The Central Dogma: From DNA to Protein

Learning Objectives

Explain the process of DNA replication, including its semi-conservative nature and key enzymes.

Describe transcription, the synthesis of RNA from a DNA template.

Detail the process of translation, where ribosomes synthesize proteins from an mRNA template.

Understand the flow of genetic information as described by the central dogma and its exceptions.

The Central Dogma of Molecular Biology

The central dogma, first articulated by Francis Crick, describes the flow of genetic information within a biological system. In its simplest form, it states that DNA makes RNA, and RNA makes protein. This pathway dictates how the genetic blueprint stored in DNA is used to create the functional machinery of the cell—proteins.

Step 1: DNA Replication - Copying the Blueprint

Before a cell divides, it must make a complete copy of its DNA. This process is called DNA replication and it is semi-conservative. This means that after replication, each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.

Key Enzymes and their Roles:

Helicase: This enzyme unwinds the DNA double helix, creating a 'replication fork' where synthesis can begin.

Primase: Synthesizes a short RNA primer, which provides a starting point (a 3' hydroxyl group) for DNA polymerase.

DNA Polymerase: The main builder. It reads the parental DNA strand and adds complementary nucleotides (A with T, C with G) to the new strand. It can only add nucleotides to the 3' end of a growing chain.

Ligase: Joins fragments of newly synthesized DNA together, particularly on the lagging strand, to create a continuous molecule.

Because DNA polymerase can only build in the 5' to 3' direction, one strand (the leading strand) is synthesized continuously. The other strand (the lagging strand) is synthesized in small, discontinuous pieces called Okazaki fragments, which are later joined by ligase.

Step 2: Transcription - From DNA to Messenger RNA (mRNA)

Transcription is the process of creating an RNA copy of a segment of DNA. This RNA copy, called messenger RNA (mRNA), carries the genetic code from the nucleus to the cytoplasm.

The main enzyme is RNA Polymerase. The process occurs in three stages:

1.Initiation: RNA polymerase binds to a specific DNA sequence called a promoter, located near the beginning of a gene.

2.Elongation: The enzyme unwinds the DNA and synthesizes a complementary RNA strand, using uracil (U) in place of thymine (T).

3.Termination: RNA polymerase reaches a terminator sequence on the DNA, which signals the end of transcription. The newly made mRNA molecule is then released.

In eukaryotes, the initial mRNA transcript (pre-mRNA) undergoes processing, including the removal of non-coding regions called introns and the addition of a 5' cap and a 3' poly-A tail.

Step 3: Translation - From mRNA to Protein

Translation is the process where the genetic code carried by mRNA is decoded to produce a specific sequence of amino acids, forming a polypeptide chain (a protein). This occurs on cellular structures called ribosomes.

Key Players in Translation:

mRNA (messenger RNA): Carries the genetic code in the form of three-nucleotide 'words' called codons. Each codon specifies a particular amino acid (e.g., AUG codes for Methionine).

tRNA (transfer RNA): Acts as a molecular adapter. Each tRNA molecule has an anticodon that is complementary to an mRNA codon, and it carries the corresponding amino acid.

Ribosomes: Composed of ribosomal RNA (rRNA) and proteins, ribosomes have binding sites for mRNA and tRNA and catalyze the formation of peptide bonds between amino acids.

The ribosome moves along the mRNA, reading codons one by one. For each codon, the correct tRNA brings the specified amino acid, which is added to the growing polypeptide chain. This continues until a stop codon is reached, at which point the ribosome releases the completed protein.

Key Terms

DNA Replication: The process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules.
Transcription: The process of creating a complementary RNA copy of a sequence of DNA.
Translation: The process in which ribosomes in the cytoplasm or endoplasmic reticulum synthesize proteins after the process of transcription of DNA to RNA in the cell's nucleus.
mRNA (Messenger RNA): A single-stranded RNA molecule that is complementary to one of the DNA strands of a gene, carrying the code for protein synthesis.
tRNA (Transfer RNA): A type of RNA molecule that helps decode a messenger RNA (mRNA) sequence into a protein by bringing specific amino acids to the ribosome.
Ribosome: A complex molecular machine found within all living cells that serves as the site of biological protein synthesis (translation).