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Chapter 13: Problem 2

Review the Chapter Concepts list on \(\mathrm{p} .342 .\) These all center around 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 \(\mathrm{RNA}\) is transcribed on a DNA template.

Short Answer

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Short Answer: The genetic code is nearly universal, redundant, non-overlapping, and possesses punctuation marks. It consists of 64 codons that encode amino acids or signal the beginning and end of translation to create proteins in living cells. DNA has a double helix structure with complementary base-pairing, which stores genetic information. This information is transcribed onto RNA molecules through the process of initiation, elongation, and termination, followed by post-transcriptional processing to create messenger RNA (mRNA) for protein synthesis.

Step by step solution

01

Introduction

In this essay, we will discuss the key properties of the genetic code and the process by which RNA is transcribed on a DNA template. The genetic code is the set of rules by which information encoded in DNA sequences is translated into proteins by living cells.
02

The Genetic Code and its Properties

The genetic code is composed of 64 codons, which are three-nucleotide (base) sequences of DNA that correspond to the 20 different amino acids used to build proteins. The properties of the genetic code include: 1. Universality: The genetic code is nearly universal, meaning that it is shared by all organisms on Earth, from bacteria to humans. 2. Redundancy: Some amino acids are represented by more than one codon, which allows for some redundancy in the code and reduces the impact of mutations on protein function. 3. Non-overlapping: Each nucleotide is part of only one codon, and the code is read in a linear fashion, with no overlapping of codons. 4. Punctuations: There are three stop codons that signal the end of a protein-coding sequence, and one start codon, AUG, which codes for methionine. 5. Specificity: Each codon specifies one (and only one) amino acid or signals a stop for translation.
03

The Structure of DNA

DNA (deoxyribonucleic acid) is a double helix structure formed by two chains of nucleotides that are complementary to each other. Each nucleotide is composed of a phosphate group, a sugar molecule (deoxyribose), and one of four nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T). The complementary nature of the two strands of DNA allows it to store genetic information by pairing A with T and G with C through hydrogen bonds, also known as base-pairing.
04

The Process of Transcription

Transcription is the process by which the genetic information stored in DNA is transferred to RNA (ribonucleic acid) molecules. Here are the steps involved in transcription: 1. Initiation: RNA polymerase, an enzyme that synthesizes RNA molecules, binds to a specific DNA sequence, known as the promoter, and unwinds the double helix. 2. Elongation: As the RNA polymerase moves along the template strand of DNA, it adds the complementary RNA nucleotides in the 5' to 3' direction, forming a growing RNA molecule that is complementary to the template DNA strand. In RNA, uracil (U) replaces thymine (T). 3. Termination: When the RNA polymerase reaches a termination sequence on the DNA, it stops transcription, releases the RNA molecule, and detaches from the DNA template. 4. Post-transcriptional processing: In eukaryotes, the newly formed RNA molecule undergoes several modifications, such as the addition of a 5' cap, 3' poly-A tail, and the removal of non-coding introns through splicing. The RNA molecule, now called messenger RNA (mRNA), is ready to be translated into a protein in a process called translation. In conclusion, the genetic code is comprised of 64 codons that specify amino acids or signal the beginning and end of translation. The genetic code is carried on the DNA molecule, which has a double helix structure and stores information through base-pairing. This information is transcribed onto RNA molecules, which are processed and then used to create proteins in living cells.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

DNA to RNA Transcription
The fundamental process of DNA to RNA transcription involves converting the genetic information stored in DNA into a format that can be used to synthesize proteins. Imagine DNA as a vast library of blueprints and RNA as a messenger carrying the instructions to the workers, or ribosomes, in the protein 'factory'.

During initiation, RNA polymerase attaches to the promoter region, a sequence that signals the start of a gene. The DNA strands then unwind to expose the coding sequence. During elongation, RNA polymerase traverses along the template strand, synthesizing a complementary RNA strand. Where DNA has thymine (T), RNA substitutes it with uracil (U). In termination, the RNA polymerase encounters a stop signal, releasing the newly synthesized RNA. This strand of RNA, in eukaryotic cells, will undergo further alterations before it's ready for translation.
Properties of the Genetic Code
The genetic code governs the translation of genetic information into functional proteins. It is composed of nucleotide triplets called codons, with each codon corresponding to a specific amino acid or a stop signal during protein synthesis.

Here are its key properties:
  • Universality: With few exceptions, the genetic code is consistent across all organisms, a fact that highlights the shared evolutionary heritage of life on Earth.
  • Redundancy: Multiple codons can encode the same amino acid, affording a buffer against mutations by reducing their potential harmful effects.
  • Non-overlapping: Codons are read one after another, without skipping any bases, ensuring precise protein synthesis.
  • Punctuation: Specific codons signal the start and end of protein synthesis, much like punctuation marks in a sentence.
  • Specificity: Each codon corresponds to one and only one amino acid (except in cases of redundancy), which guarantees the accuracy of protein construction.
RNA Polymerase Function
RNA polymerase serves as the primary enzyme responsible for transcribing DNA into RNA. Acting much like a manufacturing machine, it performs multiple tasks to ensure a faithful copy of the genetic message is created.

Its core functions include:
  • Initiating Transcription: After identifying the promoter region, RNA polymerase unwinds the DNA to prepare the template strand for transcription.
  • Elongation: It adds RNA nucleotides in an antiparallel fashion to the DNA, matching each DNA base with its complementary RNA counterpart.
  • Termination: RNA polymerase recognizes specific sequences marking the end of a gene and releases the nascent RNA transcript.
Through these mechanisms, RNA polymerase plays a crucial role in expressing genetic information.
Post-transcriptional Processing
In eukaryotic cells, the RNA transcript undergoes essential post-transcriptional processing events before it can direct protein synthesis. This process is akin to editing and refining a preliminary draft into a final, polished version.

The newly formed pre-mRNA is first ''capped'' with a modified guanine nucleotide at the 5' end, enhancing stability and aiding in translation initiation. A poly-A tail is added to the 3' end, which further prevents degradation and assists in export from the nucleus. Splicing then removes non-coding sequences, known as introns, and ligates the coding sequences, termed exons. The spliced mRNA, furnished with these modifications, exits the nucleus to meet the ribosome, where its encoded instructions will be translated into proteins.

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Most popular questions from this chapter

One form of posttranscriptional modification of most eukaryotic RNA transcripts is the addition of a poly-A sequence at the \(3^{\prime}\) end. The absence of a poly-A sequence leads to rapid degradation of the transcript. Poly-A sequences of various lengths are also added to many prokaryotic RNA transcripts where, instead of promoting stability, they enhance degradation. In both cases, RNA secondary structures, stabilizing proteins, or degrading enzymes interact with poly-A sequences. Considering the activities of RNAs, what might be general functions of \(3^{\prime}\) -polyadenylation?

A glycine residue is in position 210 of the tryptophan synthetase enzyme of wild-type \(E .\) coli. If the codon specifying glycine is GGA, how many single- base substitutions will result in an amino acid substitution at position \(210 ?\) What are they? How many will result if the wild-type codon is GGU?

Present an overview of various forms of posttranscriptional processing in eukaryotes. For each, provide an example.

Predict the amino acid sequence produced during translation by the following short hypothetical mRNA sequences (note that the second sequence was formed from the first by a deletion of only one nucleotide): Sequence 1: 5'-AUGCCGGAUUAUAGUUGA-3' Sequence \(2: 5^{\prime}-\) AUGCCGGAUUAAGUUGA-3' What type of mutation gave rise to Sequence 2 ?

It has been suggested that the present-day triplet genetic code evolved from a doublet code when there were fewer amino acids available for primitive protein synthesis. (a) Can you find any support for the doublet code notion in the existing coding dictionary? (b) The amino acids Ala, Val, Gly, Asp, and Glu are all early members of biosynthetic pathways (Taylor and Coates, 1989 ) and are more evolutionarily conserved than other amino acids (Brooks and Fresco, 2003 ). They therefore probably represent "early" amino acids. Of what significance is this information in terms of the evolution of the genetic code? Also, which base, of the first two, would likely have been the more significant in originally specifying these amino acids? (c) As determined by comparisons of ancient and recently evolved proteins, cysteine, tyrosine, and phenylalanine appear to be late-arriving amino acids. In addition, they are considered to have been absent in the abiotic earth (Miller, 1987 ). All three of these amino acids have only two codons each, while many others, earlier in origin, have more. Is this mere coincidence, or might there be some underlying explanation?
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