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Introduction to protein synthesis
Some key aspect to learn and understand
• A Protein is a polymer which consists of monomers (repeating
building blocks), called amino acids.
• A protein consists of 50 or more amino acids.
• Every protein is synthesised in accordance with instructions
contained in DNA
• The new proteins will have structural and functional
roles in cells.
•
Other substances which are also examples of proteins are
amongst others enzymes and hormones.
• Life cannot exist without proteins, enzymes, hormones and
other proteins.
Introduction to protein synthesis
• Each cell’s DNA can be seen as a book of
protein-building instructions. The alphabet used in this
book is simple A, T, G and C.
• The DNA molecules for the formation of proteins occurs
in genes on chromosomes.
• Memorise the matching of these bases and between A
and U (RNA).
• It takes two steps, transcription and translation, to
carry out a gene’s protein-building instructions.
Overview of protein synthesis
1. Transcription (takes place in nucleus)
• DNA serves as template to assemble a single
strand of mRNA from the cell’s pool of free
nucleotides. Afterward, RNA moves into cytoplasm
2. Translation (cytoplasm and ribosome)
• mRNA & tRNA control the assembly of amino acids
into proteins
Summary:
DNA
RNA
Protein
transcription
translation
Process of transcription
• The enzyme polymerase controls the process.
• The DNA unwinds and unzips by breaking the
weak hydrogen bonds between the
complementary bases.
• One of the strands of the unwound DNA acts
as a template for the formation of the
complementary strand of mRNA by using the
free nucleotides in the cytoplasm.
• A nucleotide with base C on the template will
link the nucleotide with G on the mRNA.
Adenine on DNA template links nucleotide
with Uracil in mRNA.
Diagram to show transcription
DNA unwinds and unzips. Complementary mRNA strand (Blue strand)
forms.
DNA molecule illustrates two genes (each of them with their own
codons on two different mRNAs)
Transcription (Cont.)
• If the DNA base triplet sequence is GGT, the
mRNA codon (sequence of three bases) will be
CCA.
• The sugar-phosphate bonds form to link the mRNA
nucleotides together.
• This forms a single strand with the nucleotides
in the correct sequence.
• Once the mRNA has formed, it moves away
from the DNA template and leaves the nucleus
via the nucleus membrane pore.
• The mRNA carries the ‘message’ /codons to the
ribosome.
mRNA attached to a ribosome
mRNA
Forming and movement of mRNA
From the DNA
template a mRNA is
formed in the
nucleus
Single strand mRNA
moves through the
pores in the nucleus
membrane into the
cytoplasm of a cell
Process of translation
• Translation means to change something into
another form. The code is translated into a
protein.
• The mRNA strand becomes attached to a
ribosome with the codons exposed.
• Each tRNA molecule carries one particular
amino acid from the pool of amino acids in the
cytoplasm to the ribosome.
• Here, the tRNA’s anticodon matches up with
the codon on the mRNA so that the amino
acids are placed in the correct sequence.
Process of translation (cont.)
• The amino acids were added to the
growing chain of amino acids.
• Peptides bonds are form to link adjacent amino
acids in the correct sequence (according to the
codons on the mRNA).
• Proteins consist of 50 or more amino acids.
Process of translation (Cont.)
• Once a tRNA has placed its amino acid into the
correct sequence, the tRNA molecule is released
from the ribosome.
• The tRNA molecule will then pick up another amino
acid of the same type to continue the process until
the complete protein has been formed.
• The chain of amino acids elongates one amino
acid at a step until the last particular codon,
specifying ‘stop’ codon which indicate that the
chain of amino acids is completed.
• The protein is released from the ribosome.
Amino acid
Peptide bond
Growing part of protein
Incoming tRNA with a
particular amino acid
attached
Outgoing tRNA
without
an amino acid
Anticodon
mRNA
4 Codons
Ribosome
Diagram illustrating the translation process
Diagram to show transcription
and translation
Drawings to illustrate the complete
protein synthesis process
Write the
anticodons
for the four
codons on
the mRNA.
Transcription (1 & 2)
Translation (3 & 4)
Use these drawings to describe the whole process. Underline the key
words in your four paragraphs. Memorise the key words.
Complete the following table to
show the 3 bases in each case
You don’t have to memorise the combinations of triplets. Use the
matching bases of DNA and RNA to determine the triplets.
Base triplet
on DNA
Codon on
mRNA
Anticodon on
tRNA
1
TGC
ACG
UGC
2
ATA
UAU
AUA
3
GCA
CGU
GCA
4
TGT
ACA
UGU
Memorise the following:
Pairing bases of DNA: A with T or T with A and C with G or G with C
Pairing bases of RNA: A with U or U with A and C with G or G with C
Codons for some amino acids
mRNA codon
Name of
amino acid
Abreviationg
AUG
UCC
ACU
Methionien
Serien
Threonien
Met
Ser
Thr
GUU
GCU
Valien
Alanien
Val
Ala
‘Stop’-codons (interesting information)
• Codons UAA, UAG en UGA are ‘stop’
kodons.
• These codons don’t have tRNS
molecules.
• They terminate the formation of a
specific protein and the protein is
released from the ribosome.