Genes are encoded along a pair of twisted DNA (deoxyribonucleic acid) strands, known as a double helix. Each strand is made of a sequence of four much smaller molecules, adenine (A), guanine (G), cytosine (C) and thymine (T). The strands are complementary, so that an A on one strand is matched by a G on the other strand, and a C is matched by a T. The human genome consists of more than 3 billion of these pairs, called base pairs, packaged into 23 pairs of chromosomes. A small amount of DNA also is found in the mitochondria (where energy is processed inside cells).
Humans have about 20,000–25,000 genes. The average human gene consists of about 3,000 bases. Dogs have 39 pairs of chromosomes, which contain approximately 2.5 billion base pairs, and about 19,000 genes.
The “central dogma” of genetics explains that DNA codes for RNA (ribonucleic acids, another class of long chain molecules), which in turn, encodes and manufactures all of the proteins needed by an organism. The process of creating RNA from DNA is called transcription. Even though DNA has two strands, only one strand is read during transcription of a given sequence (genes can be located on either strand). The other strand serves as a template to produce a molecule of RNA with the same sequence as the “coding” or “non-template” strand. We now know that 80% of human DNA does not code for proteins. These noncoding regions have other functions, including preserving important gene sequences and regulation of gene functions.
3. Help students understand that DNA contains a unique code that directs all of the functions of a living organism. Further, all living organisms use the same code. This code was unlocked about 60 years ago by researchers James Watson and Francis Crick. Show students the video below, in which James Watson describes how they figured out the paired, helical structure of DNA.
4. Discuss the concepts covered in the video with students: DNA consists of sequences of four molecules or nucleotide bases: adenine (A), guanine (G), cytosine (C) and thymine (T). DNA always has two complementary strands, in which an A on one strand is matched by a T on the other strand, and a G is always matched with a C. The A-T and G-C combinations are called base pairs. Since the strands are complementary, only the sequence on one of the strands is used as a template for the first step in producing a protein molecule.
Keywords: lesson | genetics | life science | biology | gene | genetic | allele | cell | chromosome | DNA | double helix | nucleotide | nucleotide base | nucleus | SNP | adenine | cytosine | guanine | thymine | genetic code | genetics | genome | genotype |
- Graphic courtesy of U.S. Department of Energy, Genomic Science Program. Public domain.
- Moreno, N. (2017) Complex Traits: Using Dogs as a Model for Modern Genetics. Baylor College of Medicine: Houston. ISBN: 978-1-994035-08-2.
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