First stage
28/12/2015
Biology
Lec 6
‫ بثينة‬.‫د‬
Cellular Activity
Proteins
Protein synthesis:
The events involved in gene expression that results in a protein
synthesis.
1- DNA contains genetic information. The sequence of its bases
determines the sequence of amino acids in a polypeptides.
2- During transcription one strand of DNA serves as a template for
formation of mRNA. The bases in mRNA are complementary to those in
DNA, every three bases is a codon that codes for amino acid.
3- Messenger RNA (mRNA) is processed before it leaves the nucleus,
during which time the introns are removed.
4- Messenger RNA (mRNA) carries a sequence of codons to the
ribosomes, which are composed of rRNA and proteins.
5- Transfer RNA (tRNA) molecules, each of which is bonded to a
particular amino acid, have anticodons that pair complementarily to the
codons in mRNA.
6- During translation, tRNA molecules and their attached amino acids
arrive at ribosomes, and the linear sequence of codons of the mRNA
determines the order in which the amino acids become incorporated
into a protein.
7- They have release factors that recognize the signals and terminate
protein synthesis.
(1)
The release factors bind to a termination codon at the A site and
stimulate hydrolysis of the bond between the tRNA and the polypeptide
chain at the P site, resulting in release of the completed polypeptide
from the ribosome. The tRNA is then released, and the ribosomal
subunits and the mRNA template dissociate.
Stages of Protein Synthesis
Protein secretion:
Proteins traveled from the Golgi apparatus to the cell surface in
secretory vesicles, which then fused with the plasma membrane to
release defined contents outside of the cell.
The secretory pathway:
RER
Golgi apparatus
secretory vesicles
cell exterior.
Plasma membrane and lysosomal proteins also travel from the RER to
the Golgi apparatus and then to their final destinations.
(2)
Proteins as well as lipids and polysaccharides are transported from the
Golgi apparatus to their final destinations through the secretory
pathway. This involves, which bud from the trans Golgi network and
deliver their contents to the appropriate cellular locations.
Lipids
Lipids synthesis & secretion:
Lipids are important energy storage molecules and the major
constituent of cell membranes. They are synthesized from acetyl CoA,
which is formed from the breakdown of carbohydrates, in a series of
reactions that resemble the reverse of fatty acid oxidation.
The major product of fatty acid biosynthesis, which occurs in the cytosol
of eukaryotic cells, is the 16-carbon fatty acid palmitate.
The principal constituents of cell membranes (phospholipids,
spingomylin, and glycolipids) are then synthesized from fatty acids in the
endoplasmic reticulum and Golgi apparatus.
(3)
The ER is the major site at which membrane lipids are synthesized in
eukaryotic cells. Because they are extremely hydrophobic, lipids are
synthesized in association with already existing cellular membranes
rather than in the aqueous environment cytosol. They are then
transported from the ER to their ultimate destination either in vesicles
or by carrier proteins.
Adipose Tissue
Lipids storage:
Lipids may accumulates as non-membrane vacuoles, which appear as
large clear spaces in the cytoplasm. Large fat vacuoles are a special
feature of fat storage cells called adipocytes. Fat also accumulates in
certain cells such as hepatocytes in the liver in response to sublethal
metabolic damage. The most common cause in chronic high alcohol
ingestion.
(4)
Carbohydrates biosynthesis:
In addition to being obtained directly from food or generated by
photosynthesis, glucose can be synthesized from other organic
molecules. In animal cells, glucose synthesis (gluconeogensis) usually
starts with lactate (produce by an anaerobic glycolsis).
Amino acid ( produced by the breakdown of proteins) or glycerol
(produced by the breakdown of lipids).
Glucose in the body undergoes catabolism in all peripheral tissue,
particularly in brain, muscle and kidney to produce ATP. Excess glucose is
charged into glycogen by the process of glycogenesis ( anabolism) and
stored as glycogen in liver and muscle or converted to fatty acid and is
stored in adipose tissue as triglycerides. Eqinephrine and glycogen
hormones are secreted to stimulate the conversion of glycogen to
glucose when blood glucose level become low. This process is called
glucogenolysis ( catabolism).
Gluconeogensis involves the conversion of pyruvate to glucoseessentially the reverse of glucolysis.
In both plant & animal cells, glucose is stored in the form of
polysaccharides (starch and glycogen, respectively).
The synthesis of polysaccharides like that of all other macromolecules is
an energey-requiring reaction.
The linkage of two sugars by glycosidic bond can be written as a
dehydration reaction, in which H2O is removed.
(5)
Carbohydrates storing:
Glycogen, a polymer and storage product of glucose, forms as granules
in the cell cytoplasm and is only visible by electron microscopy.
Demands for energy are met by conversion of glycogen to glucose.
In a certain cells, the presence of large amounts of glycogen causes pale
staining or apparent vacuolation of cell cytoplasm. Glycogen can be
stained by the PAS method.
(6)
Nucleic Acids
The precursors of nucleic acids, the nucleotides, are composed of
phosphorylated five - carbon sugars joined to nucleic acid bases.
They are obtained from:
 Dietary source
 Or reused following nucleic acid breakdown.
The starting point for nucleotide synthesis is the phosphorylated sugar
ribose-5- phosphate.
Different pathways then lead to the synthesis of purine and pyrimidine
ribonucleotides, which are the immediate precursors for RNA synthesis.
Thesis ribonucleotides are converted to deoxyribonucleotides, which
serve as the monomeric building blocks of DNA.
Watson & Crick Model:
The Watson & Crick model shows that DNA is a double helix with sugarphosphate backbones on the outside and paired bases on the inside.
Chargaff”s rules said that A=T and G=C. The model shows that A is
hydrogen-bonded to T and G is hydrogen bonded to C.
This is so-called complementary base pairing means a purine is always
bonded to pyrimidine.
(7)
DNA Replication:
It has now been confirmed that DNA is replicated by means of
complementary base pairing. During replication, each old DNA strand of
the parent molecules serves as a template for a new strand in a
daughter molecule. DNA replication is termed semiconsevative
replication because each daughter helix contains an old strand and a
new strand.
Replication requires the following steps
1- Unwinding:
The old strands that make up the parent DNA molecule are unwound
and “unzipped” (the weak hydrogen bonds between the paired bases
are broken). There is a special enzyme
called helicase that unwinds the molecule.
2- Complementary base pairing:
New complementary nucleotides, always present in the nucleus, are
positioned by the process of complementary base pairing.
3- joining:
the complementary nucleotides join to form new strands. Each daughter
DNA molecule contains an old strand and a new strand.
(8)
(9)