BIO1022 Chapter Notes - Chapter Prescribed : Guanine, Protein Structure, Frameshift Mutation
30 May 2018
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Molecular Genetics - Transcription and Translation
Concept 17.1 - 17.5
- genes dictate phenotypes through enzymes that catalyse specific chemical
reactions in the cell.
- function of a gene is to dictate a specific enzyme
- one gene - one polypeptide
- genes provide the instructions for making specific proteins.
- transcription
• the synthesis of RNA using information from DNA.
• information is transcribed/rewritten from DNA to RNA.
• resulting RNA molecule is a transcript of the gene’s protein building
instructions
• messenger RNA - carries a genetic message from the DNA to the protein
synthesising machinery of the cell.
- translation
• synthesis of a polypeptide using the information of the mRNA.
• the cell must translate the nucleotide sequence of an mRNA molecule into
the amino acid sequence of a polypeptide.
• the sites of translation are ribosomes - that facilitate the orderly linking of
amino acids into polypeptide chains.
- occur in all organisms
- genes program protein synthesis via genetic messages in the form of messenger
RNA.
- the genetic code
• codons - triplets of nucleotides
•
o triplets of nucleotide bases are the smallest units of uniform length
that can code for all the amino acids.
o flow of information from gene to protein is based on a triplet code.
o during transcription the gene determines the sequence of
nucleotide bases along the length of the RNA molecule that is being
synthesised.
o for each gene - only one of the two DNA strands is transcribed.
o
▪ template strand - provides the template for the sequence of
nucleotides in an RNA transcript.
- molecular components of transcription
• mRNA - the carrier of information from DNA to the cell’s protein
synthesising factory - is transcribed from the template strand of a gene.
• RNA polymerase pries the two strands of DNA apart and joins together
RNA nucleotides complementary to the DNA template strand - thus
elongating the RNA polynucleotide.
• RNA polymerase cab assemble a polynucleotide only in a 5-3 direction.
• the DNA sequence where RNA polymerase attaches and initiates
transcription is known as the promoter - in bacteria - the sequence that
signals the end of transcription - the terminator.
- synthesis of an RNA transcript
• the promoter of a gene includes within it the transcription start point and
extends several dozen to more nucleotide pairs from the start point.
find more resources at oneclass.com
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• RNA polymerase binds in a precise location on the promotor -
determining where transcription starts and which of the two strands of
the DNA helix is used as the template.
•
o in eukaryotes - a collection of proteins called transcription factors
mediate the binding of RNA polymerase and the initiation of
transcription.
o transcription initiation complex - complex of transcription factors
and RNA polymerase II bound to the promotor.
- elongation of the RNA strand
• as the RNA polymerase moves along the DNA it untwists the double helix,
exposing about 10-20 DNA nucleotides at a time for paring with RNA
nucleotides.
• the new RNA molecule peels away from its DNA template and DNA double
helix reforms.
- termination of transcription
• bacteria
•
o transcription proceeds through a terminator sequence in the DNA
- the transcribed terminator functions as the termination signal -
causing the polymerase to detach from the DNA and release the
transcript.
• eukaryotes
•
o RNA polymerase II transcribes a sequence on the DNA called the
polyadenylation signal sequence - which specifies a
polyadenylation signal in the pre-mRNA.
- eukaryotic cells modify RNA after transcription
• enzymes in the eukaryotic nucleus modify pre-mRNA in specific ways
before the genetic message is dispatched to the cytoplasm.
• both ends of the primary transcript are altered - certain interior sections
of the RNA molecule are cut out and the remaining parts are spliced
together.
•
o alteration of mRNA ends
o
▪ 5 end - receives a 5 cap - a modified form of guanine.
▪ 3 end - enzyme adds 50-250 more adenine forming a poly-A
tail.
▪
▪ facilities the export of the mature mRNA from the
nucleus.
▪ help ribosomes attach to 5 end.
• split genes and RNA splicing.
•
o introns/intervening sequences - noncoding segments of nucleic
acids that lie between coding regions.
o exons - expressed - usually by being translated into amino acid
sequences.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Genes dictate phenotypes through enzymes that catalyse specific chemical reactions in the cell. Function of a gene is to dictate a specific enzyme. Genes provide the instructions for making specific proteins. Genes program protein synthesis via genetic messages in the form of messenger. Termination of transcription: bacteria transcription proceeds through a terminator sequence in the dna. Ribozymes: rna molecules that function as enzymes, rna splicing can occur without proteins or additional rna molecules the intron rna functions as a ribozyme and catalyses its own excision. Translation is the rna-directed synthesis of a polypeptide. Ribosomes facilitate the specific coupling of trna anticodons with mrna codons during protein synthesis. consists of a large and a small subunit - each made up of proteins and ribosomal rnas. Termination of translation: elongation continues until a stop codon in the mrna reaches the a site of the ribosome, uag, uaa and uga nucleotide bases do not code for amino acids.
