A&P cellular biology (3) Flashcards
Protein synthesis
(creation of proteins)
1. Occurs in cells with nucleus & DNA (no nucleus = no DNA = no protein synthesis) – some cells w/out nucleus have no DNA and thus cannot divide. E.g. red blood cells. They live for approx. 120 days.2. Protein synthesis begins in the nucleus (DNA) & ends in the cytoplasm (RNA)3. The original DNA unravels, and only 1 strand is copied by RNA polymerase each time.
protein synthesis two steps
The making of protein through 2 key steps:
Transcription
Translation
DNA RNA protein
base triplet transcription codon + anti-codon translation amino acids
DNA: Double-stranded, nucleus
RNA: Single-stranded, nucleus and cytoplasm
Protein: Sequence of amino acids determines structure, anywhere in
cell
Transcription
– “reading & copying” the DNA into RNA
occurs in the NUCLEUS (step 1)
“Transcribing” the original information which is the DNA template.
Each three nucleotide bases (BASE TRIPLET) in DNA strand corresponds to one codon (RNA).
step 1
Steps:
A small section of DNA (containing the gene to be transcribed) uncoils within the nucleus (recall, it’s a db-helix). Only 1 strand is transcribed. Once done the DNA recoils back into its characteristic DB-helix.
mRNA
step 2
An enzyme called RNA-polymerase begins TRANSCRIPTION
at the promoter region (start of the gene) by matching the complementary nucleotide bases. For example,…..
(A) (U) – normally, this would be T, but in RNA, T is replaced with U)
(G) (C)
(T) (A)
(C) (G)
(G) (C) … and so forth … .
DNA new RNA
strand strand
Note: If you know what the DNA nucleobases are, you can always determine the complimentary RNA nucleobases.
step 3
- TRANSCRITPTION continues until the RNA
polymerase reaches the terminator region of the
gene, which signals to the enzyme that this is the
end of the DNA strand required for reading
(different proteins will be made from different
strands of DNA at different times depending of the
needs of the cell at any give time).
step 4
- RNA-polymerase detaches at the terminator
region and may go on somewhere else to start
another transcription process (on the same or
different DNA strand), thus leaving behind the pre-
mRNA.
step 5
The new pre mRNA strand must now be edited via snRNP’s (“snurps”) – small nuclear ribonuclear proteins, which remove the introns (useless segment) and merges the exons (useful segments). This is called SPLICING!!!!
In essence, snRNPS is the editor of pre-mRNA before it enters translation.
Alternative splicing – a process by which snRNPS will splice a pre-mRNA strand differently at different times to produce different mRNA strands which are then translated to different proteins (from the same pre-mRNA!)
step 6
- The newly edited RNA strand is now called mRNA (messenger-RNA), and goes onto the ribosomes in the cytoplasm for step #2, TRANSLATION
transcription IN NUCLEUS
Transcription takes place in the NUCLEUS starting with unraveling of DNA.
An enzyme, RNA POLYMERASE reads triplet bases of DNA, copying its compliment and making
pre-mRNA (IMMATURE)
pre-mRNA is edited and then sent out to the ribosomes in ER for translation into proteins.
Translation IN CYTOPLASM
1
– “translating/deciphering the mRNA into amino acid chains (making proteins)
occurs in the CYTOPLASM, on ribosomes.
Steps:
1. The edited mRNA from transcription coming from nucleus attaches to rRNA (ribosomal RNA) in the cytoplasm which has 2 subunits, a larger unit and a smaller subunit
2
- The smaller subunit holds the mRNA in place, while the larger unit has 2 sites, A site P site, for attachment of tRNA (transfer RNA) that carries with it, a specific amino acid (aa).
the section on the mRNA that is read is called the CODON, and this always occurs in triplet of nucleotide bases. Thus, the complimentary triplet segment on the tRNA is called the ANTICODON. Each anticodon on the tRNA encodes for a specific AMINO ACIDS, which is already attached. This is the point in which the mRNA strand is converted to an Amino Acids Chain.
Ex…AUG – the codon (on the mRNA), is matched via its complementary UAC – the anticodon (on tRNA) which produces the *aa: methionine, and is always the start of a new polypeptide chain of amino acid (ALL protein in the body starts with methionine) Thus, AUG is the initiator codon, UAC is the initiator anticodon, and the tRNA that is involved in this is called the
* initiator t-RNA
4
tRNA always binds to site A first, then shifts to site P (with the exception of UAC, which always binds to site P first, b/c it is the initiator tRNA). Then, when the next anticodon comes along with the next set of 3 aa’sthey’ll land onto the A site, then shift to the P site,… etc….
5
- this process continues to build the polypeptide chain*, with peptide bonds holding the new aa’s together, until it reaches a STOP CODON. (this stop codon can be any sequence of nucleotide base pairs. Again, it all depends on what protein the cell is trying to build based on its physiological need)
6
- Sometimes multiple ribosomes will attach to the same mRNA to produce multiple copies of the SAME aa chain. These are called polyribosomes. (this occurs if we need many of the same aa chains to produce many of the same proteins)
