From DNA to Proteins

Section 2.3BC Science Probe 9

Pages 42-47

DNA Structure

• A DNA molecule looks kind of like a twisted ladder.

DNA Structure

• A DNA molecule is made up of 2 strands of smaller molecules called nucleotides

DNA Structure

• A nucleotide is composed of:– A sugar molecule– A phosphate molecule– A nitrogenous base

molecule

DNA Structure

• The sides of the ladder are made of the sugar and phosphates.

• The rungs are the nitrogenous bases.

DNA Structure

• There are 4 nitrogenous bases:– Adenine (A)– Thymine (T)– Cytosine (C)– Guanine (G)

DNA Structure

• Adenine always pairs with thymine.

• Cytosine always pairs with guanine.

DNA Structure

• DNA replicates itself before cell division.– The molecule splits and new bases join up with

the bases on the parent strand.– The new strands are called daughter strands.

The Genetic Code

• DNA has a 4-letter code.• The 4 letters are A, T, G & C for the 4

nitrogenous bases in the nucleotides.

The Genetic Code

• These bases combine to form 3-letter words.• Each 3-letter word codes for one of the 20

different amino acids.– Amino acids are what proteins are made of.– Different combinations of amino acids make

different proteins.– Proteins determine the characteristics of

organisms.

The Genetic Code

• So all of the 3-letter words put together in a cell’s DNA form the instructions for all of the body’s cells to follow!

DNA to Genes

• Your DNA (or anything else’s) is organized into genes.– A gene is a short section of DNA that contains the

instructions to make a specific protein.– These instructions are determined by the order of

the bases.

DNA to Genes

• All of an organism’s genes (all of its DNA) is called the genome.

• The human genome is contained in 23 pairs of chromosomes.

Genes to Proteins

• The making of proteins is called protein synthesis.

• The code for the protein comes from the DNA.• Protein synthesis takes several steps:

Genes to Proteins

• 1. mRNA is made from DNA. – RNA is

ribonucleic acid, a single stranded molecule similar to DNA.

Genes to Proteins

• 2. mRNA is transferred from the nucleus (the region that stores DNA) to the ribosomes (the site where protein synthesis occurs). This brings the mRNA to somewhere useful.

Genes to Proteins

• 3. Ribosomes match up each set of nucleotides in the mRNA to a matching tRNA, starting at the "AUG" (ATG, the start sequence). The matching tRNA is carrying the amino acid for that specific sequence.

Genes to Proteins

• 4. The amino acid on the tRNA is added on to the growing polypeptide (protein) chain.

Genes to Proteins

• Eventually, the ribosomes hit a point in the RNA that codes for "STOP". This completes the synthesis of the protein.

Genes to Proteins

• Proteins have many functions:– Enzymes– Hormones– Insulin– Hemoglobin– Fibrinogen– Keratin– Collagen

Genes and Variation

• Genes are responsible for all of the characteristics that make up a species.– Different species have different genes. (Ex. tree vs.

human)

Genes and Variation

• All members of the same species have the same number and types of genes.

• There are variations among genes.– Ex. The genes for eye

colour can code for blue eyes, brown eyes or green eyes.

Genes and Variation

• Different versions of genes produce different variations or traits.– Some are controlled by only one gene.– Some are controlled by a combination of genes.