Understanding DNA and RNA: From Structure to Gene Expression

DNA: Structure and Essential Functions

Deoxyribonucleic Acid (DNA) is the genetic material, composed of chains of deoxyribonucleotides (Adenine, Guanine, Cytosine, Thymine – A, G, C, T). Except in some viruses, DNA typically comprises two nucleotide chains.

It is found in the nucleus of eukaryotic cells (as linear molecules) and in the cytoplasm of prokaryotic cells, some viruses, and certain organelles like mitochondria and chloroplasts (where it is circular in shape).

DNA Structure

The structure of DNA can be described at different levels:

• Primary Structure: Consists of a linear sequence of nucleotides.
• Secondary Structure: The renowned Double Helix Model, proposed by Watson & Crick in 1953. Key features include:
  • Double chains of deoxyribonucleotides.
  • Complementary chains regarding the sequence of bases: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C) via hydrogen bonds (A=T; G≡C). Each nucleotide in one chain is linked to a complementary nucleotide in the other chain.
  • The double helix structure positions the nitrogenous bases in the interior, arranged in parallel.
  • For the chains to separate, they must unroll.

DNA Function

DNA's primary functions are:

• It contains and transmits hereditary information in the form of a base sequence, which is used to synthesize proteins.
• It governs cellular functions by directing protein synthesis.

RNA: Structure, Functions, and Types

Ribonucleic Acid (RNA) is made up of chains of ribonucleotides (Adenine, Guanine, Cytosine, Uracil – A, G, C, U). Generally, RNA consists of a single chain of nucleotides.

It is found in the nucleus of eukaryotic cells, the cytoplasm of both prokaryotic and eukaryotic cells, ribosomes, and some viruses.

RNA Functions

RNA is primarily responsible for synthesizing proteins, utilizing the genetic information from DNA.

Types of RNA

There are several types of RNA, each with specific roles in protein synthesis:

• mRNA (messenger RNA): Carries genetic information from DNA to ribosomes.
• rRNA (ribosomal RNA): A structural component of ribosomes, where protein synthesis occurs.
• tRNA (transfer RNA): Transports specific amino acids to the ribosome during protein synthesis.

Gene Expression: Transcription and Translation

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product, such as a protein. This process involves two main stages: transcription and translation.

Transcription: DNA to mRNA

Transcription is the process where the genetic information from DNA is copied into an mRNA molecule:

1. The two DNA strands unwind and separate.
2. The information (base sequence) stored in the template strand is transferred as mRNA by the enzyme RNA polymerase (transcriptase).
3. The mRNA is constituted by complementary nitrogenous bases to the template strand, but thymine (T) is replaced by uracil (U) (A=U; G≡C).

Translation: mRNA to Protein

Translation is the process where the information transcribed in the mRNA is used to synthesize a protein in the ribosomes (located in the cytoplasm):

1. mRNA information passes from the nucleus to the cytoplasm, where it binds to the ribosomes.
2. At the same time, free amino acids in the cytoplasm bind to their respective tRNA molecules, which transport them to the mRNA.
3. Each amino acid, bound to its tRNA, recognizes a specific sequence of three mRNA bases (a codon). This recognition occurs because the tRNA has an area of three complementary bases (an anticodon) for each mRNA codon. The tRNA then links to the mRNA, creating a protein chain.
4. The newly synthesized protein separates from the ribosome.