Edited by simon
2025-11-29 23:48 · Updated content
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DNA, Genes, and Chromosomes
This section introduces the molecular instructions of life: DNA, genes, and chromosomes.
It explains what they are, how they are organized, what they do, and whether they can be seen under a standard optical microscope (spoiler: almost never, except in special cases).
1. What Is DNA?
DNA (Deoxyribonucleic Acid) is the long molecule that stores all the genetic information of a cell.
- It is made of nucleotides (A, T, C, G).
- It forms a double helix.
- It contains instructions for building proteins.
- All living organisms (in onion cells, yeast cells, bacteria, humans) use DNA.
Scale:
A single DNA molecule can be millions of nucleotides long, but only 2 nanometers wide — far below optical resolution.
Optical visibility:
- Onion cell: DNA is not visible, but during mitosis you can sometimes see condensed chromosomes as thick fibers.
- Yeast cell: DNA is never visible with standard light microscopy.
2. What Are Genes?
A gene is a specific sequence of DNA that provides the instructions to make a functional molecule:
- usually a protein,
- sometimes a functional RNA (tRNA, rRNA, etc.).
A gene includes:
- coding regions (exons)
- non-coding regions (introns in eukaryotes)
- promoters and regulatory sequences
- signals for starting and stopping transcription
Yeast vs. human genes:
- Yeast genes: short, often no introns.
- Human genes: large, with many introns.
Optical visibility:
Genes are far too small to see under an optical microscope.
3. Chromosomes
A chromosome is one long DNA molecule wrapped around proteins called histones.
Chromosomes:
- organize DNA
- protect the genetic material
- allow accurate cell division
- contain thousands of genes
Number of chromosomes:
- Human somatic cells: 46
- Yeast (S. cerevisiae): 16
- Onion cells: 16 (but huge in size compared to yeast)
Condensed vs. uncondensed:
- During normal life, DNA is in a loose form called chromatin (invisible).
- During mitosis, chromatin condenses into thick, visible X-shaped structures.
Optical visibility:
- Onion cells:
- Chromosomes can be seen only during mitosis when they condense.
- Ideal for beginners learning microscopy.
- Yeast cells:
- Condensed chromosomes are too small to be resolved by standard optical microscopes.
4. Chromatin: How DNA Is Packaged
DNA is wrapped around histone proteins, forming a structure called chromatin.
Two forms:
- Euchromatin – loose, active, genes are expressed
- Heterochromatin – compact, inactive, silent regions
Chromatin structure influences gene activity (epigenetics).
Optical visibility:
Not visible under light microscopes (only the whole nucleus is seen).
5. Telomeres
Telomeres are repetitive DNA regions at the ends of chromosomes.
They act as protective caps, preventing DNA loss during replication.
- They shorten with each cell division (in many species).
- Yeast and germline cells maintain length using telomerase.
- Telomere shortening could be linked to aging in humans.
Optical visibility:
Not visible; require molecular techniques (FISH, sequencing).
6. DNA Replication (How DNA Copies Itself)
Before a cell divides, it must copy its DNA.
Key steps:
- Helicase unwinds the double helix
- DNA polymerase builds new strands
- Leading and lagging strands are formed
- Proofreading ensures accuracy
- Final ligation closes gaps
Replication happens in the nucleus (eukaryotes) or cytoplasm (prokaryotes).
Optical visibility:
Cannot be seen with a light microscope.
7. What Is a Mutation?
A mutation is a permanent change in the DNA sequence.
Types include:
- Substitution
- Insertion
- Deletion
- Duplication
- Inversion
Causes:
- Copying errors
- Radiation
- Reactive oxygen species
- Chemicals
- Natural DNA damage
Effects range from harmless to severe, or even beneficial.
8. Epigenetics (Beginner Overview)
Epigenetics refers to chemical modifications that affect gene activity without changing DNA sequence.
Main mechanisms:
- DNA methylation
- Histone modification (acetylation, methylation, etc.)
- Chromatin remodeling
- Non-coding RNAs
Epigenetic states influence:
- aging
- cell identity
- cancer
- reprogramming (e.g., OSK factors)
9. Summary Table: DNA, Genes, Chromosomes Visibility
| Structure | Visible in Onion Cells? | Visible in Yeast Cells? |
|---|---|---|
| DNA (uncondensed) | No | No |
| Genes | No | No |
| Chromosomes (condensed) | Yes (mitosis only) | No |
| Chromatin | No | No |
| Telomeres | No | No |
Additional Notes for Beginners
- DNA is information.
- Chromosomes are storage and organization.
- Genes are instructions.
- Proteins are the workers.
- Epigenetics controls which instructions are read.
Together, these explain most of cell biology, aging, development, and biotechnology.