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DNA, Heredity and Drug Resistance

Chromosomes are molecules of DNA that provide the essential genetic code for all living organisms, and it is the code that directs the synthesis of proteins that define each organism's structure and function. Genetic factors contribute to causation of many diseases, such as breast cancer and heart disease. Understanding genetics also enables one to understand the major threats to health as a result of the development of drug resistance.

Learning Outcomes

  1. Explain how DNA encodes genetic information and the role of messenger RNA and transfer RNA
  2. Explain how DNA directs protein synthesis and roles of DNA and proteins in regulating cell function
  3. Demonstrate how to predict the possible genotypes that could occur in an offspring provided one knows the genotype of both parents
  4. Explain what a mutation is and give examples of how it might occur

DNA

Deoxyribonucleic acid (DNA) is an extremely long polymer made from units called nucleotides.

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The backbone of each molecule is composed of alternating sugars (the pentagon with "S") and each sugar is also covalently bonded with one of the following nucleotide bases:

  • Adenine (A)
  • Thymine (T)
  • Cystosine (C)
  • Guanine (G)
  • Uracil (U)

DNA and RNA differ in several ways:

  • DNA is double stranded, while RNA is single stranded (though RNA forms loops by hydrogen-bonding to itself)
  • DNA contains the sugar deoxyribose while RNA has the sugar ribose
  • RNA contains the base uracil in place of thymine

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The illustration above shows the 46 chromosomes that contain the human genome. There are 22 homologous pairs and 2 sex chromosomes in humans. One chromosome in each pair is inherited from one's mother and one from the father. Each chromosome is a single molecule of DNA. If we were to take the physical human chromosome and stretch it out, it would be about 5 centimeters, or all 46 would be about 2 meters laid end to end.

With the exception of red blood cells (which lost their nucleus) each cell in the human body has all 46 chromosomes, so there are about 3 billion base pairs. The single chromosome of a bacterium is located diffusely in the cytoplasm, but chromosomes of higher species are contained with a membrane-bound nucleus.

One of the helical strands of DNA is the coding strand. The coding strand of a chromosome has thousands of genes along its length (segments which contain genetic code for specific cellular proteins).

Genes and Inheritance

Each chromosome contains thousands of genes. This takes up up only 3-5% of our cellsDNA, haswhile the rest of DNA is considered "non-coding areas." Altogether 23 pairs of chromosomes carry the code for 20k-25k genes. Most genes are transcribed into "messenger RNAs" (mRNA) that provide a template that is used to translate the code into specific proteins. About 100 genes are transcribed into "ribosomal RNAs" and "transfer RNAs" that also play a vital role in the synthesis of proteins.

The sequence of bases in DNA can be though of as "letters" that provide the basis for all genetic code for proteins synthesized by our bodies, which provide the basis of structure of all our cells, enzymes and our traits and characteristics. The production of cellular proteins requires two major processes: transcription followed by translation.

 

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