology, the term ‘gene’ has two primary meanings: it can refer to a basic unit of heredity (Mendelian gene) or a sequence of nucleotides in DNA that transcribes into functional RNA molecules.

Types of Molecular Genes

There are two main types of molecular genes: protein-coding genes and non-coding genes. Protein-coding genes produce proteins, while non-coding genes do not directly code for proteins but play crucial roles in gene regulation.

Gene Expression and Functionality

When DNA is copied into RNA, this process is known as gene expression. The RNA can be functional or serve as an intermediate template for protein synthesis. This intricate dance of genetic material determines the traits we see in living organisms.

The Transmission of Genes and Phenotypic Traits

Genetic traits are influenced by polygenes (multiple genes) and gene-environment interactions, with some traits being instantly visible while others may take time to manifest. A gene can acquire mutations leading to different variants or alleles, which might cause distinct phenotypical traits.

The Evolution of Genes

Genes evolve through natural selection and genetic drift, with alleles changing over generations. The concept of genes has evolved significantly since the early 19th century when Gregor Mendel first suggested discrete inherited units in his groundbreaking work.

The Structure and Function of Genes

A gene is a DNA sequence that codes for a diffusible product, which may be protein or RNA. The crucial feature is that the product diffuses away from its site of synthesis to act elsewhere. A gene corresponds to a transcription unit and produces both mRNA and noncoding RNAs, with regulatory sequences controlling expression.

Early Speculations on Gene Size

Early speculations on the size of genes were based on protein and RNA sizes, with 1500 base pairs considered reasonable. However, the discovery of introns revealed that many eukaryotic genes are much larger than initially thought.

The Concept of Genes Over Time

The term ‘gene’ was introduced by Wilhelm Johannsen in 1909, while Eduard Strasburger used the term ‘pangene.’ The discovery of DNA as the molecular repository of genetic information began in the 1940s and 1950s with the study of its structure by Rosalind Franklin and Maurice Wilkins.

Advances in Understanding Genes

The modern synthesis integrated Mendelian genetics with Darwinian evolution, recognizing random genetic drift as a major player. The development of chain-termination DNA sequencing improved the efficiency of sequencing and turned it into a routine laboratory tool.

The Structure of DNA and RNA

DNA consists of long strands with four types of nucleotide subunits: 2-deoxyribose, phosphate groups, and adenine, cytosine, guanine, and thymine bases. The double helix structure features specific base pairing between adenine and thymine, and cytosine and guanine.

Chromosomes and Genomes

Chromosomes store genes in an organism or cell, making up its genome. A chromosome consists of a single DNA helix on which thousands of genes are encoded. The region where a gene is located is called its locus and can contain multiple alleles.

The Role of Regulatory Sequences

Prokaryotes store their genomes on a single circular chromosome, while eukaryotes often have regions of DNA without an identified function. This ‘junk DNA’ makes up the majority of human genome but may be expressed.

Gene Expression and Regulation

The promoter is recognized and bound by transcription factors to initiate transcription. Gene promoters can vary in strength and complexity, with some forming strong associations with transcription factors to initiate high rates of transcription.

Messenger RNA and Protein Synthesis

Protein-coding genes consist of elements such as introns and untranslated regions, and all genes require regulatory sequences like promoters. Messenger RNA produced from protein-coding genes contains untranslated regions, introns, and exons that are connected through splicing.

The Genetic Code

The genetic code specifies the amino acid sequence of a protein through nucleotide sequences that correspond to codons. Transcription produces a complementary single-stranded RNA molecule known as messenger RNA, which acts as an intermediate between DNA and protein product.

Genetic Inheritance

Organisms inherit their genes from parents, with asexual organisms inheriting a complete copy of the parent’s genome and sexual organisms inheriting two sets of chromosomes. Mendelian inheritance describes the relationship between an organism’s genotype and phenotype.

DNA Replication and Cell Division

During DNA replication, enzymes make a duplicate copy of every gene using semiconservative synthesis. The process is essential for growth, development, and reproduction. Cell division involves separating the two genome copies and physically dividing into two distinct cells.

The Role of Non-Coding Genes

Non-coding genes have different transcription termination mechanisms and often lack poly(A) tails. Prokaryotic genes are organized into operons, which are transcribed as a continuous polycistronic mRNA controlled by repressors.

Eukaryotic Gene Regulation

Eukaryotic promoter regions are more complex than prokaryotic ones, with regulatory regions that can alter expression and act as enhancers or silencers. Enhancers increase transcription by binding activator proteins, while silencers bind repressor proteins to make DNA less available for RNA polymerase.

Gene Expression and Post-Translational Modification

The process of gene expression involves two steps: transcription to mRNA and translation to protein. The genetic code is read three nucleotides at a time via interactions with tRNA molecules, and the ribosome attaches amino acid cargo to the polypeptide chain.

Regulation of Gene Expression

Genes are regulated so that they are expressed only when needed, drawing on limited resources. Regulation can occur from transcriptional initiation to post-translational modification of proteins. Some RNAs have enzymatic or regulatory functions, while others serve as intermediates in protein synthesis.

RNA Genes and Non-Coding RNA

RNA genes are copied into RNA from DNA and can be functional products themselves, such as ribosomal RNA and transfer RNA. Non-coding RNA genes encode sequences transcribed from DNA. Some viruses store their genomes entirely in RNA, allowing for rapid protein synthesis upon infection.

Asexual vs. Sexual Reproduction

Organisms inherit their genes from parents, with asexual organisms inheriting a complete copy of the parent’s genome and sexual organisms inheriting two sets of chromosomes. Mendelian inheritance describes the relationship between an organism’s genotype and phenotype.

The Human Genome Project

The human genome contains approximately 19,000 protein-coding genes, with essential genes estimated at around 2000. These genes include housekeeping genes critical for basic cell functions and genes expressed at different times during development or life cycle.

Genetic Engineering

Genetic engineering is the modification of an organism’s genome through biotechnology. Techniques developed since the 1970s allow for specific gene addition, removal, and editing. Recent advances use engineered nuclease enzymes to create targeted DNA repair.

Applications of Genetic Engineering

Many organisms have been genetically modified for applications in agriculture, industrial biotechnology, and medicine. In multicellular organisms, the embryo is often engineered which grows into the adult organism. Gene therapy techniques can also be used to edit cell genomes to treat genetic diseases.

Understanding genes and their functions is crucial for unraveling the mysteries of life. From Mendel’s early speculations to the complex structures we know today, genes continue to fascinate scientists and researchers alike. As technology advances, our ability to manipulate and understand these genetic building blocks grows, opening up new possibilities in medicine, agriculture, and beyond.