Protein Synthesis Pt. I:Transcription

There are 2 parts to protein synthesis (making a protein)

Transcription & Translation

Helpful hint

• Protein synthesis is complex and occurs in 2 parts

1. Transcription2. Translation Always in that order…. To help you remember the order, think alphabetical…I have underlined the first letters after trans. C comes before L in the alphabet.

Protein Synthesis

• Remember, DNA provides the instruction for making proteins.

• To make proteins, there are 3 different types of slightly different nucleic acids, called RNA, and the 2 steps we already mentioned.– mRNA(messenger RNA)– rRNA (ribosomal RNA)– tRNA (transfer RNA)

RNA

• RNA (ribonucleic acid) is made up of many nucleotides assembled in exactly the same way as DNA, except…– RNA is mostly single stranded – The sugar is Ribose (not deoxyribose)– The N.Base Thymine is replaced by Uracil• Uracil will pair with Adenine since there is no Thymine

in RNA

– Other 3 bases (A,C,G) are the same

RNA vs. DNA

Transcription (Part 1)

• The first step in using the DNA code to make proteins is transcription

• This happens in the NUCLEUS• DNA unzips and a new enzyme, RNA

polymerase, attaches and makes a strand of RNA (instead of DNA, like in replication)

• Only one side of DNA is transcribed– To transcribe is to write, or to make a written copy

of something

Transcription (Part 1)

• Once the RNA has “transcribed” or copied 1 strand of DNA, the DNA zips back up

• The newly made RNA transcription exits the nucleus and into the cytoplasm

• All of this MUST happen in the nucleus because DNA is too big, and SO PRECIOUS and IMPORTANT, that it must stay in the nucleus where it is protected.

Transcription (Part 1)

• Once the RNA is out of the nucleus and in the cytoplasm (liquid portion of the cell)

• All 3 types of RNA are made this way…let’s talk more about those types of RNA

mRNA (messenger RNA)

• mRNA - messenger RNA is a messenger because it carries a code of what amino acids are to be put together in what order to make a protein

• mRNA varies in length (# of bases) determined by the number of amino acids required to make the protein for which it carries the code

• Goes through the nuclear pore, into the nucleus, makes a copy of 1 strand of DNA and comes back out

rRNA (ribosomal RNA)• Important component of the ribosome, which is the

organelle of the cell that actually makes the protein• Ribosomes contain two major rRNAs and 50 or more

proteins. The rRNAs form 2 subunits, the large subunit (LSU) and the small subunit (SSU)

tRNA (transfer RNA)

• Transfers the amino acids to the ribosome for assembly into proteins

• Very different, “peculiar” structure• At least 1 different tRNA for each amino acid!• All tRNA end in CCA (cytosine, cytosince,

adenine)on 3’ end – That 3’ end serves as the attachment point for the

amino acid that is going to be transfered

tRNA (transfer RNA)

• Very different, “peculiar” structure - Looks like a hairpin with 3 loops of unpaired bases– See next slide for picture

• Center loop has 3 unpaired bases called the anti-codon – Anti-codon is the recognition code that assures

that the particular tRNA is attracted to ONLY 1 AMINO ACID (so it doesn’t bring the wrong one)

– See next slide for picture

tRNAAmino Acid Attachment siteNOTE: ACC

Anti-codon

Note the 3 unpaired loops & notice the rest of the tRNA bases are paired up with their complimentary bases.

Q: what does the anti-codon determine?

To sum it up

• These notes introduced you to transcription and the 3 types of RNA

• Basically, Transcription is the process of making an RNA copy of a gene sequence. This copy, called a messenger RNA (mRNA) molecule, leaves the cell nucleus and enters the cytoplasm, where it directs the synthesis of the protein, which it encodes.– For our purpose, mRNA is the only RNA molecule involved

in the transcription portion of protein synthesis, (of course, transcription is how rRNA and tRNA are made…)