RNA, Ribosome, Cell Cycle

1. Different Types of RNA, Ribosomes
&
Cell Cycle
Dr. Ifat Ara Begum
Associate Professor
Department of Biochemistry
Dhaka Medical College, Dhaka

2. RNA

3.  RNA is one of the three major biological macromolecules
that are essential for all known forms of life
(Other two macromolecules are DNA and proteins)

4.  Polymer of ribonucleotides
 Ribonucleotide
=
Ribose sugar + phosphate +
nitrogen base
 RNA with role as enzymes to
speed chemical reactions:
Ribozymes

5. Structure of RNA

7.  RNA is typically single stranded (a)
 Also can fold upon itself, with the
folds stabilized by short areas of
complementary base pairing within
the molecule, forming a three-
dimensional structure (b)

8. Different types of RNA

10. 1. Messenger RNA
(mRNA)

11.  A single-
stranded molecule of RNA that
corresponds to the genetic
sequence of a gene and is read by
a ribosome in the process
of synthesizing a protein
 Represent 2-5% of cellular RNA
 Are found in nucleus & cytoplasm
 Monocistronic (one mRNA contains
one gene
 Half life: Short (hours to days)
Are produced by post
transcriptional modification of
primary transcript of a gene called
hnRNA
 Most heterogeneous in size &
base sequence: It depends on size
of gene

12.  It contains only the coding information (exons) of the coding region
of a gene flanked by 5’ UTR (5’ leader sequence) & 3’ UTR (3’ trailer
sequence)

13.  Function:
mRNA conveys genetic information from DNA and then acts
as a template to faithfully translate genetic information for
protein synthesis.

14. 2. Transfer RNA
(tRNA)

15.  Transfer RNA is an adaptor molecule that serves as the physical link
between the mRNA and the amino acid sequence of proteins.
It does this by carrying an amino acid to ribosome as directed by a codon
in a mRNA.
As such, tRNAs are a necessary component of translation

16.  Represent 10-20% of cellular RNA
 Smallest RNA found in cytoplasm
 Sixty different types exist
 Shows a cover leaf structure by
internal base pairing
 Contains highest % of unusual
(modified) bases

17.  Attaches with specific amino acid at its
3’ end
 Carry anticodon (triplet base) at its
anticodon loop , which is complimentary
to the specific template codon of mRNA
to which it specifically attaches
D arm is a 4- to 6- bp stem ending in a
loop that often contains dihydrouridine
T arm is a 4- to 5- bp stem
contains pseudouridine (Ψ), a
modified uridine

18.  Function:
tRNA acts as an adaptor molecule to recognize a definite
codon on one hand & a specific AA on other hand. Thus
carries amino acid to the ribosome for protein synthesis.

19. Anticodon: The three-base
sequence in tRNA
complementary to a codon
on mRNA.

20. Codon Anticodon
Found on the coding
strand of double-stranded
DNA and in the (single-
stranded) mRNA.
It is read 5' to 3' and is
part of the 'genetic code'
Found on the tRNA and is
the part that base-pairs
with the codon (on the
mRNA) in order to bring
the appropriate amino
acid to the ribosome to be
added to the growing
peptide chain.

21. 3. Ribosomal RNA
(rRNA)

22.  A type of non-coding RNA which is the primary component
of ribosomes, essential to all cells.
 rRNA associates with protein to form ribosome where
rRNA stands for two third of its bulk
 Represent 70-80% of cellular RNA
 Found in ribosome & nucleolus

23.  The ribosomal RNAs form two subunits
of ribosome, the large subunit (LSU) and
small subunit (SSU)
During translation, mRNA is sandwiched
between the small and large subunits
 The LSU rRNA acts as a ribozyme,
catalyzing peptide bond formation

24.  Depending on their sedimentation
velocity coefficient measured in
Svedberg (S) unit, there are 4
types of rRNA
 28S rRNA
 18S rRNA
 5.8S rRNA
 5S rRNA

25.  Function:
rRNA acts as a platform where mRNA and tRNA interact for
protein synthesis.

26. Other RNAs

27. Small nuclear RNA (snRNA)
 <1% of cellular RNA
 30 different types of snRNA
exist
 Function :
 Facilitates post transcriptional
modification of RNA
 Helps in gene regulation
Small interfering RNA (siRNA)
Control gene expression.
Micro RNA (miRNA)
Control gene expression.

28. Summary on three main
types of RNA

29. Point mRNA tRNA rRNA
Content 2-5% 10-20% 70-80%
Site Nucleus, Cytoplasm Cytoplasm Ribosome,
Nucleolus
Size Heterogenous Homogenous Heterogeneous

30. Point mRNA tRNA rRNA
Binds with 40S ribosome Amino Acid tRNA & mRNA
Function Contains genetic
code & acts as
template for
protein synthesis
Contains anticodon &
carries AA to
ribosome for protein
synthesis
Acts as platform
for interaction
between mRNA
& tRNA in protein
synthesis

31. Ribosome

32.  Cytoplasmic nucleoproteins (rRNA
+ Protein)
 Act as platform where mRNA &
tRNA interact for protein synthesis
using mRNA as a template
 In cytoplasm, ribosomes remain
quite stable & capable to carry out
many translation cycles in
cytoplasm

33.  Has 2 subunits (60S &
40S) which join together
to make the total
ribosome (80S
ribosome)

34. a) 60S ribosome :
 Large subunit
 Protein+28S rRNA+5.8S
rRNA+5S rRNA
b) 40S ribosome :
 Small subunit
 Protein + 18S rRNA

35. Binding sites for tRNA:
 A site / Aminoacyl site:
Aminoacyl tRNA binds here
 P site / Peptidyl site: Peptidyl
tRNA binds here
 E site / Exit site: Empty tRNA
exits through it

36. Difference between DNA & RNA

37. Characteristics DNA RNA
Structure Polymer of deoxy
ribonucleotide
Polymer of ribonucleotide
Bases & Sugar Adenine, guanine, thymine
and cytosine.
Deoxyribose
Adenine, guanine, uracil and
cytosine.
Ribose
Strands Generally double stranded;
single stranded DNA is found
in some viruses.
Generally single stranded;
double stranded RNA is found in
some viruses.
Molecular size Large Small
Location Nucleus (>99%), mitochondria
(<1%)
Cytoplasm (90%), nucleus (10%)

38. Characteristics DNA RNA
Number of bases Sum of purine bases is equal
to sum of pyrimidine bases
Not so
A / T and G / C
ratio
1.0 Not so
Hydrolysis by
alkali
No Yes
UV damage Susceptible to UV damage. Relatively resistant to UV
damage.
Function Chemical basis of heredity.
Store, replicate & transmit
genetic information
Translate genetic information into
protein synthesis.
Some RNA act as catalyst . e.g.
Ribozyme

39. Cell Cycle

40.  Cyclical sequence of events
involving
replication of cellular
components, dividing in half,
expanding & doing the same
again
in response to extracellular
growth factors
with a view to produce two
identical daughter cells.
 It is controlled by alternate
activation and deactivation of the
key enzyme, CDK (cyclin dependent
protein kinase) & its coenzyme
cyclin
 To ensure the proper division of
the cell, there are control
mechanisms known as cell cycle
checkpoints

41. Typically, a cell cycle consists of
four distinct phases
1. G1 phase (Pre synthetic phase)
2. S phase (DNA synthetic phase)
3. G2 phase (Premitotic phase)
4. M phase (mitotic phase)

42.  G1 phase, S phase &
G2 phase are
collectively known
as interphase
 M phase is composed of:
a) Karyokinesis: Division of
cell's chromosomes
b) Cytokinesis: Division of
cell's cytoplasm
ultimately forming two
daughter cells.

43.  Activation of each phase is
dependent on the proper
progression and completion
of the previous one.
 Cells that have temporarily
or reversibly stopped
dividing are said to have
entered a state of
quiescence called G0 phase

44. Interphase:
 It is the period between cell
divisions which is extended
from G1 phase to G2 phase
through S phase
 Cells spend most of their
time here to carry out
normal metabolic activities
 Here, chromosomes are
biologically active
showing
 replication,
 transcription,
 Translation
 repair of DNA damage &
error of replication

45. State Phase Abbrev. Description
Resting Gap 0 G0
(years to
indefinite time)
 A phase where the cell has left the cell
cycle and has stopped dividing.
 e.g. neuron
Inter-phase Gap 1 /
Pre-
synthetic
phase
G1
(Most variable in
duration.
Hours to years)
 Preparatory phase for replication
 Cells increase in size. Brief cellular growth
occurs featured by transcription, translation
& repair of DNA damage. Full complement
of proteins for daughter cells are produced
 Absent in rapidly dividing cells
 Cells are responsive to both +ve & -ve
growth factors to allow controlled cell
division
 The G1 / 1st checkpoint control mechanism
ensures that everything is ready
for DNA synthesis.

46. State Phase Abbrev. Description
Interphase
Synthetic
phase
S
(7 hours)
 Replication of chromosomal DNA &
histone occurs
Gap 2 /
Pre-
mitotic
phase
G2
(Up to 5
hours)
 Preparatory phase for mitosis
 The cell will continue to grow with further
protein and organelle synthesis
 Repair of any error of replication occurs
 The G2 / 2nd checkpoint control mechanism
ensures that everything is ready to enter the
M (mitotic) phase and divide.

47. State Phase Abbrev. Description
Cell division Mitosis
M
(1 - 2 hours)
Cell growth stops
 Mitosis occurs
 Cellular energy is focused on the orderly
division into two daughter cells
Chromosomes are inactive here
It is absent in neuron
 Metaphase Checkpoint ensures that the
cell is ready to complete cell division.