DNA and RNA relationshipDNA and RNA relationship A gene is a genetic A gene is a genetic

sequence thatsequence that codes for an RNAcodes for an RNA..

In protein coding In protein coding genes, RNA genes, RNA codes codes for a protein.for a protein.

During protein synthesis a working During protein synthesis a working copy of the code is made from RNA copy of the code is made from RNA

Overall scheme: DNA -> RNA -> Overall scheme: DNA -> RNA -> proteinprotein

HOW ?HOW ?

Processes involving the Processes involving the two..two..

TRANSCRIPTION = The transfer of the TRANSCRIPTION = The transfer of the coded informationcoded information from the from the nucleus to the nucleus to the cytoplasm.cytoplasm.

TRANSLATION = The TRANSLATION = The conversion of that conversion of that infoinfo into polypeptides on ribosomes. into polypeptides on ribosomes.

Both these reaction involves DNA + RNABoth these reaction involves DNA + RNA..

TranscriptionTranscription

• nucleus.nucleus.• major components = DNA, RNA major components = DNA, RNA

polymerase polymerase • Importance = Gene is activated. Importance = Gene is activated.

= A copy of the code is made from RNA = A copy of the code is made from RNA (m-RNA)(m-RNA)

= m-RNA leaves the nucleus, goes to = m-RNA leaves the nucleus, goes to cytoplasm. cytoplasm. 

Transcription

Formation of m-RNA moleculeRNA polymerasesSeparate the 2 DNA strands and link RNA

nucleotides as they base pair along the DNA template

Can only add nucleotides to the 3' end; thus mRNA goes in the 5' to 3' direction

TranslationTranslation

cytoplasm, on the ribosomes m-RNAcytoplasm, on the ribosomes m-RNAMajor components = RibosomesMajor components = Ribosomes

Peptidyl transferase enzyme Peptidyl transferase enzyme Transfer RNAs Transfer RNAs 

Importance = Ribosome decodes the Importance = Ribosome decodes the m-RNA and makes the correct m-RNA and makes the correct protein.protein.

TranscriptionTranscription

a Working Copy of the Code is Made From a Working Copy of the Code is Made From RNARNA

The RNA copy of the code is The RNA copy of the code is complementary:complementary:

  DNA Base RNA BaseDNA Base RNA Base A U, C G, G C, T A A U, C G, G C, T A Note that U replaces T in RNA (U = uracil) Note that U replaces T in RNA (U = uracil) RNA leaves the nucleus and goes into the RNA leaves the nucleus and goes into the

cytoplasm, attaches to a ribosome to cytoplasm, attaches to a ribosome to make protein (translation) make protein (translation)

Transcription…Transcription…

DNA acts as DNA acts as templatetemplate for the synthesis of RNA for the synthesis of RNA RNA polymeraseRNA polymerase attaches to the double helix. attaches to the double helix. H+ bondsH+ bonds are broken = DNA unwinds. are broken = DNA unwinds. Coding strand acts as a templateCoding strand acts as a template and is and is copiedcopied

by base pairing of nucleotides.by base pairing of nucleotides. A complementary polynucleotide strand of A complementary polynucleotide strand of

mRNAmRNA is built. is built. The The mRNA detachesmRNA detaches and and leaves the nucleusleaves the nucleus

through the nuclear membrane pore.through the nuclear membrane pore. In the cytoplasm, mRNA becomes attached to In the cytoplasm, mRNA becomes attached to

ribosomesribosomes..

Close up…Close up…

Eukaryotic genes Eukaryotic genes contains 2 regions: contains 2 regions: “exons”“exons” and and “introns”.“introns”.

Exons – code for amino Exons – code for amino acids.acids.

Introns - Introns - DO NOT .DO NOT . Fx? Fx? During transcription, During transcription,

both E+I are copied both E+I are copied from the DNA.from the DNA.

DNA sequence coding DNA sequence coding for proteins for proteins exons exons may may be interrupted by non be interrupted by non coding DNA icoding DNA introns.ntrons.

During RNA synthesis, non-coding sequences of base pairs needs to be

subtracted from the coding sequences of a gene in order to transcribe DNA

into (mRNA.)

So that..

mRNA that leaves now consists of a mRNA that leaves now consists of a continous coding regioncontinous coding region

Before mRNA leaves the nucleus, introns Before mRNA leaves the nucleus, introns are removed are removed

RNA splicing..RNA splicing.. Process to remove Process to remove

unwanted materials.unwanted materials. i.i. In the nucleus, DNA In the nucleus, DNA

that includes the E + I is first that includes the E + I is first transcribedtranscribed into a into a complementary nuclear RNA complementary nuclear RNA copy copy (nRNA)(nRNA)

ii.ii. Introns are removedIntrons are removed from nRNA by a process from nRNA by a process called RNA splicing.called RNA splicing.

The edited sequence is The edited sequence is called called mRNAmRNA

Vital steps

InitiationInitiation ElongationElongation TerminationTermination

TranscriptionInitiation

RNA polymerases bind to DNA at promoters (initiation site); Enzyme separates the 2 DNA strands and transcription begins

Elongation RNA polymerases move along the DNA 1- untwists and opens a short segment of DNA (DNA template) 2- links incoming RNA nucleotides to the 3' end the elongating strand 1 at a time in the 5' to 3' direction the non-coding strand of DNA reforms a DNA-DNA double helix by

pairing with the coding strand

Termination

RNA polymerases reaches a termination site on the DNA

RNA molecule is then released