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protein synthesis Flashcards

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1
Q

what are proteins and what is used to create it?

A
  • Proteins are polymers made from amino acid monomers
  • The order of bases in the genetic code in DNA codes for the appropriate sequence of amino acids

Transcription (DNA to RNA) and translation (RNA to protein) are important processes for protein synthesis

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2
Q

Amino Acid Assembly by tRNA

A
  • tRNA molecules have complementary base triplets that match up to the mRNA code
  • tRNA molecules have amino acids attached
  • Matching of tRNA and mRNA means that the amino acids are assembled in the correct sequence
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3
Q

what are the following:

Aminoacyl tRNA molecule

deacylated tRNA

peptidyl tRNA

A

Aminoacyl tRNA molecule, also known as charged tRNA

tRNA molecule which has had its amino acid removed is known as deacylated or uncharged tRNA

tRNA molecule bound to growing polypeptide chain is known as peptidyl tRNA

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4
Q

Genetic Code and Proteins:

what are base triplets?

where are codons found ?

why is genetic code degenerate ?

importance of degeneracy ?

A
  • DNA base triplets represent each amino acid
  • Base triplets in mRNA are called codons
  • More than one codon codes for each of the 20 amino acids (the genetic code is degenerate)
  • First two bases in the codon are the most crucial:
  • Gives tolerance against mutations
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5
Q

what is mRNA start codon?

stop codon?

A
  • Some mRNA codons have special roles
  • START codon:

AUG (methionine)

• STOP codons:

UAG
UAA
UGA

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6
Q

mutation on the genatic code.

A
  • DNA triplet sequence is important
  • Small changes such as insertion or deletion of bases can cause a shift in the reading frame
  • Likewise, substitutions of bases changing the codon can change the corresponding amino acid
  • This can change protein structure significantly and may even result in the creation of stop codons in the wrong places
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7
Q

why cannot DNA be used directly for translation.

A
  • Protein synthesis takes place outside the nucleus
  • DNA is too big to leave the nucleus
  • DNA code is transcribed into mRNA as it is small and mobile
  • mRNA leaves nucleoplasm via nuclear pores in the nuclear envelope and enter the cytoplasm
  • mRNA can then travel to ribosomes for translation of the DNA code into proteins
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8
Q

Transcription

A
  • RNA polymerase breaks apart the DNA strands
  • A single strand of mRNA is transcribed from the template strand using the base pair rule
  • In mRNA, the nucleobase T is substituted by U
  • Required nucleotides for mRNA synthesis are found freely in the nucleus
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9
Q

Pre-mRNA Processing:

what does it requre?

what is the cap made of?

how it degradation of mRNA prevented.

A
  • Early mRNA (pre-mRNA) is not in its final form
  • Pre-mRNA needs a 5’ cap
  • The cap is composed of phosphorylated 7-methyl guanosine which is added to the 5’ end of the mRNA by guanyltransferase
  • Ensures mRNA is exported out of nucleus
  • Blocks degradation of mRNA by 5’ exonucleases
  • Promotes translation
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10
Q

Pre-mRNA needs a 3’ poly-A-tail. why?

A
  • The pre-mRNA is cleaved by an endonuclease near a signal AUAAAA sequence at the 3’ end
  • 200 adenosine residues are then added at the cleavage site by poly-A polymerase (polyadenylation)
  • This poly-A-tail protects the mRNA from degradation by 3’ exonucleases
  • The poly-A-tail also aids in termination of transcription, ensures export from the nucleus and is important in translation
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11
Q

Splicing of Pre-mRNA:

exons?

introns?

splice variation

A
  • Pre-mRNA contains untranslated regions and also open reading frame regions (which code for proteins) known as exons
  • Pre-mRNA also contains unused base sequences known as introns which need to be spliced off to produce final mRNA
  • Splice variants mean that a single gene can code for many different proteins
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12
Q

Alternative Splicing

A

Alternative splicing of pre-mRNA sequence can produce different proteins from the same gene

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13
Q

Mature mRNA structure

A

5’cap, 5’UTR,coding reagion,3’UTR, poly(A) tail

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14
Q

Translation- ribosomes

A

Ribosomes:

  • Are composed of ribosomal RNA (rRNA) and ribosomal proteins
  • Consist of a 60S subunit and a smaller 40S subunit
  • Can be free or attached to the rough endoplasmic reticulum
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15
Q
  1. Initiation
A

binding of ribosome to 5’ end of mRNA and hydrogen binding of the anticodon of an aminoacylated tRNA carrying methionine on the AUG start codon

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16
Q
  1. Elongation
A

the addition of further amino acids to the growing polypeptide chain brought by corresponding aminoacylated tRNAs. Peptidyl transferase creates covalent peptide bonds between the amino acids

17
Q
  1. Termination
A

when the stop codon is reached and the peptide and ribosomal subunits are released

18
Q

Fate of Synthesised Polypeptide

A
  • Has to acquire 2° structure (α-helices and β-pleated sheets)
  • Has to fold into 3° structure
  • Has to assemble into 4° structure if appropriate
  • Proteins destined for use within the cytoplasm are synthesised on free ribosomes
  • Proteins destined for secretion out of the cell are synthesised on ribosomes attached to the RER
19
Q

Secreted Proteins -RER

A
  • Synthesised on ribosomes attached to the RER
  • RER is a system of flattened cavities
  • Lined by thin membrane running from nuclear envelope and into the cytoplasm and with many ribosomes on its surface
  • Provides compartment for protein synthesis
  • Secreted proteins have special signal sequences which interact with the RER membrane
  • The proteins are incorporated into vesicles (small spherical compartments made from RER membrane) for transport to the Golgi apparatus
20
Q

Secreted Proteins- golgi apparatus

A

Secreted Proteins

  • Vesicles move to Golgi apparatus (Golgi complex)
  • GA is a system of flattened plate-like cavities
  • Lined by a thin membrane
  • Post-translational modification of proteins occurs in the GA cavities. For example, glycosylation of membrane spanning proteins
  • The now modified protein traverses the GA and is packaged into secretory vesicles
  • Protein containing secretory vesicles move to the cell membrane, fuse with it and expel their contents into the extracellular space (exocytosis)
  • Some specialised GA vesicles called lysosomes contain enzymes which can digest old organelles
21
Q

Mutation in Sickle Cell Anaemia

A

The change in amino acid sequence causes haemoglobin to crystallise when oxygen levels are low, causing the sickle shape which gets stuck in small blood vessels
glutamic acid -to valine

biology (25 decks)

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