Genetics Basics - Lesson 1: The basics of DNA and genetic disease, including phenotype and genotype
1.1 The Units of Heredity
Genes are the units of heredity and are the instructions that make up the body’s blueprint. They code for the proteins that determine virtually all of a person's characteristics.
Humans have an estimated 35,000 genes.
Most genes come in pairs and are made of strands of genetic material called deoxyribonucleic acid, or DNA.
Genetic disorders are caused by one or more changes, or mutations, in the instruction code of a particular gene(s), preventing the gene(s) from functioning properly.
The study of human genetics is the study of human variation that is carried in the genes.
Genes are organized in structures called chromosomes.
1.2 Structure of DNA
The DNA molecule is a double helix, and it is composed of three main parts:
Five-carbon sugar (deoxyribose)
One of four nitrogen-containing bases
The double helix is like a ladder. Two anti-parallel strands are comprised of sugars and phosphates, and the bases comprise the "rungs" of the "ladder."
The message encoded in DNA is made up of the four-letter alphabet A, G, C, and T, with each letter representing one of the bases.
1.3 Codons and Building Proteins
DNA is the code used to produce proteins.
First, the DNA is replicated, so that there are two identical copies of the DNA. Then, the DNA is transcribed into RNA, which is then translated, or read, by tRNA to make the proteins.
This process of replication, transcription, and translation, is called the “central dogma”.
Just like DNA, RNA is composed of a four-letter alphabet. However, the thymine (T) in DNA is replaced by a uracil (U) in RNA.
The message encoded in RNA is read in three-letter words called codons.
Codons code for specific amino acids, which are the building blocks of proteins. Therefore, by knowing the sequence of bases in a gene, it is possible to predict the codons and, ultimately, the amino acid sequence of the protein the gene makes.
There are twenty possible amino acids. However, most amino acids can be coded for by more than one codon.
The beginning of a coding sequence is signaled by a start codon, a unique sequence for this purpose; the start codon also codes for a methionine.
There are three codons that indicate the end of the amino acid sequence. These are called stop codons.
1.4 Codons and Building Proteins
The RNA message (mRNA) is read in three-letter “words” called codons. According to the instructions in the RNA message, amino acids are added sequentially to create a protein chain as the message is read.
Each codon signals for a certain amino acid to be added to the protein chain. In the example shown the codon ‘GUG’ signals for the amino acid valine. Therefore, an RNA message reading ‘GUGGAGUUU’ would code for a protein chain of valine, glutamic acid, and phenylalanine.
In order to translate the RNA message (m-RNA), transfer RNA (tRNA) must bring the appropriate amino acid to the mRNA template. The tRNA’s contain the complementary RNA code (in the case of valine, CAC). One by one, amino acids are then added together to form a polypeptide chain.