B3.3 Genetics · KS3 Science

DNA, genes and proteins

DNA is a long molecule made of two strands twisted into a double helix. It is found in chromosomes in the nucleus.
A gene is a section of DNA that codes for a particular protein. The genome is all of an organism's genetic material.
In protein synthesis, the code in a gene is copied, and ribosomes use it to join amino acids in the right order.
Characteristics depend on genes and on the environment.
Gregor Mendel's experiments with pea plants in the 1860s showed that characteristics are passed on in "units" (genes). In 1953, Watson and Crick worked out the double helix, using X-ray images made by Rosalind Franklin.

Mitosis makes two identical body cells, for growth and repair.
Meiosis makes sex cells (gametes) with half the number of chromosomes: 23 in humans. It makes four cells that are all genetically different.
Fertilisation joins two gametes, so the fertilised egg has the full 46 chromosomes.
Sexual reproduction gives variation, which helps a species survive changes, but needs two parents and takes longer. Asexual reproduction is quick and needs one parent, but gives no variation.

Alleles are different versions of the same gene. A dominant allele (a capital letter, B) shows even if there is only one copy. A recessive allele (a lower-case letter, b) only shows if there are two copies.
The genotype is the alleles an organism has (BB, Bb or bb). The phenotype is the characteristic you can see (brown or blue eyes).
Homozygous means two identical alleles (BB or bb). Heterozygous means two different alleles (Bb).
A Punnett square predicts offspring: Bb × Bb gives BB, Bb, Bb and bb, a 3 : 1 ratio. There is a 75% chance of the dominant characteristic and a 25% chance of the recessive one.

Cystic fibrosis is caused by a recessive allele; it makes thick mucus build up in the lungs. Polydactyly (extra fingers or toes) is caused by a dominant allele.
Humans have 23 pairs of chromosomes; one pair decides sex. Females are XX and males are XY.
The father's sperm carries either an X or a Y, so each baby has a 50% chance of being a boy or a girl.

DNA: The molecule that carries genetic information, shaped like a double helix.
Gene: A section of DNA that codes for a protein.
Allele: A different version of the same gene.
Dominant: An allele that shows even if there is only one copy.
Recessive: An allele that only shows if there are two copies.
Genotype: The alleles an organism has, such as Bb.
Phenotype: The characteristic that shows, such as brown eyes.
Homozygous: Having two identical alleles, such as BB or bb.
Heterozygous: Having two different alleles, such as Bb.
Chromosome: A long strand of DNA in the nucleus.
Meiosis: Cell division that makes sex cells with half the chromosomes.
Gamete: A sex cell, such as a sperm or an egg.
Punnett square: A diagram that predicts the alleles of offspring.
Genome: All the genetic material of an organism.