Gene expression is the process by which information from a gene is used to produce a functional product like proteins or RNA. It involves two main steps: transcription of DNA into mRNA and translation of mRNA into proteins. Transcription involves unwinding the DNA and synthesizing mRNA complementary to the template strand. Translation uses ribosomes to read the mRNA codon by codon and add the corresponding amino acids to produce a protein according to the genetic code. It occurs through three phases: initiation, elongation, and termination.

1. Gene Expression
Made by: Sara Hassan

2. Gene expression
Gene expression is the process by which
information from a gene is used in the
synthesis of a functional gene product. These
products are often proteins, but in non-protein
coding genes such as transfer RNA
(tRNA) genes, the product is a functional RNA
according to the Cental Dogma.

4. The Link Between DNA and Protein
• DNA contains the molecular blueprint of every
cell
• Proteins are the “molecular workers” of the cell
• Proteins control cell shape, function,
reproduction, and synthesis of biomolecules
• The information in DNA genes must therefore
be linked to the proteins that run the cell

5. The Genetic Code
• The base sequence in a DNA gene dictates the
sequence and type of amino acids in
translation
• Bases in mRNA are read by the ribosome in
triplets called codons
• Each codon specifies a unique amino acid in
the genetic code
• Each mRNA also has a start and a stop codon

8. Overview of Transcription
• Transcription of a DNA
gene into RNA has
three stages
– Initiation
– Elongation
– Termination

9. Initiation
• Initiation phase of transcription
1. DNA molecule is unwound and strands are
separated at the beginning of the gene
sequence
2. RNA polymerase binds to promoter region at
beginning of a gene on template strand

11. Elongation
1. RNA polymerase synthesizes a sequence of
RNA nucleotides along DNA template strand
2. Bases in newly synthesized RNA strand are
complementary to the DNA template strand
3. RNA strand peels away from DNA template
strand as DNA strands repair and wind up

13. Elongation
• As elongation proceeds, one end of the RNA
drifts away from the DNA; RNA polymerase
keeps the other end temporarily attached to
the DNA template strand

15. Termination
– RNA polymerase reaches a termination
sequence and releases completed RNA strand

20. Translation process

21. Translation
Translation is the process by which ribosomes read the genetic
message in the mRNA and produce a protein product according
to the message's instruction.
Figure 1.

22. Requirements of Translation
 Ribosomes
 tRNA
 mRNA
 Amino acids
 Initiation factors
 Elongation factors
 Termination factors
 Aminoacyl tRNA synthetase enzymes
 Energy source

23. Ribosomes
 Eukaryotic ribosomes are larger. They consist of two
subunits, which come together to form an 80S particle.
 60S subunit holds (three rRNAs 5S, 5.8S, 28S and about 40
proteins).
 40S subunit contains (an18S rRNA and about 30 proteins).
Figure 2.

24. The large ribosomal subunit contains three tRNA binding sites,
designated A, P, and E.
The A site binds an aminoacyl-tRNA (a tRNA bound to an amino
acid).
P site binds a peptidyl-tRNA (a tRNA bound to the peptide
being synthesized).
The E site binds a free tRNA before it exits the ribosome.
Figure 3.

25. Steps for protein synthesis
 First, aminoacyl tRNA synthetase joins amino acid to their
specific tRNA.
 Second, ribosomes must dissociate into subunits at the end of
each round of translation.

26. The protein synthesis occur in 3 phases
 Accurate and efficient initiation occurs; the ribosomes binds
to the mRNA, and the first amino acid attached to its tRNA.
 Chain elongation, the ribosomes adds one amino acid at a
time to the growing polypeptide chain.
 Accurate and efficient termination, the ribosomes releases
the mRNA and the polypeptide.

27. Initiation
 The initiation phase of protein synthesis requires over 10
eukaryotic Initiation Factors (eIFs).
 Factors are needed to recognize the cap at the 5'end of an
mRNA and binding to the 40s ribosomal subunit.
 Binding the initiator Met-tRNAiMet (methionyl- tRNA) to the
40S small subunit of the ribosome.
 Scanning to find the start codon by binding to the 5'cap of the
mRNA and scanning downstream until they find the first AUG
(initiation codon).

28.  The start codon must be located and positioned correctly in
the P site of the ribosome, and the initiator tRNA must be
positioned correctly in the same site.
 Once the mRNA and initiator tRNA are correctly bound, the
60S large subunit binds to form 80s initiation complex with a
release of the eIF factors.

30. Elongation
 Transfer of proper aminoacyl-tRNA from cytoplasm to A-site of
ribosome.
 Peptide bond formation; Peptidyl transferase forms a peptide
bond between the amino acid in the P site, and the newly
arrived aminoacyl tRNA in the A site. This lengthens the
peptide by one amino acids.
 Translocation; translocation of the new peptidyl t-RNA with
its mRNA codon in the A site into the free P site occurs. Now
the A site is free for another cycle of aminoacyl t-RNA codon
recognition and elongation.

31. Each translocation event moves mRNA, one codon length through
the ribosomes.
Figure 5.

32. Termination
Translation termination requires specific protein factors
identified as releasing factors, RFs in E. coli and eRFs in
eukaryotes.
The signals for termination are the same in both prokaryotes
and eukaryotes. These signals are termination codons present
in the mRNA. There are 3 termination codons, UAG, UAA and
UGA.
After multiple cycles of elongation and polymerization of
specific amino acids into protein molecules, a nonsense codon =
termination codon of mRNA appears in the A site.

33.  The is recognized as a terminal signal by eukaryotic releasing
factors (eRF) which cause the release of the newly
synthesized protein from the ribosomal complex.
Figure 6.

34. Polysomes
 Most mRNA are translated by more than one ribosome at a
time; the result, a structure in which many ribosomes
translate a mRNA in tandem, is called a polysomes.
Figure 7.

35. Post Translational Modifications