Chromosomes, Genes & DNA
Describe the relationship between chromosomes, genes, and DNA in heredity, including the double helix structure of DNA and the historical roles of Watson, Crick, Franklin, and Wilkins
What a learner can do afterwards
- Explains that a gene is a section of DNA and that chromosomes package DNA with proteins inside a typical human cell's nucleus
- Describes how many genes provide instructions for proteins, and explains that several genes and the environment can influence one trait
- States that a typical human body cell has 46 chromosomes in 23 pairs, while mature red blood cells have none
- Describes the contributions of Watson, Crick, Franklin, and Wilkins to working out DNA structure
1 · Read
Human skin cells and nerve cells have a nucleus. Most of their DNA is inside it. The nuclear DNA in a typical human body cell would stretch to about two metres. Chromosomes package DNA with proteins. This packaging becomes especially tight when a cell prepares to divide.
A gene is a section of DNA with instructions for a working molecule. Many genes provide instructions for proteins, which build structures or do jobs in cells. One trait can involve several genes and the environment. A chromosome contains many genes, DNA outside genes and packaging proteins. DNA is its material, so a chromosome and its DNA molecule are not separate levels in a size ranking.
A typical human skin cell has 46 chromosomes in 23 pairs, one chromosome of each pair from each biological parent. To count from pairs, multiply by two. Mature human red blood cells have no nucleus and no chromosomes.
DNA's usual shape is a double helix: two strands twisted around each other. Sugar and phosphate form the sides of the ladder-like molecule. Pairs of chemical bases form its rungs. The bases have letter names: A pairs with T, and G pairs with C.
Franklin and Wilkins studied DNA using X-ray diffraction patterns, which give clues about molecular shape. Watson and Crick used X-ray evidence and other researchers' findings to propose a double helix model in 1953. Watson, Crick and Wilkins received the 1962 Nobel Prize in Physiology or Medicine. Franklin had died before the award.
Genes are DNA sections. Chromosomes package DNA with proteins. A typical human body cell has 46 chromosomes in 23 pairs. X-ray evidence helped scientists work out DNA's double helix.
Lesson, questions and flashcards written for this dataset. OpenStax records were studied for scientific background.
Readings written for this dataset. Video link credits TED-Ed.
More to read
DNA as material, genes as sections
A gene and DNA are not different substances. A gene is a particular section of a DNA molecule. A chromosome packages DNA together with proteins. It contains many genes and also DNA outside genes.
In a human skin cell, the nuclear chromosomes are inside the nucleus. Their packaging becomes especially compact when the cell prepares to divide. A chromosome does not stop being a chromosome whenever its DNA becomes less tightly packed.
Many genes provide instructions for proteins. Proteins can form structures or carry out jobs in cells. This does not mean that every trait has exactly one gene. Several genes and environmental effects can contribute to a trait.
Using evidence to work out DNA's shape
Scientists cannot read an X-ray diffraction pattern as though it were an ordinary photograph. They interpret the pattern to learn about a molecule's structure. Rosalind Franklin and Maurice Wilkins studied DNA with this method.
James Watson and Francis Crick used X-ray evidence together with other researchers' findings to propose their double helix model in 1953. The model has two strands wound around each other. Sugar and phosphate form the sides, and paired bases lie between them.
Watson, Crick and Wilkins received the 1962 Nobel Prize in Physiology or Medicine. Franklin had died before the award. The recipients were not the only people whose work contributed to understanding DNA.
Chromosomes and inherited instructions
A typical human skin cell has 23 pairs of chromosomes. One chromosome of each pair came from each biological parent. Inheriting chromosomes also means inheriting the gene instructions carried in their DNA.
A pair contains two chromosomes. To count all the chromosomes, multiply 23 by two: 46. This usual count does not apply to every human cell. A mature red blood cell has no nucleus and no chromosomes.
Genes occupy sections of chromosomes, but a chromosome is not made only of genes. It also contains other DNA and proteins that help package the DNA. A chromosome count alone cannot tell you all of a person's traits.
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