Chapter 17 What is gene expression? What are the two main stages of gene express

ID: 270506 • Letter: C

Question

Chapter 17 What is gene expression? What are the two main stages of gene expression? What are some differences in gene expression between prokaryotic and eukaryotic cells? What is transcription? o What is a primary transcript? o What is the triplet code? Codons? Anticodon? Template strand? Coding strand? Reading frame? Genetic code? o What enzym o What are the three stages of transcription Be able to es are involved in transcription? describe the process of transcription at each of the three stages o What is a transcription factor? Promoter? o How is the mRNA processed in eukaryotic cells before it can leave the nucleus? Why are these modifications important? o What is RNA splicing? What is alternative RNA splicing? What is translation? o What are the three main types of RNA? (They are all involved in the process of translation.) Be able to define each and describe their function o What is a ribosome? What does it do? What is its structure? What are the two different kinds of ribosomes? What kinds of proteins does each type of ribosome specialize in producing and where/how? o What are the three stages of translation? Be able to describe the process of translation at each of the three stages o What is a polyribosome? What are the different kinds of mutations which we discussed and how do they affect polypeptide production? What is a mutagen and what is mutagenesis?

Explanation / Answer

Answer 1

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as transfer RNA (tRNA) or small nuclear RNA (snRNA) genes, the product is a functional RNA.

The process of gene expression is used by all known life—eukaryotes (including multicellular organisms), prokaryotes (bacteria and archaea), and utilized by viruses—to generate the macromolecular machinery for life.

Stages of gene expression

This process involves two major steps: transcription and translation.

Simplified schematic of central dogma, showing the sequences of the molecules involved.

The two strands of DNA have the following sequences:

5'-ATGATCTCGTAA-3' 3'-TACTAGAGCATT-5'

Transcription of one of the strands of DNA produces an mRNA that nearly matches the other strand of DNA in sequence. However, due to a biochemical difference between DNA and RNA, the Ts of DNA are replaced with Us in the mRNA. The mRNA sequence is:

5'-AUGAUCUCGUAA-5'

Translation involves reading the mRNA nucleotides in groups of three; each group specifies an amino acid (or provides a stop signal indicating that translation is finished).

3'-AUG AUC UCG UAA-5'

AUG ightarrow?right arrowMethionine AUC ightarrow?right arrowIsoleucine UCG ightarrow?right arrowSerine UAA ightarrow?right arrow"Stop"

Polypeptide sequence: (N-terminus) Methionine-Isoleucine-Serine (C-terminus)

Thus, during expression of a protein-coding gene, information flows from DNA ightarrow?right arrow RNA ightarrow?right arrow protein. This directional flow of information is known as the central dogma of molecular biology. Non-protein-coding genes (genes that specify functional RNAs) are still transcribed to produce an RNA, but this RNA is not translated into a polypeptide. For either type of gene, the process of going from DNA to a functional product is known as gene expression.

Differences in the Regulation of Gene Expression of Prokaryotic and Eukaryotic Organisms

Prokaryotic organisms

Eukaryotic organisms

Lack nucleus

Contain nucleus

RNA transcription and protein translation occur almost simultaneously

RNA transcription occurs prior to protein translation, and it takes place in the nucleus. RNA translation to protein occurs in the cytoplasm.

RNA post-processing includes addition of a 5? cap, poly-A tail, and excision of introns and splicing of exons.

Gene expression is regulated primarily at the transcriptional level

Gene expression is regulated at many levels (epigenetic, transcriptional, post-transcriptional, translational, and posttranslational)

Answer 2

Transcription is the first step in gene expression, in which information from a gene is used to construct a functional product such as a protein. The goal of transcription is to make a RNA copy of a gene's DNA sequence. For a protein-coding gene, the RNA copy, or transcript, carries the information needed to build a polypeptide (protein or protein subunit). Eukaryotic transcripts need to go through some processing steps before translation into proteins.

The DNA strand that mRNA is built from is called the template strand because it serves as a template for transcription. It is also called the antisense strand. The template strand runs in a 3’ to 5’ direction.

The strand of DNA not used as a template for transcription is called the coding strand, because it corresponds to the same sequence as the mRNA that will contain the codon sequences necessary to build proteins. The only difference between the coding strand and the new mRNA strand is instead of thymine, uracil takes its place in the mRNA strand. The coding strand is also called the sense strand. The coding strand runs in a 5’ to 3’ direction.

The region of the nucleotide sequences from the start codon (ATG) to the stop codon is called the Open Reading frame.

Gene finding in organism specially prokaryotes starts form searching for an open reading frames (ORF). An ORF is a sequence of DNA that starts with start codon “ATG” (not always) and ends with any of the three termination codons (TAA, TAG, TGA). Depending on the starting point, there are six possible ways (three on forward strand and three on complementary strand) of translating any nucleotide sequence into amino acid sequence according to the genetic code .These are called reading frames.

Being a very essential process in the cell, Transcription has multiple enzymes being involved.

Thus, transcription being a very complicated mechanism continuously occurring within our body composes of multiple enzymes depending the the gene to be transcribed and its location on the genome.