Transcript Protein Synthesis

Synthesis and Release
of Protein
Learning Outcomes
• State that proteins can be classified as
either fibrous or globular, describe their
structure and function and give examples
of each type.
Proteins
• Proteins contain the elements Carbon (C),
Hydrogen (H), Oxygen (O) and Nitrogen
(N).
• They also often contain sulphur (S).
• Proteins are build up from subunits called
amino acids.
Proteins
• Amino acids are joined together by
peptide bonds to form polypeptides.
• Each polypeptide chain will have a
particular sequence of amino acids.
• Once the polypeptide chains are formed
they can then be arranged in different
ways to form different types of proteins.
Proteins
• The sequence of amino acids determines
the structure and function of the protein.
• Proteins have many important roles within
an organism
–
–
–
–
–
Enzymes - amylase, pepsin, lipase
Structural - membranes, hair
Hormones – insulin
Defence – antibodies
Carriers – haemoglobin
Fibrous Protein
• Fibrous proteins are insoluble, and play a
structural or supportive role in the body,
and are also involved in movement (e.g. in
muscle).
• Collagen – strong and elastic found in
ligaments and skin.
• Actin and myosin – a contractile protein
found in muscle cells.
Keratin is a major component of hair,
nails and skin.
It has a rope-like structure.
Globular proteins
• Once the polypeptide chains have been
formed they are folded into a spherical
shape to form a globular protein.
• Globular proteins are found in the plasma
membrane (structural proteins).
• They can also be hormones , enzymes and
antibodies.
Hormones
• Chemical messengers
made of protein.
• Insulin is involved in
the regulation of
blood sugar levels.
• Insulin is regarded as
a globular protein.
Enzymes
Active site
Molecule
being digested
Conjugated Proteins
• Sometimes a globular protein can have a non
organic component as part of their structure.
These are known as conjugated proteins.
• Haemoglobin has iron in the structure and is
used to transport oxygen.
• Chlorophyll has magnesium in the structure and
is an essential component of photosynthesis.
Questions
1. What elements are present in protein?
2. What determines the structure and function of a
protein?
3. Give 2 examples of a fibrous protein and state
their functions.
4. Name 2 types of globular protein and state their
function.
5. What is a conjugated protein? Give an example.
Learning Outcome
• Describe the structure of a nucleotide and
recall the names of all the bases found in
them.
• Give an account of the structure of DNA,
describing the positioning of the
nucleotides, the base pairs established,
and the coiling of the molecule to form the
“double helix”.
DNA
• Chromosomes are thread-like structures
found inside the nucleus of a cell.
• They contain deoxyribonucleic acid (DNA).
• A molecule of DNA consists of 2 strands,
each made up of repeating units called
nucleotides.
Nucleotide
• A nucleotide consist of 3 parts
 ribose or deoxyribose sugar compound
 an organic base
 a phosphate group
DNA Bases
• There are 4 different DNA bases:
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–
–
–
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
• So there are 4 different nucleotides,
depending on what type of base they have.
Nucleotides
– This diagram shows how
several nucleotides are
linked together
– They are joined by strong
bonds between the
phosphate group of one
nucleotide and the
deoxyribose sugar of
another.
Chemical
bond
DNA Double Strand
• DNA is made up of
two parallel
strands.
• The two DNA
strands are joined
together by
hydrogen bonds
between their
bases.
DNA
The bonds
between the
bases become a
ladder-like
structure. The
bases are the
rungs and the
sugar-phosphates
are the uprights.
The ladder is
twisted to form a
double-helix.
Double Helix
DNA Bases
• Each base can only join with one other
type of base.
• Adenine (A) will only join with Thymine (T).
• Cytosine (C) will only join with Guanine (G).
Learning Outcomes
• Show an understanding of DNA
replication, identifying the stages
and substances required for the
process to take place.
Replication of DNA
• DNA is able to reproduce itself
exactly, this is called replication.
• There are a number of stages in DNA
replication.
• Stage 1: The original DNA molecule
becomes unwound.
Replication of DNA
• Stage 2: Weak hydrogen bonds between
the bases break and cause the 2 strands
of DNA to ‘unzip’ and expose their bases.
• Stage 3: Pairing of 2 bases allows a free
DNA nucleotide to find and line up with its
complementary nucleotide on the open
chain.
Replication of DNA
• Stage 4: Weak hydrogen bonds are forming
between complementary base pairs.
• Stage 5: A strong chemical bond is forming
between the sugar of one nucleotide and the
phosphate of the next one.
• The linking of nucleotides into a chain is
controlled by an enzyme called DNA
polymerase.
Replication of DNA
• Stage 6: The newly formed molecule of
DNA is about to wind up into a double
helix.
• The new DNA molecule is identical to the
original DNA molecule.
DNA Replication
DNA replication
• For DNA replication to occur the
nucleus must contain:
– DNA (to act as a template for the new
molecule)
– A supply of the 4 types of DNA
nucleotide
– The appropriate enzymes (DNA
polymerase)
– A supply of ATP to provide energy
Importance of
Replication
• DNA replication ensures that an exact
copy of the species genetic information is
passed from cell to cell and from
generation to generation.
• It ensures that daughter cells have
exactly the same genetic information as
the parent cells.
RNA
• The second type of nucleic acid is called
ribonucleic acid (RNA).
• The structure of RNA is similar to DNA,
but differs from it in 3 important ways.
Comparing DNA and RNA
DNA is found only in the nucleus of the cell
RNA can be found in the nucleus or the cytoplasm of the cell
DNA
• Double stranded
• Deoxyribose sugar
• Bases –
– Cytosine
– Guanine
– Adenine
– Thymine
RNA
• Single stranded
• Ribose sugar
• Bases –
– Cytosine
– Guanine
– Adenine
– Uracil
RNA
• There are 2 types of RNA:
– mRNA (messenger RNA)
– tRNA (transfer RNA)
• For protein synthesis to occur, DNA
has to be transcribed into mRNA.
RNA Polymerase
• Separates the DNA molecule by breaking the
H-bonds between the bases.
• Then moves along one of the DNA strands and
links RNA nucleotides together.
Transcription
• The genetic information carried on DNA
makes contact with structures responsible
for protein synthesis via a messenger.
• This go-between is called messenger RNA
(mRNA).
• mRNA is formed (transcribed) from one of
the DNA strands using free RNA
nucleotides present in the nucleus.
mRNA
• mRNA strand is made in the nucleus during
protein synthesis, but moves out into the
cytoplasm.
• Made up of codons (sequence of three bases).
Each codon, is specific for an amino acid.
• To allow the mRNA to be synthesised, a section of
the DNA molecule must unzip.
mRNA synthesis
• Once the DNA strand has partially unzipped ,
RNA nucleotides will enter the nucleus.
• They will line up beside the complementary bases
on the exposed section of DNA.
• The RNA nucleotides now link up between the
phosphate group and the sugars.
• This is called transcription ie a copy of the
genetic code is made in the nucleus and is then
taken into the cytoplasm and attaches to the
ribosome.
mRNA
Transcription
• The transfer of information in the nucleus from a
DNA molecule to an RNA molecule.
• Only 1 DNA strand serves as the template
1.RNA polymerase unwinds and unzips (hydrogen bonds
between bases break) part of a chromosome (ATP
supplies the energy needed)
2. Base pairing occurs between free RNA nucleotides and
DNA template
3. Strong chemical bond forms between the RNA
nucleotides.
4.mRNA separates from DNA and leaves nucleus.
Messenger RNA (mRNA)
start
codon
A U G G G C
U C C A U C
G G C G C
codon 1
codon 2
codon 3
codon 5
codon 6
protein methionine glycine
serine
glycine
alanine
mRNA
codon 4
isoleucine
A U A A
codon 7
Primary structure of a protein
aa1
aa2
aa3
peptide bonds
aa4
aa5
aa6
stop
codon
tRNA
• A second type of RNA is found in the cells
cytoplasm. This is called transfer RNA (tRNA).
• Picks up the appropriate amino acid floating in
the cytoplasm.
• Transports amino acids to the mRNA.
• Have anticodons that are complementary to
mRNA codons.
• Recognises the appropriate codons on the mRNA
and bonds to them with H-bonds.
tRNA
• Once an amino acid reaches the ribosome
the tRNA places it in position by matching
the tRNA anti-codon with its mRNA codon.
• The amino acids should now be arranged in
the correct sequence required to
synthesise the protein.
• The amino acids now join together using
peptides bonds.
Transfer RNA (tRNA)
Ribosomes
• Ribosomes are found free floating in the
cytoplasm and attached to ER.
• They are the site of translation of mRNA
into protein.
• Each ribosome contains enzymes needed
for protein synthesis. Large numbers of
ribosomes are found in growing cells which
need to produce large quantities of
protein.
Translation
• A ribosome becomes attached to one end of the
mRNA molecule about to be translated.
• Inside the ribosome, there are sites that tRNA
molecules can attach to, which allows the
anticodon to line up with the mRNA codon.
• As this happens along the molecule, it allows amino
acids to line up and become joined together by
peptide bonds.
Translation
• Synthesis of proteins in the cytoplasm
• Involves the following:
1. mRNA (codons)
2. tRNA (anticodons)
3. ribosomes
4. amino acids
Translation
End Product
• The end product of this part of protein
synthesis is a polypeptide.
– A sequence of amino acid bonded
together by peptide bonds.
aa2
aa1
aa3
aa4
aa5
aa199
aa200
Re-use of RNA
• Each tRNA molecule becomes attached to
another molecule of amino acid, ready to
repeat the process.
• The mRNA is often also reused to produce
further molecules of the same polypeptide.
• Protein synthesised in ribosomes is for use
in the cell. Protein synthesised in
ribosomes attached to ER is for export.
Link between DNA and
proteins
•
•
•
•
DNA is the template for mRNA
mRNA is used as a template for tRNA
One end of tRNA is attached to an amino acid
Therefore DNA is transcripted and translated
into protein
• A gene is a section of DNA (about 1000
nucleotides) that codes for one protein
 DNA sequence A- G- T
 mRNA codon U- C- A
 tRNA anticodon A- G- U
Definitions
Term
Definition
Transcription
Copying of the DNA code
onto the mRNA
Converting the information
given on the mRNA into a
sequence of amino acids
Sequence of bases
specifying one amino acid
Translation
triplet
Codon
Triplet of bases on mRNA
Anticodon
Triplet of bases on tRNA
Decoding the gene
The main stages of decoding a gene are first
Transcription and then translation.
transcription
translation
DNA
mRNA
Protein
Sequence of
sequence of
sequence of
Bases
bases
amino acids
Translation
aa1
aa2
2-tRNA
1-tRNA
anticodon
hydrogen
bonds
U A C
A U G
codon
G A U
C
U A C U U C G A
mRNA
peptide bond
aa3
aa1
aa2
3-tRNA
1-tRNA
anticodon
hydrogen
bonds
U A C
A U G
codon
2-tRNA
G A A
G A U
C U A C U U C G A
mRNA
aa1
peptide bond
aa3
aa2
1-tRNA
3-tRNA
U A C
(leaves)
2-tRNA
A U G
G A A
G A U
C U A C U U C G A
mRNA
Ribosomes move over one codon
aa1
peptide bonds
aa2
aa4
aa3
4-tRNA
2-tRNA
A U G
G A U
C U A
3-tRNA
G C U
G A A
C U U C G A A C U
mRNA
Golgi Apparatus
• Proteins synthesised on the rough ER are
secreted by the cell. These proteins are
passed on to the Golgi apparatus for
packaging and secretion.
• This happens when vesicles containing the
protein pinch off from the ER and fuse
with the Golgi apparatus.
Golgi Apparatus
• The Golgi apparatus then processes the
protein.
• Vesicles containing the finished protein
become pinched off from the Golgi
apparatus.
• The vesicle then moves towards the cell
membrane and fuses with it, discharging
its contents.
Questions
1.
What are the subunits of DNA called? Draw and
label one.
2.
What type of bond holds 2 strands of DNA
together?
3.
Where in a cell does mRNA synthesis occur?
4.
Where does the mRNA go once it has been
synthesised?
5.
What are the triplets of bases called on mRNA
and tRNA?
Questions
1.
What must happen to amino acids to form a
polypeptide chain?
2.
Which organelle in a cell transports protein
once it has been synthesised?
3.
Which organelle processes the protein for
secretion?
4.
Once processed, how does it leave the cell?