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How Genes are Made
Context:
The alikeness of identical twins can be astonishing, driven by their identical genetic makeup. Genes, the fundamental units of heredity, play a central role in defining individuality.
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- While identical twins share all their genes, non-identical twins differ in about 50% of their genetic material, leading to greater variation.
- The study of genes not only explains human diversity but also reveals how new genetic material evolves.
The Foundations of Genetics
- Humans have 24 distinct DNA molecules corresponding to the 24 chromosomes, including the numbered chromosomes (1–22) and the sex chromosomes, X and Y.
- These DNA molecules are organised into two sets of genomes—one inherited from the mother and the other from the father.
- Each DNA molecule comprises two strands held together by base-pair bonds, forming a total of 3.2 billion base pairs in the human genome.
- Genes, which are segments of DNA, are typically a few thousand base pairs long.
- When a gene is expressed, its sequence is transcribed into messenger RNA (mRNA), which is then translated to produce proteins.
- Humans have approximately 20,000 protein-coding genes and an equal number of non-coding genes involved in regulating gene expression.
How New Genes Emerge
- Gene Duplication: In 1970, biologist Susumu Ohno proposed that new genes primarily arise through duplication.
- When a gene is duplicated, one copy can retain its original function while the other is free to mutate and potentially acquire a new function.
- However, Ohno’s hypothesis did not address the issue of protein overproduction, which can harm the organism.
- The University of Nevada, Reno researchers tackled this problem by comparing duplicated and non-duplicated genes in humans and mice, which last shared a common ancestor 75 million years ago.
- They discovered that promoters of duplicated genes exhibited higher levels of DNA methylation—a chemical modification that reduces gene expression.
- This mechanism likely mitigated the risks of protein overproduction, allowing duplicated genes to persist and evolve new functions.
- Interestingly, the higher methylation rates also increased the likelihood of mutations, further facilitating the evolution of new genes.
- When a gene is duplicated, one copy can retain its original function while the other is free to mutate and potentially acquire a new function.
- Random DNA Sequences: The team from the Max Planck Institute explored another mechanism by which new genes might form.
- They introduced random DNA sequences into human-derived cell populations and observed their effects over 20 days.
- They found that random DNA sequences could influence cell growth, with 53% of cell types decreasing in abundance, 8% increasing, and 40% remaining unchanged.
- These findings suggest that random DNA sequences can behave like “incipient genes,” affecting cellular functions and becoming potential substrates for evolutionary processes.
The Fate of Genes
- For a gene to be retained in the genome, it must either serve a functional purpose or mutate into a form that evolution deems useful.
- However, determining the utility of specific genes can be challenging.
- Consider the ABO blood group system.
- Humans can have blood types A, B, AB, or O, depending on the variants of the ABO gene they inherit.
- Each variant corresponds to a specific protein, yet the O variant produces a protein with no known function and a markedly different amino-acid sequence.
- Despite this, primates and humans have retained all three variants (A, B, and O) over millions of years.
- While the precise evolutionary advantage remains unclear, it underscores the complexity of genetic retention.
Recent research highlights the intricate processes that lead to the creation and retention of new genes. These insights deepen our understanding of how genomes adapt and evolve, shaping the incredible diversity of life on Earth.
