D. Genetic Processes · Genetic ProcessesDNA, Genes and Chromosomes: The Language of Heredity
D3.2 D2.1 A2.2 A1.12About 70 minutes
Learning goals- I can describe how DNA, genes and chromosomes are related.
- I can explain what homologous chromosomes, loci and alleles are.
- I can tell diploid (2n) cells from haploid (n) cells and give the human numbers.
- I can read a karyotype and tell autosomes from sex chromosomes.
Words to know
heredity
The passing of traits from parents to offspring.
DNA (deoxyribonucleic acid)
The molecule that carries genetic information. Two strands twisted into a double helix, joined by base pairs (A with T, C with G).
gene
A section of DNA at a particular place on a chromosome that carries the code for a protein (or another product) that affects a trait.
chromosome
One long, tightly packed DNA molecule plus proteins. Humans have 46 in most body cells.
genome
All of the DNA in one set of an organism's chromosomes.
homologous chromosomes
A matching pair of chromosomes, one from each parent, carrying the same genes in the same order.
locus (plural loci)
The position of a gene on a chromosome.
allele
One version of a gene. For example, the pea seed-colour gene has a yellow allele and a green allele.
diploid (2n)
Having two sets of chromosomes, one from each parent. Human body cells: 2n = 46.
haploid (n)
Having one set of chromosomes. Human eggs and sperm: n = 23.
autosome
Any chromosome that is not a sex chromosome. Humans have 22 pairs.
sex chromosomes
The X and Y chromosomes. Usually XX in females and XY in males.
karyotype
A picture of a cell's chromosomes arranged in homologous pairs by size and shape.
Why do you look like your family, but not exactly?
Maybe you have your mother's curly hair and your grandfather's ears, but nobody else in your family has your exact face (unless you are an identical twin). How can traits be passed down so faithfully, yet get shuffled into a brand-new combination in every child?
That question is what this unit is about. To answer it, we need a clear picture of where the instructions live and what they are made of. This first lesson sets up the vocabulary you will use in every other lesson of the Genetic Processes unit.
Remember from Grade 10
In Grade 10 Science (Tissues, Organs, and Systems) you learned that the DNA in a nucleus is divided into chromosomes, that human body cells have 46 chromosomes (23 pairs), and that before a cell divides each chromosome is copied into two sister chromatids joined at a centromere. You followed mitosis (prophase, metaphase, anaphase, telophase) and saw that it makes two genetically identical daughter cells. Grade 10 ended by saying there is another kind of division, meiosis, that makes sperm and egg cells. That is where this unit is heading.
DNA: the instruction molecule
DNA is a very long molecule shaped like a twisted ladder, the double helix. Each side of the ladder is a chain of sugars and phosphates. The rungs are pairs of bases. There are four bases: adenine (A), thymine (T), cytosine (C) and guanine (G). A always pairs with T, and C always pairs with G.
The information is in the order of the bases, the same way the meaning of a sentence is in the order of its letters. The human genome has about 3 billion base pairs in one set of chromosomes.
Genes: the useful sentences
A gene is a section of DNA that carries the code for a particular product, usually a protein. Proteins do most of the work in cells: they build structures, act as enzymes and carry signals. Humans have roughly 20 000 protein-coding genes.
Here is the chain that links DNA to a trait you can see:
DNA base sequence → gene → protein → trait
For example, a gene carries the code for an enzyme that helps make melanin, a dark pigment. The amount of working enzyme affects the colour of skin, hair and eyes.
Chromosomes come in pairs
A chromosome is one DNA molecule wrapped around proteins and packed tightly. In most of your body cells there are 46 chromosomes in 23 pairs. In each pair, one chromosome came from your biological mother (in her egg) and one from your biological father (in his sperm).
The two chromosomes of a pair are called homologous chromosomes (homo = same). They are the same length, have the centromere in the same place and carry the same genes in the same order. The place where a gene sits is its locus (plural loci).
But the two homologues do not have to carry the same version of each gene. Different versions of a gene are called alleles. For example, the gene for seed colour in pea plants has a yellow allele and a green allele.
- If both homologues carry the same allele, the organism is homozygous for that gene (AA or aa).
- If they carry different alleles, it is heterozygous (Aa).
You will use these words constantly from the Mendel lesson onward.
Diploid and haploid
A cell with two sets of chromosomes (one from each parent) is diploid, written 2n. A cell with one set is haploid, written n.
- Human body (somatic) cells are diploid: 2n = 46.
- Human eggs and sperm (gametes) are haploid: n = 23.
- When a sperm fertilizes an egg, the new cell, the zygote, is diploid again: 23 + 23 = 46.
Every species has its own number. The number of chromosomes says nothing about how complex an organism is:
| Organism | Diploid number (2n) | Haploid number (n) |
|---|
| Fruit fly (Drosophila) | 8 | 4 |
| Garden pea | 14 | 7 |
| Corn (maize) | 20 | 10 |
| Human | 46 | 23 |
| Dog | 78 | 39 |
Autosomes, sex chromosomes and karyotypes
Of the 23 pairs in humans, 22 pairs are autosomes, numbered 1 to 22 roughly from largest to smallest. The 23rd pair is the sex chromosomes. Most females have XX and most males have XY. The X is a large chromosome with many genes; the Y is much smaller and carries fewer genes, including one that starts male development.
A karyotype is a picture of all of a cell's chromosomes, photographed during cell division when they are condensed, and arranged in pairs by size, centromere position and banding pattern. Doctors use karyotypes to count chromosomes and spot large changes. You will read real karyotypes in the lesson on chromosomal disorders.
Worked exampleExample: Using the vocabulary
A cat's body cells have 38 chromosomes.
(a) What is the diploid number? The haploid number? (b) How many chromosomes are in a cat egg cell? (c) How many pairs of autosomes does a cat have, if it has one pair of sex chromosomes like humans? (d) A cat is Bb for a gene. Is it homozygous or heterozygous? Where are the two alleles?
- Body cells are diploid, so 2n = 38. One set is half: n = 19.
- An egg is a gamete, so it is haploid: 19 chromosomes.
- 38 chromosomes = 19 pairs. One pair is the sex chromosomes, so 19 − 1 = 18 pairs of autosomes.
- B and b are different alleles, so the cat is heterozygous. One allele is on each chromosome of a homologous pair, at the same locus.
2n = 38, n = 19; an egg has 19 chromosomes; 18 pairs of autosomes; heterozygous, with the two alleles at the same locus on the two homologues.
Scientists behind the story
We know DNA is the genetic material partly thanks to Oswald Avery, a scientist born in Halifax, Nova Scotia. In 1944, working in New York with Colin MacLeod and Maclyn McCarty, he showed that DNA (not protein) could transfer a hereditary trait from one strain of bacteria to another. In 1953 James Watson and Francis Crick proposed the double-helix model, using X-ray images made in the laboratory where Rosalind Franklin and Maurice Wilkins worked; Franklin's images were key evidence for the helix shape.
Just for funYour DNA is extremely long and extremely thin. If you stretched out the DNA from just one of your cells, it would be about 2 metres long, yet it fits inside a nucleus far too small to see without a microscope. That is some serious packing.
Hands-on activityExtract DNA from a strawberry
Grown-up note: an adult should handle the rubbing alcohol: keep it cold, away from any flame or heat, and in a ventilated room. Nobody drinks or tastes anything in this activity.
You need: 2 or 3 strawberries, a zip-top bag, 2 tablespoons of water, 1 teaspoon of dish soap, a pinch of salt, a coffee filter or paper towel, a clear glass, about 60 mL of rubbing alcohol chilled in the freezer for an hour (adult), and a wooden skewer or toothpick.
- Mix the water, soap and salt in a cup. This is your extraction buffer.
- Put the strawberries in the bag, add the buffer, seal it and squish for 2 minutes. The soap breaks down cell and nuclear membranes (they are made of fats). The salt helps the DNA clump together.
- Strain the mush through the filter into the glass.
- The adult slowly pours the cold alcohol down the side of the glass so it forms a layer on top.
- Wait 1 minute. Cloudy white strands appear where the two layers meet. That is DNA (millions of molecules clumped together). Lift some out with the skewer.
Think about it: Strawberries have many copies of each chromosome per cell, which is one reason they give so much DNA. Why can you see clumped DNA here, but never a single DNA molecule? (A single molecule is only about 2 nm wide, far too thin to see.)
Your turn
1Put these in order from largest to smallest: gene, nucleus, chromosome, base pair.
nucleus → chromosome → gene → base pair. A nucleus holds chromosomes; a gene is one section of a chromosome; a gene is made of many base pairs.
2One strand of DNA reads A T T G C A. Write the bases on the matching strand.
T A A C G T (A pairs with T and C pairs with G).
3What is the difference between a gene and an allele?
A gene is a section of DNA at a particular locus that affects a trait (for example, seed colour). An allele is one version of that gene (yellow or green).
4A corn plant has 2n = 20. How many chromosomes are in a corn pollen grain's sperm nucleus?
Gametes are haploid: n = 10.
5Classify each genotype as homozygous or heterozygous: TT, Tt, tt.
TT homozygous (dominant); Tt heterozygous; tt homozygous (recessive).
6Why are the two chromosomes in a homologous pair not identical, even though they carry the same genes?
They came from different parents, so at many loci they carry different alleles. Same genes, possibly different versions.
More practice: warm-up
1How many chromosomes are in a typical human body cell?
2What are the four DNA bases?
Adenine (A), thymine (T), cytosine (C), guanine (G).
Having one set of chromosomes (n).
4Which sex chromosomes does a typical human female have?
5What is the position of a gene on a chromosome called?
More practice: on your own
1A fruit fly body cell has 8 chromosomes. How many homologous pairs is that, and how many chromosomes are in a fruit fly egg?
8 ÷ 2 = 4 pairs. An egg is haploid, so it has 4 chromosomes, one from each pair.
2Explain why a zygote is diploid even though it came from two haploid cells.
Each gamete gives one set (n). Fertilization joins them: n + n = 2n. In humans, 23 + 23 = 46.
3Describe the chain from DNA to a visible trait, using melanin as an example.
The base sequence of a gene codes for an enzyme (a protein) that helps make melanin. How much working enzyme a person makes affects how much pigment is in their skin, hair and eyes, which is the trait we see.
4A karyotype shows 22 matched pairs plus one large chromosome and one small one. Is the person most likely male or female? Explain.
Male. The large and small unmatched chromosomes are an X and a Y. A female would have two X chromosomes (a matched pair).
5Does a dog (2n = 78) have more genes than a human because it has more chromosomes? Explain.
Not necessarily. The number of chromosomes depends on how a species' DNA is divided into pieces, not on how many genes it has or how complex it is. Dogs and humans have a broadly similar number of genes.
Challenge
1A gene is about 1500 base pairs long. Each set of 3 bases codes for one amino acid in the protein. About how many amino acids long is the protein? (Ignore any parts of the gene that are not used.)
1500 ÷ 3 = 500 amino acids. (Real genes also contain non-coding sections, so this is a simplified estimate.)
2The human genome has about 3 billion base pairs and about 20 000 protein-coding genes. If the average coding section is about 1500 base pairs, roughly what percent of the genome codes for protein?
20 000 × 1500 = 30 000 000 base pairs = 3 × 10⁷. Then 3 × 10⁷ ÷ 3 × 10⁹ = 0.01 = about 1%. Most of the genome does not code for protein (it includes regulating sections, repeated sequences and parts whose roles are still being studied).
3A cell has 2n = 6 chromosomes. Make a sketch (in words) showing how you would label the three homologous pairs, and state how many different chromosomes an egg cell from this organism would contain.
Label the pairs 1, 1′; 2, 2′; 3, 3′, where each number is one size and shape and the prime shows the chromosome came from the other parent. An egg is haploid: it has 3 chromosomes, one from each pair (for example 1, 2′, 3). It never has both members of one pair.
Quiz
1. Which statement correctly describes the relationship between these terms?
2. Which base always pairs with cytosine (C) in DNA?
5. An organism with genotype Gg is:
6. How many pairs of autosomes do humans have?
Big idea: This lesson covers the vocabulary of D2.1 and the DNA–gene–chromosome–allele part of D3.2. Everything later in the unit (meiosis, Punnett squares, disorders) depends on these words.
How to help: Ask your teen to explain the difference between homologous chromosomes and sister chromatids; this is the most common mix-up in the whole unit. Homologues are a pair from two parents (they can carry different alleles); sister chromatids are identical copies made when DNA is copied. The strawberry activity is safe if an adult handles the cold rubbing alcohol away from any flame.
Common difficulties:
- Mixing up gene and allele. Gene = the topic (seed colour); allele = the version (yellow or green).
- Thinking more chromosomes means a more complex organism (dogs have 78).
- Forgetting that "n" means one set, not one chromosome.