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Chapter 12: Gene Expression

Page history last edited by Derek Weber 12 years, 5 months ago


Learning Objectives 

12.1 Overview of Gene Expression

  • Describe the flow of information from the genetic material to a functional protein molecule. Describe the evidence for the one-gene/one-polypeptide hypothesis.
  •  Distinguish between transcription and translation.

 

12.2 Transcription

  • List the sequence of events during transcription of a gene.
  • Describe the roles of the different eukaryotic RNA polymerases.
  • List similarities and differences between the process of transcription discussed in this section and the process of DNA replication discussed in Chapter 11.

 

12.3 RNA Processing in Eukaryotes

  • Explain what happens to a pre-mRNA molecule during process of RNA splicing.
  • Describe the importance of end modification to eukaryotic mRNA molecules.

 

12.4 Translation and the Genetic Code

  • Summarize the experiments that revealed the genetic code.
  • Describe the characteristics of the genetic code.
  • Identify the relationship between codons and amino acids.

 

12.5 The Machinery of Translation

  • List the roles played by RNA in gene expression.
  • Describe the structure of a ribosome including the functional regions and explain how they contribute to the process of translation.
  • Explain the interaction between mRNA, tRNA, and rRNA during translation.

 

12.6 The Stages of Translation

  • Compare and contrast start and stop signals for transcription and translation, and note relevant differences between bacteria and eukaryotes
  • Explain the elongation cycle of translation.
  • Trace the fate of a eukaryotic mRNA molecule from initial transcription through its translation.
  • Compare translation on the RER and in the cytoplasm.

 

Chapter Summary

The current model of heredity states that individual genes on chromosomes code for particular polypeptides, which are then assembled into complex proteins. This is basically a two-step process with the DNA coding for various forms of RNA, which are then used to produce a polypeptide. There are three general classes of RNA derived from DNA; messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). The first step, DNA to RNA, is called transcription. The language of the molecules is basically the same (excluding the substitution of uracil for thymine), the product is mRNA. The second step, RNA to protein, is called translation. There is a change in language from the nucleic acid sequence of the RNA to the amino acid sequence of the protein product. All three RNAs are involved in this process; mRNA is the blueprint, rRNA is the assembly line machinery, and tRNA is the robot that delivers the amino acids from the supply room to the assembly line. Both transcription and translation are ultimately controlled by various assembly enzymes that recognize specific nucleotide sequences.

 

The genetic code that translates base pair sequence into amino acid sequence was deciphered by several researchers, including Crick. Crick postulated that each letter of the code was a block of three nucleotides, called a codon. Experimental data confirmed this and indicated that the code was a simple linear arrangement not punctuated by intervening nucleotides. Each of the 64 possible codons codes for a particular amino acid, a start or a stop signal. A few amino acids are represented by several codons, while others are represented by only one or two. Individual activating enzymes recognize certain short sequences in a specific region of the tRNA. Alteration of these sequences attaches an amino acid other than that associated with its anticodon onto the tRNA.  The genetic code is almost universal.  Except for a few exceptions, all organisms use the same genetic code.

 

Transcription, preparing the mRNA blueprint from the master DNA information, is completely dependent on the complex molecule RNA polymerase. Unlike DNA replication, RNA transcription does not require any primer. There are three main steps to transcription: initiation, elongation, and termination. RNA polymerase recognizes a specific site on the coding strand of DNA and causes the double helix to unwind forming a transcription bubble. The transcription bubble travels down the length of the gene until an appropriate stop signal is reached and transcription is terminated. Unlike DNA replication, RNA transcription has no proofreading capabilities.  In eukaryotes, the initial RNA transcript is modified before it leaves the nucleus.  These modifications include addition of a 5’ cap, 3’ tail and removal of intervening sequences, known as introns.  With the knowledge gained from the Human Genome Project, it is apparent that introns may be removed in different patterns depending on the cells expressing the gene.  This phenomenon is alternative splicing.

 

The mechanism of protein synthesis is controlled by several enzymes and initiation factors that accurately place the mRNA within the rRNA of the ribosome. Positioning is critical throughout the process to ensure proper reading of the sequences so the polypeptide is made correctly. The ribosome moves along the mRNA sequentially, reading the codon and adding a new amino acid to the growing chain. When a stop signal is reached, the entire complex disassociates its components free to be used elsewhere.

  

PowerPoint Presentations

Chapter 12 PowerPoint (.pdf)

  

Reading Assignments and Homework

Please access the ConnectPlus site for Health Science Academy to access our reading assignments and homework.

 


 

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