Science 9: Unit A – Biological Diversity Topic 5: When Plans Change

DNA is the Genetic Code for Life DNA = Deoxyribonucleic acid. DNA is a chemical found in the nucleus of all cells. DNA forms long strands called chromosomes. Every cell in a human has 46 chromosomes in its nucleus. 23 chromosomes come from our fathers, and 23 from our mothers.

The Structure of DNA DNA is in the form of a coiled ladder. The ‘rungs’ of the ladder are made up of pairs of chemicals called nitrogen bases. There are four different nitrogen bases.

The Four Nitrogen Bases The chemical rungs are four nitrogen bases: Adenine (A), Thiamine (T), Guanine (G), and Cytosine (C). The rungs of DNA are made up of alternating pairs of these bases. Adenine can only pair up with thiamine, and cytosine with guanine. The pattern of these repeating pairs makes up our entire genetic code.

DNA, Proteins, and Genes DNA forms chromosomes. Different sections of chromosomes act as blueprints for different proteins to be made. A section of a chromosome that codes for a single protein is called a GENE. A single chromosome can have hundreds or thousands of genes, made up of billions of base pairs. All cells and tissues are made up of proteins.

The Discovery of DNA Canadian scientist, Oswald Avery discovered DNA, but could not figure out its structure or see it in a microscope. Watson and Crick, an English and American duo discovered DNA’s structure. Their model explains how just four nitrogen bases can account for all of the biological diversity in the world.

Layout of Chromosomes DNA is organized into chromosomes in a cell’s nucleus. The chromosomes are organized into pairs. So humans have 23 pairs of chromosomes, one from each parent. Each chromosome in a pair has genes that code for the same protein, however, only the dominant one is switched on.

What’s an allele? An allele is a form of a gene. For example, the number of fingers on your hands is based on a gene. There is more than one allele for that gene. You can have 5 fingers (recessive allele) or 6 fingers (dominant allele) Something like hair color can feature many genes so that a child from brown and blonde haired parents will have dirty blonde hair. So for every gene you have, you have an inactive allele.

Mitosis is Cell Reproduction for Somatic Cells Is how somatic cells (non-gametes) reproduce in our body. It works like binary fission. Step 1: The chromosomes in the parent cell nucleus clone themselves. Step 2: The chromosomes all line up in the centre of the nucleus. Step 3: The nucleus splits up and the cell stretches. Step 4: The chromosome twins split apart, going in opposite directions. Step 5: The cell membrane pinches off, producing two daughter clone cells, each with the same number of chromosomes as the parent cell.

Meiosis is Cell Reproduction to Form Gametes Is how gametes are formed. Gametes must have half the # of chromosomes of a regular cell since they will be combining with each other in sexual reproduction. This process is achieved by having a regular cell divide twice. This produces four gamete cells each with only half the chromosomes of the parent cell.

Meiosis and Mutations During meiosis, genes are swapped between chromosomes so that the chromosomes are now different. This further increases variation in a species. Your father’s chromosome may have contained the allele for brown hair, but during meiosis, this may have been switched with the allele (from Mom’s chromosome) for red hair.

Genetic Engineering Is the process of taking the genes of one species and transplanting them into the chromosomes of another organism. AKA Gene splicing. Eg. Certain plants have been given the gene for disease resistance from insect species. These new species of GMOs (genetically modified organisms or foods) are extremely controversial as people do not like the idea of scientists tampering with nature.