4. 1. DNA
• Double helix with 2 complementary strands
• 4 bases: A, C, G, T
• Has two strands that complementary to each
other and joined by hydrogen bond
• Contains gene that are hereditary units
• Has one strand that can act as the template to
synthesise a molecule of mRNA
7. 2. Messenger RNA (mRNA)
Function: mRNA is transcribed from DNA to carry the gene message out
to the ribosomes for translation into an amino acid sequence
• It is made in the nucleus using one strand of DNA as a template before
moves to cytosol.
• It is a single stranded molecule consisting of mRNA nucleotides
• Sets of three bases of DNA are termed codon.
• Similar to DNA except 2 differences:
1. The deoxyribose sugar is replaced by a ribose
sugar
2. Thymine (T) is replaced by Uracil (U). So the
bases are A, C, G, U
8.
9. mRNA and Codons
When mRNA is transcribed from a molecule of DNA:
Adenine(DNA) bonds to Uracil (mRNA) : A-U
Thymine(DNA) bonds to Adenine (mRNA) : T-A
Guanine(DNA) bonds to Cytosine (mRNA) : G-C
Cytosine(DNA) bonds to Guanine (mRNA) : C-G
10.
11. DNA VS RNA
RNA
• Ribose sugar
• Bases: A, U, C, G
• Single stranded
• Not helical
DNA
• Deoxyribose sugar
• Bases: A, T, C, G
• Double stranded
• Helical
12. 3. Transfer RNA (tRNA)
• Has a 3D clover leaf shape
• Function: place amino into its correct sequence
in the polypeptide being synthesized, as specified
by the sequence of bases on the mRNA.
• Each type of tRNA will carry only one specific
amino acid molecule
• Each tRNA has two binding site:
– Where amino acid binds (acceptor stem)
– Anticodon consisting 3 bases that are complimentary
to the mRNA codon.
13.
14.
15. 4. Ribosomal RNA (rRNA)
• Function of ribosomes:
– Sites of protein synthesis
– move along the mRNA, translating the mRNA code into a
sequence of amino acids in a polypeptide molecule.
• Combined with proteins to make up ribosomes in the
endoplasmic reticulum or the cytoplasm
• Ribosomes are made of ribosomal RNA (rRNA)
16.
17. 5. Amino Acids (aa)
• Monomers or building blocks of polypeptides
and protein molecules
• Cells make protein molecules from
combinations of 20 different kinds of amino
acids
• Proteins differ in their number type and
sequence of these amino acids.
18.
19. GENETIC CODE
• Three bases are called codon
• Protein molecules can consist of one
polypeptide chain folded into precise shape or
more than one.
• Triplet bases are the smallest units necessary
to code for all 20 amino acids (found in mRNA)
20.
21. Amino Acid Abbreviations
The 20 amino acids can be
abbreviated by a single
letter or three letters a
shown in the table
23. Pathway of Protein Synthesis
DNA
RNA
Amino acids
Polypeptides
PROTEIN
Transcription
Translation
24.
25. How to bring DNA to ribosomes?
DNA stored
in nucleus
Ribosomes
(protein
factories)
in the
cytoplasm)
26. Protein Synthesis
• In order to get information from the nucleus
to the ribosomes, cell makes copy of the DNA
instruction. This copy called mRNA.
• But DNA is too big. It can’t go through
nucleus. RNA (in the other hand) is half the
size of DNA. Thus, TRANSCRIPTION is needed
to copy DNA to RNA so that the information in
DNA can be delivered to ribosome.
27. Two Steps of Protein Synthesis
Step 1: Transcription
Step 2: Translation - initiation
- elongation
- termination
28. Key Points for Transcription
• It occurs in the nucleus
• The process where an mRNA molecule is formed
that has a sequence of bases complementary to a
portion of one DNA strand
• Because DNA is double stranded, only one strand
is used for the coding of protein.
• This strand is called the template strand
(Antisense)
• The other strand is called the complementary
strand (sense)
29. The Process of Transcription
1. A segment of the double stranded DNA helix unwinds and unzips
to separate the two strands. This is done by an enzyme called RNA
Polymerase
2. Using the template strand of DNA, RNA polymerase will bring RNA
nucleotides and synthesize an mRNA strand complementary to
the template DNA.
3. RNA polymerase joins the RNA nucleotides together in the 5’3’
direction.
4. That means, RNA polymerase can only add RNA nucleotides at
the 3’end of an mRNA
5. The newly synthesized mRNA strand(RNA transcript) will undergo
some processing steps before it leaves the nucleus and goes into
the cytoplasm for the next step of protein synthesis
32. mRNA Splicing
• The newly synthesized mRNA strand is actually divided into
segments of introns and exons
• Exons (coding DNA) – DNA in a gene that is transcribed into mRNA
and translated into polypeptide / protein.
• Introns (Non-coding DNA) - DNA in a gene that is only transcribed
into an RNA molecule. It Does not code for protein.
• Before the newly synthesized mRNA can leave the nucleus, it will
undergo a processing step called mRNA splicing
• During mRNA splicing, the introns are removed and the exons are
joined together. This is done by a protein complex called
spliceosome
• Only processed mRNA will leave the nucleus to complete protein
synthesis
33.
34. Key Points for Translation
• Occurs in ribosomes at the cytoplasm or the
endoplasmic reticulum. (where ribosomes are found)
• The process where the sequence of codons on the
mRNA at a ribosome directs the formation of a
sequence of amino acids or a polypeptide
• Molecules that are involved: mRNA, ribosomes, tRNAs
• Can be divided into 3 steps:
1. Initiation
2. Elongation
3. Termination
35.
36. 1. Initiation
• Starts when the mRNA strand associates with
a ribosome
• Translation is initiated by the “start” codon,
AUG
• The initiator tRNA with the anti codon UAC
will bring the amino acid methionine (met)
which corresponds to the “start” codon (AUG)
38. 2. Elongation
• Once the first amino acid, methionine, is brought to
the ribosome, the ribosome will start to “read” the
mRNA strand
• Ribosomes “read” the mRNA strand one codon at a
time(3 bases at a time)
• While “reading” the mRNA strand, the ribosome will
direct the correct tRNA to the mRNA so that the
correct amino acid will be synthesized
• So elongation can be said as a process which a long
amino acid chain is produced from mRNA
40. 3. Termination
• Occurs when the ribosome reaches one of the stop
codons (UAA, UAG or UGA)
• When this happens, the ribosome will stop reading the
mRNA and stop bringing in tRNAs with amino acids
• The mRNA will then detach from the ribosome
• The newly synthesized amino acid chain or polypeptide
chain is also released for further modifications or
processing
