‫المرحلة األولى‬
‫كلية الطب‬
‫المحاضرة الخامسة‬
The mitochondria:-
called the “powerhouses” of the cell.
- present in all areas of each cell’scytoplasm, but the total number per cell varies from less
than a hundred up to several thousand, depending on the amount of energy required by the
cell.
-The basic structure , is composed of two lipid bilayer–protein membranes: an outer
membrane and an inner membrane. Many infoldings of the inner membrane form shelves
onto which oxidative enzymes are attached.
- The inner cavity is filled with a matrix that contains large quantities of dissolved enzymes
that are necessary for extracting energy from nutrients.
-These enzymes operate in association with the oxidative enzymes on
the shelves to cause oxidation of the nutrients entering the cell, forming carbon dioxide and
water and at the same time releasing energy. in the process called the citric acid cycle.
-The liberated energy is used to synthesize a “high-energy” substance called adenosine
triphosphate (ATP). , a highly reactive chemical that can diffuse through the cell to release it
energy whenever it is needed for performing cellular functions.
-Mitochondria are self-replicative, that one mitochondrion can form a second one, a third
one, and so on, whenever there is a need in the cell for increased amounts of ATP.
-The mitochondria contain DNA similar to that found in the cell nucleus.
Cytoskeleton
-The fibrillar proteins are precursor protein molecules synthesized by ribosomes in the
cytoplasm.of the cell and organized into filaments or tubules.
-filaments ,a large numbers of actin filaments form an elastic support for the cell membrane.
Also, in muscle cells, actin and myosin filaments are organized into a special contractile
machine for muscle contraction.
- microtubules is act as a cytoskeleton, providing rigid physical structures for certain parts of
cells. like the flagellum of a sperm. and both the centrioles and the mitotic spindle of the
mitosing cell.
The nucleus
-act as the control center of the cell, contains large quantities of DNA, called the genes.
-The genes determine the characteristics of the cell’s proteins, including the structural
proteins and the intracellular enzymes that control cytoplasmic and nuclear activities.
-The genes control reproduction of the cell itself. In which the genes first reproduce to give
two identical sets of genes; then the cell splits by a special process called mitosis to form two
daughter cells, each of which receives one of the two sets of DNA genes.
-The nuclear membrane, is bilayer membranes. The outer membrane is continuous with the
endoplasmic reticulum of the cell cytoplasm, and the space between the two nuclear
membranes is also continuous with the space inside the endoplasmic reticulum.
-The nuclear membrane is penetrated by several thousand nuclear pores. each pore is about 9
nanometers in diameter.
-During mitosis, the chromatin material organizes in the form of highly structured
chromosomes.
Nucleoli
-the nuclei of most cells contain one or more highly staining structures called nucleoli.
-The nucleolus ,does not have a limiting membrane, it is an accumulation of large amounts
of RNA and proteins of the types found in ribosomes.
-Ribosome RNA is stored in the nucleolus and transported through the nuclear membrane
pores to the cytoplasm , where it is used to form mature ribosome’s that play an important
role in the formation of proteins.
-genes, control heredity from parents to children, the same genes also control day-today
function of all the body’s cells.
-Each gene, which is a nucleic acid called deoxyribonucleic acid (DNA), which control the
formation of another nucleic acid, ribonucleic acid (RNA)
- this RNA then spreads throughout the cell to control the formation of a specific protein.
Gene (DNA)
|
RNA formation
|
Protein formation
|
Cell structure
Cell enzymes
|
|
Cell function
- there
are more than 100,000 different genes in each cell, it is possible to form a very large
number of different cellular proteins.
-Some of the cellular proteins are structural proteins ,others are enzymes that catalyze, and
promote all the oxidative reactions that supply energy to the cell, and they promote synthesis
of all the cell chemicals, such as lipids, glycogen, and adenosine triphosphate (ATP).
-Each gene is a double – strands helical molecule of DNA composed of multiple units of
sugar deoxyribose , phosphoric acid and four nitrogenous bases including two purines ,
)adenine and guanine) and two pyrimidines( thymine and cytosine).
The helical, double-stranded structure of the gene. The outside
strands are composed of phosphoric acid and the sugar deoxyribose.
The internal molecules connecting the two strands of the
helix are purine and pyrimidine bases; these determine the “code” of the gene
-Nucleotides, combine one molecule of phosphoric acid, one molecule of deoxyribose, and
one of the four bases to form an acidic nucleotide, Four separate nucleotides form
DNA.
-The phosphoric acid and deoxyribose form the two helical strands that are the backbone of
the DNA molecule, and the nitrogenous bases lie between the two strands and connect
them by weak cross-linkage hydrogen bonds.
-Three successive bases in the DNA strand are each called a code word ( adenine , thymine ,
guanine );( cytosine , guanine , thymine ). in RNA the same bases as in DNA, except that
uracil in RNA replaces thymine in DNA
-These code words control the sequence of amino acids in the protein to be formed in the
cytoplasm.
-The sequence of successive code words on the DNA strand is known as the genetic code.