Reading notes for A Level Biology

1. Protein synthesis
By
Wodrick P. Bukuku
Dip ED, BSc ED, MSc BC

2. Protein synthesis
 Protein synthesis is a process of forming
protein molecule in the cells
 The sequence of amino acids in a given
protein is determine triplet codes on the
DNA molecule
 In the process the amino acids are linearly
arranged into proteins through the
involvement of ribosomal RNA, transfer
RNA, messenger RNA, and various
enzymes.

3. Stages of protein synthesis
 There are four main stages that are
involved in protein synthesis.
a) Synthesis of amino acid.
b) Transcription (formation of mRNA
c) Amino acid activation
d) translation

4. Synthesis of amino acid
 In plants formation of amino acids
occurs in mitochondria and chloroplasts
in a series of the following stages
a) Absorption of nitrates from the soil
b) Reduction of nitrates into amino group
c) Combination of amino groups with
carbohydrate skeleton e.g.
ketogluterate from Kreb’s cycle

5. d) Transfer of amino groups from one
carbohydrate skeleton to another
through the process called
transamination.

6. Transcription
 Transcription is the process in which a
complementary mRNA copy is made
of the specific region of the DNA
molecule which code for a
polypeptide.
 During transcription the following
events occur;
i. A specific region of the DNA called
citron unwinds.

7. ii. The unwinding of the DNA results in
the breaking of hydrogen bonds
between base pairs in the DNA
double helix and is catalyzed by
helicase enzyme. This exposes the
bases along the strand.

8.  Each base along one strand attracts a
complementary RNA nucleotide where
guanine is paired with cytosine and
uracil is paired with adenine.
 The enzyme called RNA polymerase
moves along the DNA adding one
complementary RNA nucleotide at a
time to newly unwound portion of
DNA.

9.  The region of base pairing between
the DNA and RNA is only around 12
base pairs at one time as the DNA
reform behind the RNA polymerase.
 The DNA thus acts as a template
against which mRNA is manufactured.

10.  Many mRNA molecules are made before
the RNA polymerase leaves the DNA.
 The mRNA leaves from the nucleus to
cytoplasm through the nuclear pores.
The mRNA is then attracted to the
ribosome.
 Along the mRNA are the triplet codes
which have been determined by the DNA
and each triplet is called codon.

11. Amino acid activation
 Activation is the process by which
amino acids combine with tRNA in the
presence of ATP.
 Each type of tRNA binds with a
specific amino acid.
 There are at least 20 types of tRNA
and each type of tRNA differs in the
composition of a triplet bases called
anticodon.

12.  All tRNA molecules have a common
free end which terminates in the
triplet.
 The tRNA molecules with attached
amino acids now move towards the
ribosomes.

13. Translation
 Translation is the process in which a
specific sequence of amino acids is
formed in accordance with the codons
on the mRNA.
 A group of ribosomes becomes
attached to the mRNA to form a
polysome.

14.  The complementary anticodons of a
tRNA-amino acid complex are
attached to the first codon (start
codon) on the mRNA.
 The ribosomes acts as a framework
which holds the mRNA and tRNA-
amino acid complex together until the
two have combined to form a peptide
between each other.

15.  Once they have combined the
ribosomes moves along the mRNA to
hold the next codon-anticodon
complex until the third amino acid is
linked with the second.
 Once each amino acid is linked, the
tRNA which carries it to the mRNA is
released into the cytoplasm, hence
free again to combine with its specific
amino acid.

16.  The ribosome continues to move
along the mRNA until it reaches one
of the non-sense codes at the point
the polypeptide is cast off.
 The polypeptides so formed are
assembled into protein.

18. The genetic codes
 Genetic codes are the biochemical
instructions that translate the genetic
information present as a linear
sequence of nucleotide triplets in
mRNA into the correct linear
sequence of amino acids for the
synthesis of a particular peptide chain
or protein.

19. Features of genetic codes
a) The genetic are triplets of bases
 There are 64 codons. Only 61 triplets
or codons code for amino acids. Three
of the 64 codons terminate protein
synthesis and release the polypeptide
from the translation machinery.
These triplets are called stop codons
or nonsense codons or terminator
codons. These are UUA, UAG, and
UGA.

20. b) The code is a degenerative code. This
is because several codons code for
the same amino acid.
c) It is universal i.e. the triplets codes
for the same amino acids in all
organisms
d) It is non-overlaping i.e. there is no
punctuation between each codon

21. e) The reading frame is set at the
beginning of the gene.
 Frame shift mutations can be caused
by the ADDITION or DELETION of
only one or two bases.

22. f) The reading of mRNA is always in the
same direction 5' to 3' (the same way
as transcription and replication).

23. THANK YOU FOR LISTENING
AND
GOD BLESS YOU