Introduction to transcription and translation Flashcards
Passed on unchanged from cell to descendants by DNA replication process
Sequence of nucleotides
DNA is made up of 1000s of genes
which are short sequences of deoxyribonucleotides in DNA (segments within DNA)that are genetic instructions to make proteins (or other RNA molecules)
The 2003 human genome project estimated that humans have
between 20,000 and 25,000 genes now it is known to be in excess of >45,000 genes which code for regulatory RNAs
Genes are
short sequences of DNA that contain genetic instructions (deoxyribonucleotides) to produce an RNA molecule (Ribonucleotides) and/or a protein (amino acids)
The genetic code refers to
a triplet sequence of ribonucleotides (RNA) that codes for a specific amino acid
Examples of proteins
Fibrous proteins: such as collagen, elastin, keratin (skin and hair)
Enzymes: digestive enzymes such as amylase, protease, lipases, pepsin, trypsin, DNAses, RNAses
metabolic enzymes: oxidase, carboxylase, dehydrogenase, lipogenases, oxidoreductases, kinases, lyases, transferases
Immune system: Antigens, antibodies, cytokines
From gene to protein requires 2 steps
Transcription and translation
Nuceic acids:
DNA and RNA (comprised of nucleotides) and proteins (comprised of amino acids) contain information written in 2 different chemical languages
Transcription
portion of DNA nucleotide sequence (gene) copied into RNA nucleotide sequence (same language) - occurs in nucleus for eukaryotic cells and cytoplasm of prokaryotic cells
Translation
synthesis of polypeptide under direction of RNA (change of language) - occurs in cytoplasm of all cells
The flow of genetic information
DNA -> transcription -> RNA -> translation -> protein
Genes within DNA uses RNA
as a temporary carrier of genetic information to direct the synthesis of proteins
The “Central Dogma” of biology
DNA Double-stranded (ds) (4 nucleotides: A,C,G,T)
[transcription of genes]
RNA single stranded (ss) (4 nucleotides: A,C,G,U)
[Translation of mRNA]
Protein Single-stranded (20 amino acids)
Differences between transcription and translation in eukaryotic vs prokaryotic cells [eukaryotic]
-DNA protected in nucleus therefore transcription occurs in the nucleus
-But mRNA most exit the nucleus to go to the cytoplasm for translation
-DNA contains non-coding DNA (introns) which must be cut out of DNA between the 2 processes therefore mRNA processing required
Differences between transcription and translation in eukaryotic vs prokaryotic cells [prokaryotic]
-No nucleus and DNA lies in cytoplasm and therefore both processes occur in tandem (quickly) = no transport needed
-DNA is mostly coded (exons) and therefore processing of non-coding DNA is not required
Prokaryotic cell
in a cell lacking a nucleus, mRNA produced y transcription is immediate translated without additional processing
Eukaryotic cell
The nucleus provides a separate compartment for transcription. The original RNA transcript, called pre-mRNA, is processed in various ways before leaving the nucleus as mRNA
Portions of DNA sequence are transcribed into RNA: transcription [RNA]
-nts (Nucletides) Ribonucleotides (G,C,A, Uracil) (G-C and A-U base pairs)
-It is a single stranded linear polymer of the 4 its linked together by phospherdiester bonds
-sugar = ribose
-RNA: single stranded, short polymer
-DNa: double stranded, long polymer
messanger RNA (mRNA) molecules
RNA molecules copied from genes which direct synthesis of proteins
other RNA sub-types are also synthesised by transcription
These RNA’s serve as
Enzymatic and structural components for a wide variety of processes in the cell ( do not direct protein synthesis)
These RNA’s include
Transfer RNA (tRNA)
Ribosomal RNA (rRNA)
Small nuclear RNA ( snRNA)
All subtypes of RNA are
Synthesised by transcription but are not translated into a protein (only mRNA is involved in translation)
Transcription produces
RNA complementary to one strand of DNA
Transcription begins with
opening and unwinding of small portion of DNA double helix to expose bases on each DNA strand
only 1 of the 2 strands acts as a template for the synthesis of RNA (the other is the coding strand)
Ribonucleotides added
one-by-one to growing RNA chain
As in DNA replication
nt sequence of RNA chain is determined by complementary base-pairing with DNA template (therefore continues code from DNA to RNA)
Each incoming ribinucleotire is
covalently linked to growing RNA chain in enzymatically catalysed reaction
The RNA strand is
elongated one nt at a time
RNA chain produced is called a
transcript, and has a nt sequence exactly complementary to strand of DNA used as template
RNA polymerase responsible for
synthesis of mRNA in transcription
RNA pol moves
stepwise along DNA, unwinding DNA helix just ahead to expose a new region of the template strand for complementary base-pairing
RNA chain extended
one nt at a time in 5’ to 3’ direction
RNA pol uses
ribonucleoside triphosphate (ATP,CTP, UTP, GTP) whose high-energy bonds provide the energy that drives reaction forward
Unlike DNA pol
RNA pol can stat new RNA chain without a primer
RNA makes approx 1 mistake every 10^4 nts compared to DNA pol: approx 1 mistake every 10^7 nts
Points to note
one template strand of DNA
Closed DNA outside of RNA polymerase, DNA only one within RNA plolymerase
Temporary base pairing of ribonucleotodes (red) to deoxyribonucleotodes (blue) on template strand
single strand newly synthesis mRNA (red) is left exposed from 5’ end
