Protein Synthesis
Central Dogma After discovering the double helix structure Crick went on to study how DNA serves as the hereditary molecule of life. Both he and Watson reasoned that DNA must code for proteins.
Central Dogma One puzzle that they had to figure out, though, was how DNA in the nucleus could code for proteins in the cytoplasm…
Central Dogma
Crick proposed the ‘central dogma’ of biology which states that RNA serves as the carrier molecule between the DNA in the nucleus and the ribosomes in the cytoplasm.
DNA and RNA both have a sugar phosphate backbone.
However, DNA and RNA have a different sugar -
DNA and RNA also each have 4 nitrogenous bases –However… DNA’s bases are A, T, C, G RNA’s bases are A, U, C, G
Further, RNA is single stranded while DNA is a double helix!
Crick Animation dex.html dex.html Go to note sheet and do transcription
TRANSCRIPTION of DNA
How does RNA polymerase know where a gene starts and stops? Enzyme binds to places with specific DNA sequences called _______________. PROMOTERS tell _________________ where to start. promoters RNA polymerase
RNA At this point we know RNA is the key. Information flows from DNA to RNA to PROTEIN. But how? –How is RNA made? –How does RNA make proteins?
Animation of Transcription
Types of RNA Messenger RNA (mRNA) Transfer RNA (tRNA) Ribosomal RNA (rRNA)
How & Where is mRNA formed? All RNA is formed in the nucleus in a process called transcription. –Once mRNA is formed it is edited and sent out to the ribosome… –The mRNA transcript has a cap and a tail added. –The mRNA transcript has the region coding for introns removed. »DNA is composed of both introns and exons. »Exons are segments of DNA that are expressed as genes. »Introns are ‘junk DNA.’ They are not expressed and must be removed before the mRNA leaves the nucleus.
What is the typical shape of mRNA? –mRNA (single short strand of RNA)
How and where does mRNA function? mRNA – serves as a messenger carrying messages from the DNA in the nucleus to the ribosome in the cytoplasm –mRNA functions in the nucleus and cytoplasm (remember it carries the message)
mRNA requires editing before leaving the nucleus… Image by Riedell ExonsIntrons “expressed ” Exons Exit the nucleus!
Think break – wait, WHY WASTE IT? May allow same gene to be used in different ways in different kinds of cells. May have a role in evolution… allows small changes in genes to have a big effect.
How & Where is tRNA formed? All RNA is formed in the nucleus in a process called transcription. –Once tRNA is formed it heads right to the cytoplasm
What is the typical shape of tRNA? –tRNA (folded strand of RNA, forms a t shape)
How and where does tRNA function? tRNA – transfers amino acids from the cytoplasm to the ribosome –tRNA functions in the cytoplasm (remember it gathers the amino acids)
How & Where is rRNA formed? All RNA is formed in the nucleus in a process called transcription. –Once rRNA is formed it goes to the nucleolus to be modified into a ribosome.
What is the typical shape of rRNA? –rRNA (folded strand of RNA, forms a ribosome) –Consists of 2 subunits
How and where does rRNA function? Found in cytoplasm and on rough ER rRNA – functions as the ribosome (makes proteins) –rRNA functions in the cytoplasm (remember it is the ribosome)
Animation of Translation
v=H5udFjWDM3E Chef animation
Images modified from © Pearson Education Inc, publishing as Pearson Prentice Hall. All rights reserved Amino Acid tRNA anticodon mRNA strand rRNA Step 2: Translation
The 3 Types of RNA work together to make proteins… Let’s check it out! (overview of t & t) Let’s check it out! This analogy often helps people remember the roles of RNA – Think of 3 people baking a cake… –mRNA – brings the recipe –tRNA – brings the ingredients –rRNA – puts the ingredients together using the recipe / it is the kitchen (chef)!
Protein (polypeptide) Translation Transcription DNA replication DNA Nucleus mRNA codon rRNA
The m-RNA Code 64 possible codons Some amino acids have more than one codon. Codons code for different amino acids UCG-CAC-GGU Serine- Histidine- Glycine