This document provides an overview of gene expression and protein synthesis. It discusses how DNA is transcribed into mRNA which is then translated into protein. Transcription occurs in the nucleus, while translation occurs in the cytoplasm. The genetic code is composed of three nucleotide mRNA codons that each specify one amino acid. Transcription includes initiation, elongation, and termination, while translation involves initiation, elongation, termination and the joining of amino acids into a protein chain. The protein then undergoes processing and folding into its final functional structure.

1. Biology for medical students
lecture 2.
Gene expression. The genetic code.

2. DNA (deoxyribonucleic acid) takes part in
some very important processes in cell:
• Self-replication
• Transcription
• Recombination
• Repair
• Mutation

3. The central dogma of molecular
biology
transcription
translation

4. Graphics Gallery Index

5. Protein (polypeptide) is a polymer
built up of amino acids
amino acid

7. Peptide bond forming between 2 amino
acids
С—N
O H

8. Structures of a protein

9. 2 types of
secondary structure
x

10. Examples of quaternary structure of protein

11. • Gene expression is the process by which inheritable
information from a gene, such as the DNA
sequence, is made into a functional gene product,
such as protein or r- and tRNA.
• Non-protein coding genes (e.g. rRNA genes, tRNA
genes) are transcribed, but not translated into
protein.

12. In a prokaryotic cell, transcription and translation
are coupled; that is, translation begins while the
mRNA is still being synthesized. In a eukaryotic cell,
transcription occurs in the nucleus, and translation
occurs in the cytoplasm.

13. Lac-operon E.coli
РНК-
полимераза
zpromoter
genes

14. Transcription and translation are
spatially and temporally separated in
eukaryotic cells; that is, transcription
occurs in the nucleus to produce a pre-
mRNA molecule.
The pre-mRNA is typically processed to
produce the mature mRNA, which exits
the nucleus and is translated in the
cytoplasm.

16. What is a gene?
• Johannsen, 1909: a hereditary factor that
constitutes a single unit of hereditary
material.
• These days: a segment of DNA that codes
for synthesis of a single polypeptide chain
or t- and rRNA.

17. Typical eukaryotic gene
leader trailer
Exons
Introns

18. A promoter is a region of DNA that facilitates the
transcription of a particular gene. Promoters are typically
located near the genes they regulate, on the same strand and
upstream (towards the 5‘ region of the sense strand).

19. There are 4 stages in the gene
expression process :
• Transcription
• Processing of RNA
• Translation
• Post-translational modifications of a protein
(processing of a protein).

20. Protein processingProtein processing
1
2
3
4

21. I. Transcription

22. Transcription – synthesis of RNA
from DNA template
• Initiation
• Elongation
• Termination

23. Transcription in eukaryotes

24. 1. Initiation - RNA polymerase binds to
the promoter
Complex of
transcription-
regulating
proteins (factors)
RNA polymerase
(RNAP)
promoter

25. 2. Elongation – new nucleotides are
added to growing RNA

26. mRNA
(Hairpin*)
3. Termination – a hairpin structure is formed

27. *) a hairpin structure

28. 2. Processing of RNA

29. 2. Processing of mRNA
• The pre-mRNA molecule undergoes three
main modifications. These modifications
are 5’capping, 3' polyadenilation, and RNA
splicing, which occur in the cell nucleus
before the RNA is translated.

31. Protein processingProtein processing
1
2
3
4
Processing of RNA :
1. “Cap” is added to 5’-end.
2. Introns are cut off .
3. Exons are spliced
4. Poly-A tail is added to 3’-end.
nuclear membrane

32. 3. Translation

33. Translation – synthesis of a
polypeptide on RNA template. It
requires:
• Ribosomes
• tRNA
• mRNA
• Amino acids
• Energy of ATP

35. tRNA

36. most important are
nucleotides 1 and 2

37. • The genetic code is the set
of rules by which
information encoded in
genetic material (DNA or
RNA sequences) is
translated into proteins
(amino acid sequences)

38. The genetic code

39. Properties of the genetic code
• The genetic code is composed of nucleotide triplets. In other
words, three nucleotides in mRNA (a codon) specify one amino
acid in a protein.
• The code is non-overlapping. This means that successive
triplets are read in order. Each nucleotide is part of only one
triplet codon.
• The genetic code is unambiguous. Each codon specifies a
particular amino acid, and only one amino acid. In other words,
the codon ACG codes for the amino acid threonine, and only
threonine.
• The genetic code is degenerate. In contrast, each amino acid can
be specified by more than one codon.
• The code is nearly universal. Almost all organisms in nature
(from bacteria to humans) use exactly the same genetic code. The
rare exceptions include some changes in the code in
mitochondria, and in a few protozoan species.

40. Translation:
1. Initiation
2. Elongation
3. Termination

41. АUGGGGUUUАААCCC……………………….UGА
«cap»
methionine
Met-tRNA
5’ 3’
mRNA
Small subunit
Large subunit
UAC
codon
anticodon
Initiation

42. АUGGGGUUUАААCCC……………… ……………UGА5’ 3’
mRNA

43. АUGGGGUUUАААCCC………………….UGА5’ 3’
UAC
CCC
proline
Elongation

44. АUGGGGUUUАААCCC……………………….UGА5’ 3’
UAC
Centre of a ribosome
Р А

45. АUGGGGUUUАААCCC……………………….UGА5’ 3’
UACCCC
First peptide bond is
formed and first tRNA
leaves the ribosome

46. АUGGGGUUUАААCCC……………………….UGА5’ 3’
CCC
ААА
lysine
new amino acid comes
to A-site
Р А

47. АUGGGGUUUАААCCC……………………….UGА5’ 3’
CCC
New peptide bond is
formed and ribosome
moves along mRNA
ААА

48. АUGGGGUUUАААCCC……………………….UGА5’ 3’
The polypeptide chain
grows until one of stop-
codons comes to A-site of
ribosome
ААА
None tRNA is complementary
to stop codon and translation
is over
Termination of translation

49. One mRNA can be translated by many
ribosomes

50. Processing of a protein

51. 4. Post-translational modifications of
protein
• Secondary structure
• Tertiary structure
• Quaternary structure (for some proteins)
folding
Active
protein
+ co-factor

52. Folding

53. Hemoglobin

54. Quaternary structure of insulin

55. The end.
Thank you for coming.
Next lecture will be about
recombination, repair and
mutations of DNA