HUMAN MOLECULAR GENETICS - INTRODUCTION

MOLECULAR GENETICS 

•  Molecular genetics basically revolves around the interrelationship between 2 nucleic acids – DNA and RNA.
• The large amount of DNA in the nucleus is also composed of repetitive sequences that have been found to support chromosomal function. There is also a good amount of defective copies of functional genes. 
• The main story that is dealt with is the synthesis of polypeptides by these 2 nucleic acids.
• It is hypothesized that RNA may have been the hereditary material during early evolution.
• However, now most organisms contain their genetic information in DNA molecules that are more chemically stable.
• This genetic information is copied and transmitted to daughter cells.
• Some viruses still contain RNA as the genetic material. Some of them are Ebola virus, SARS coronavirus, Influenza virus, Hepatitis C virus (HCV), West Nile virus, Polio virus, Measles virus.
• DNA molecules are found mainly in the chromosome of the nucleus, and also in each mitochondrion. In plants, DNA molecules are also found in chloroplasts.
• A gene is a part of a DNA molecule that serves as a template for making a functional RNA molecule.
• In bacteria and other simple organisms, the DNA is packed within genes.
• In Eukaryotes, the nucleus contains tens of thousands of genes packed in chromosomes, whereas the mitochondrion or chloroplast contain only a few genes.

RNA Molecules

• There are many types of RNA molecules and can generally be divided into 2 broad classes.
• Coding RNA – contain coding sequence that can be decoded to generate a corresponding polypeptide sequence.
• Messenger RNA – carries genetic information from DNA to the protein synthesis machinery. (mRNA)
• The mRNA made in the nucleus has to be transported to the cytoplasm to interact with ribosomes and make proteins.
• The mRNA in the mitochondrion and chloroplasts make protein within the organelles.
• Functional end point of information stored in DNA – protein synthesis.

Central Dogma

• One-way direction of genetic information flow – DNA> RNA> Polypeptide
• Universality of this gene flow is why it is referred to as the central dogma.
• The process flow – Transcription> Translation. The 2 essential processes in cellular organisms.
• Transcription – DNA id used as a template by an RNA polymerase to synthesize a particular type of RNA.
• Translation – mRNA is decoded to make polypeptides at ribosomes.
• Ribosomes are large RNA-protein complexes that are found in the cytoplasm and also in mitochondria and chloroplasts.
• Genetic information is written as a linear sequence of nucleotides in DNA
• The sequence is decoded in group’s three nucleotides to give a linear sequence of nucleotides in RNA. Here the Thymine found in DNA is replaced by Uracil in RNA.
• This is again decoded in groups of three to give a linear sequence of amino acids in forming the polypeptide.
•  Eukaryotic cells, including mammalian cells contain nonviral chromosomal DNA sequences, such as members of the mammalian Line-1 repetitive DNA family that encode cellular reverse transcriptases.
•  Reverse transcriptases can produce DNA from an RNA template.
• Hence, the central dogma is not strictly valid.