Question

Review the Chapter Concepts list on p. 231. These all center on how genetic information is stored in DNA and transferred to RNA prior to translation into proteins. Write a short essay that summarizes the key properties of the genetic code and the process by which RNA is transcribed on a DNA.

Short answer

Question: Explain the key properties of the genetic code and describe the process of RNA transcription on a DNA template. Answer: The genetic code is a set of rules defining how DNA or RNA sequences are translated into amino acid sequences in proteins. It has three main properties: 1) Triplet code - codons or groups of three nucleotides code for specific amino acids; 2) Degeneracy - most amino acids are coded by more than one codon, allowing mutation tolerance; 3) Universality - the same codons code for the same amino acids in almost all organisms. Transcription, the process of transferring genetic information from DNA to RNA, comprises three stages: Initiation - RNA polymerase binds to a DNA promoter, unwinding the double helix; Elongation - RNA polymerase moves along the DNA template, adding complementary RNA nucleotides; Termination - transcription ends upon encountering a DNA terminator sequence, releasing the RNA molecule.

Step-by-step solution

Introduction

Genetic information is stored in DNA and transferred to RNA, which is later translated into proteins, the functional molecules in cells. In this essay, the key properties of the genetic code and the process of RNA transcription on a DNA template will be summarized.

Key Properties of the Genetic Code

The genetic code is the set of rules that determine how the nucleotide sequence in DNA or RNA is translated into the amino acid sequence in proteins. There are three main properties of the genetic code: 1. Triplet code: The genetic code is a triplet code, with each group of three nucleotides (called a codon) coding for a specific amino acid. There are 64 possible codons (4^3 combinations of the four nucleotides), of which 61 code for the 20 amino acids and 3 function as stop codons, signaling the end of translation. 2. Degeneracy: The genetic code is degenerate, meaning that most amino acids are coded for by more than one codon. This redundancy allows for some tolerance of mutations, as a single nucleotide change may not necessarily result in a different amino acid. 3. Universality: The genetic code is largely universal, with few exceptions, meaning that the same codons code for the same amino acids in almost all organisms. This shared genetic language supports the concept of a common ancestor for all life on Earth.

RNA Transcription Process

Transcription is the process by which genetic information stored in DNA is transferred to a complementary RNA molecule. This process occurs in three main stages: 1. Initiation: Transcription begins when an enzyme called RNA polymerase binds to a specific sequence on the DNA (the promoter). This binding unwinds the DNA double helix, exposing the template strand that will be used for RNA synthesis. 2. Elongation: RNA polymerase moves along the template strand, "reading" the DNA sequence and adding complementary RNA nucleotides to the growing RNA chain. Adenine (A) in DNA binds to uracil (U) in RNA, whereas cytosine (C) and guanine (G) bind as they do in DNA. 3. Termination: Transcription ends when RNA polymerase encounters a specific DNA sequence (the terminator) that signals the end of the gene. At this point, the RNA molecule is released, and the DNA double helix reforms. In summary, the genetic code is a triplet, degenerate, and universal code that translates nucleotide sequences in DNA or RNA into amino acid sequences in proteins. The transcription process transfers genetic information from DNA to RNA, which later undergoes translation to produce functional proteins.

Key concepts

Transcription Process

Transcription is a fundamental biological process that serves as the first step in gene expression. It involves copying a gene's DNA sequence into a complementary RNA sequence. This process occurs in all living cells and plays a crucial role in transferring genetic information from DNA to RNA.
The transcription process comprises three main stages: initiation, elongation, and termination.

• Initiation: Transcription begins when the enzyme RNA polymerase attaches to a specific region on the DNA known as the promoter. This binding causes the DNA to unwind, making the template strand accessible for the synthesis of RNA.
• Elongation: Once initiation is complete, RNA polymerase travels along the template strand, catalyzing the creation of an RNA strand by adding RNA nucleotides. Each nucleotide in the DNA sequence pairs with a complementary RNA nucleotide – adenine pairs with uracil and cytosine with guanine. This creates a growing RNA chain that is a mirror image of the DNA template strand.
• Termination: When the RNA polymerase meets a termination signal, a specific DNA sequence, the transcription process ends. The newly formed RNA strand detaches, and RNA polymerase leaves the DNA template. At this stage, the DNA helix reforms its original structure.

Understanding transcription is essential because it is how genetic instructions are conveyed to produce proteins, which are essential for various physiological processes.

Codons and Amino Acids

The genetic code is essentially the set of rules by which the information encoded in genetic material (DNA or RNA sequences) is translated into proteins by living cells.
In this code, sequences of three nucleotides, known as codons, correspond to specific amino acids or signal stop commands during protein synthesis.

• Triplet Code: The genetic code is based on triplet codons. Each codon consists of three nucleotides, allowing for 64 possible codons. Given that there are only 20 amino acids, this means multiple codons can encode the same amino acid.
• Degeneracy: This redundancy is referred to as the degeneracy of the genetic code. It means that several different codons can specify the same amino acid, offering a protective buffer against mutations.
• Universality: The genetic code is nearly universal across all organisms, meaning that a given codon typically corresponds to the same amino acid in different species. This is a testimony to the shared evolutionary heritage of life on Earth.

Knowing how codons translate to amino acids helps us understand how proteins are formed. Each protein's unique sequence is dictated by the order of codons, ultimately determining its structure and function.

RNA Polymerase

RNA polymerase is an essential enzyme in the transcription process. Its primary role is to synthesize RNA from a DNA template, enabling gene expression.
Without RNA polymerase, the transcription process could not occur.
Here are the key functions and features of RNA polymerase:

• Binding to DNA: RNA polymerase initiates transcription by attaching to a promoter region on the DNA. This binding is critical for the beginning of RNA synthesis.
• RNA Chain Elongation: The enzyme traverses the DNA template strand, catalyzing the formation of phosphodiester bonds between RNA nucleotides. This adds to a growing RNA strand, extending the RNA molecule one nucleotide at a time in the direction of 5’ to 3’.
• Proofreading: Although less rigorous than DNA polymerase, RNA polymerase performs a proofreading function to minimize errors in RNA synthesis.

RNA polymerase is vital for the expression of genes, thus directly impacting the synthesis of proteins. Its function underscores the importance of enzymes in biochemical processes and cellular functions.