Protein Synthesis

1
Protein Synthesis

• Central Dogma in Biology
• DNA ? RNA ? Protein
• The information in DNA always flows from DNA into
  proteins, and never the reverse! The 1
  structure of DNA defines and directs the
  production of proteins. Proteins do not define
  DNA.

2
Protein Synthesis

• Protein synthesis is composed of two phases.
• DNA ? RNA ? Protein
• 1. Transcription 2. Translation
• 1. Transcription is making a duplicate copy of
  something. The building blocks of DNA and RNA
  are almost exactly the same (nucleotides). DNA
  and RNA therefore are made from the same
  language.
• 2. Translation is a process of taking information
  from one language to another. The building
  blocks of a protein are amino acids. The
  sequence of nucleotides in DNA and RNA is
  translated into the language of proteins (aa).

3
NUCLEUS The DNA code is transcribed into
mRNA. RIBOSOMES The mRNA is translated to
give instructions for proteins synthesis.
4
Transcription

• Three different kinds of RNA are transcribed.
• Messenger RNA (mRNA)
• Contains the information for making the protein.
• Sometimes called the message or transcript.
• Contains the sequence of codons necessary for
  translation
• Ribosomal RNA (rRNA)
• Makes the ribosome.
• Composed of a large and small subunit.
• The ribosome is where translation takes place.
• Transfer RNA (tRNA)
• Transports amino acids to the ribosome/mRNA
  complex for translation.
• Contains the sequence of anticodons necessary for
  translation.
• Also contains the amino acid attachment site.

5
Transcription RNA Polymerase

• An enzyme that oversees the synthesis of RNA
• Unwinds the DNA template
• Adds complementary ribonucleoside triphosphates
  on the DNA template
• Joins these RNA nucleotides together
• Encodes a termination signal to stop transcription

6
Transcription
7
Figure 3-18
Transcription of a gene from the DNA template to
RNA transcript. RNA triplets are called
codons.
8
Post-transcriptional Modification of mRNA

• The newly form transcript contains introns and
  exons.
• Introns are noncoding segments while exons are
  coding segments.
• mRNA processing is accomplished in the nucleus
  before export to the cytosol.
• Because of alternate splicing one gene may code
  for more than one protein. It also explains how
  isozymes are developed.

9
Genetic Code

• RNA codons code for amino acids according to a
  genetic code

Figure 3.36
10
Figure 3-20
Codon sequence on mRNA pairs with anticodon
of tRNA to determine which amino acid gets put
into the new protein.
11
Arrow indicates movement of the ribosome along
the mRNA.
12
Formation of Multiple Protein Chains
An mRNA molecule may have several ribosomes on it.
13
Large proteins can be cut into smaller proteins.
14
Secretory Pathway

• Proteins synthesized on R.E.R. enter a secretory
  pathway mediated by vesicular transport and
  exocytosis.

15
Protein Degradation

• Nonfunctional organelle proteins are degraded by
  lysosomes
• Ubiquitin attaches to soluble proteins and they
  are degraded in proteasomes