BIO1022 Chapter Notes - Chapter Prescribed : Guanine, Protein Structure, Frameshift Mutation
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.
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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.
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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.