Human Physiology Cell structure and genetic control

1. Cell Structure and Genetic Control
Human
physiology

2. Cell
• Basic unit of structure and function of the body.
▫ Highly organized molecular factory.
• Great diversity of function.
▫ Organ physiology derived from complex functions of
the cell.
• 3 principal parts:
▫ Plasma membrane.
▫ Cytoplasm and organelles.
▫ Nucleus.

3. Plasma Membrane
• Is selectively permeable.
• Composition:
▫ Double layer of phospholipids due to hydrophobic/hydrophilic
parts.
 Restrict passage of H20 and H20 soluble ions.
▫ Proteins span or partially span the membrane.
 Provide structural support, transport molecules, serve as receptors.
▫ Negatively charged carbohydrates attach to the outer
surface.
 Involved with regulatory molecules.

4. Plasma Membrane (continued)

5. Cytoplasm, Organelles, Nucleoli
• Cytoplasm:
▫ Aqueous content of the cell.
• Organelles:
▫ Sub-cellular structures within the cytoplasm.
• Nucleus:
▫ Is a large spheroid body.
▫ Largest of the organelles.
▫ Contains the genetic material (DNA).
▫ Nucleoli:
 Centers for production of ribosomes.

6. Cytoplasm, Organelles, Nucleoli (continued)

7. Bulk Transport
• Phagocytosis:
▫ Phagocytic cells use pseudopods to surround and engulf particles.
▫ Pseudopods join, fuse, and surround ingested particle (food
vacuole).
 Lysosomes digest food vacuole.
▫ Protects from invading organisms.
▫ Removes debris.
• Endocytosis:
▫ Pinocytosis:
 Nonspecific process.
 Plasma membrane invaginates, fuses, vesicle containing ECF pinches off,
and vesicle enters cell.

8. Bulk Transport (continued)
• Receptor-mediated endocytosis:
▫ Interaction of molecules in ECF with specific membrane receptor
proteins.
▫ Membrane invaginates, fuses, pinches off and forms vesicle.
▫ Vesicle enters cell.
• Exocytosis:
▫ Process by which cellular products are secreted into extracellular
environment.
▫ Proteins and other molecules to be secreted are packaged in vesicles
by Golgi complex.
▫ Vesicles fuse with plasma membrane and release contents into
extracellular environment.

9. Cilia, Flagella, Microvilli
• Cilia:
▫ Tiny hair-like structures that project from the
surface of the cell.
 Stroke in unison.
 Respiratory tract, uterine tube.
• Flagella:
▫ Simple whip-like structure that propels sperm
through its environment.
• Microvilli:
▫ Numerous folds (finger-like projections) increase
surface area.
 Aid absorption.

10. Cytoplasm and Cytoskeleton
• Cytoplasm:
▫ Jelly-like matrix within
the cell.
▫ Includes organelles and
cytosol.
▫ Highly organized
structure with
microtubules and
microfilaments that
function as cytoskeleton.
• Cytoskeleton:
▫ Actin and myosin
(microfilaments).
▫ Spindle apparatus
(microtubules).

11. Lysosomes
▫ Primary:
 Contain only digestive enzymes.
▫ Secondary:
 Primary lysosome fuses with food vacuole or organelle.
 Contain partially digested remnants of other organelles and organic
material.
▫ Residual body:
 Contain undigested wastes.
▫ Autophagy:
 Process that destroys worn-out organelles, so that they can be
continuously replaced.
▫ Apoptosis (programmed cell death):
 Lysosomes release digestive enzymes into the cell.

12. Peroxisomes
• Membrane-enclosed organelles.
▫ Contain specific enzymes that promote
oxidative reactions.
▫ Oxidize molecules and form H202.
• Catalase: converts H202 H20 + 02.
• Oxidation of toxic molecules by peroxisomes
is an important function of liver and kidney
cells.

13. Mitochondria
• Sites for energy
production of all cells;
but mature RBCs.
• Contain own DNA, can
reproduce themselves.
• Structure:
▫ Outer membrane: smooth.
▫ Inner membrane: cristae.
▫ Cristae and matrix
compartmentalize
mitochondrion space.
 Have different roles in
energy generation.

14. Ribosomes
• Protein factories:
▫ Proteins produced according to genetic information
contained in mRNA.
▫ Located in cytoplasm and on the surface of
endoplasmic reticulum.
• rRNA molecules serve as enzymes (ribozymes)
required for protein synthesis.
▫ Contains 2 subunits composed of rRNA and
proteins.

15. Endoplasmic Reticulum (ER)
• Granular (rough) ER:
▫ Bears ribosomes on
surface, in cells active in
protein synthesis.
 Proteins enter cisternae
are modified for secretion.
• Agranular (smooth) ER:
▫ Provides site for enzyme
reactions in steroid
hormone production and
inactivation.
▫ Storage of Ca2+
in striated
muscle cells.

16. Golgi Complex
• Stacks of hollow, flattened
sacks with cisternae.
▫ One side of sack faces site for
entry of vesicles from ER that
contain cellular products.
▫ Other site faces towards
plasma membrane and
releases vesicles of chemically
modified products.
• Modifies proteins, separates
according to destination, and
packages into vesicles.

17. Cell Nucleus
Most cells have single nucleus.
Enclosed by inner and outer membrane
(nuclear envelope).
◦ Outer membrane is continuous with ER.
Nuclear pore complexes fuse inner and outer
membranes together.
◦ Selective active transport of proteins and RNA.
 Regulation of gene expression.
 Transport of mRNA out of nucleus to ribosomes.
Nucleoli:
◦ DNA contains the genes that code for the production
of mRNA.

18. Chromatin
• DNA within nucleus combines with protein (histones) to
form chromatin.
▫ Thread-like material that makes up the chromosomes.
▫ Histone proteins are positively charged and form spools around
which the negatively charged DNA strands wrap.
• Euchromatin:
▫ Active in genetic transcription.
• Heterochromatin:
▫ Contains genes that are permanently inactivated.

19. Chromatin (continued)

20. RNA Synthesis
• One gene codes for one polypeptide chain.
▫ Each gene is several thousand nucleotide pairs long
(DNA).
• Each gene contains the code for the
production of a particular type of mRNA.
▫ For the genetic code to be translated into synthesis
of a particular protein, the DNA code is copied onto
a strand of RNA (genetic transcription).

21. Genetic Transcription
• RNA-polymerase breaks weak
hydrogen bonds between paired
bases of DNA.
▫ Regulatory molecules act as
transcription factors by binding to
promoter region of gene, activating
the gene.
• Double stranded DNA separates at
region to be translated.
▫ One freed strand of DNA serves as
guide.
 Freed bases pair with complementary
RNA nucleotide bases.
• RNA detaches.

22. Types of RNA
4 types of RNA produced within nucleus by
transcription.
◦ Precursor mRNA pre-mRNA):
 Altered in nucleus to form mRNA.
◦ Messenger RNA (mRNA):
 Contains the code for synthesis of specific
proteins.
◦ Transfer RNA (tRNA):
 Decodes genetic message contained in mRNA.
◦ Ribosomal RNA (rRNA):
 Forms part of the ribosome structure.

23. Pre-mRNA
Contains excess bases
within the
pre-mRNA.
Introns:
◦ Regions of non-coding
DNA within a gene.
Exons:
◦ Coding regions.
Introns are removed
and the ends of exons
spliced by snRNPs to
produce mRNA.

24. Protein Synthesis
• Each mRNA passes through ribosomes forming
a polyribosome.
• Association of mRNA with ribosomes is needed
for genetic translation.
• Translation:
▫ Production of specific protein according to code
contained in mRNA base sequence.

25. Protein Synthesis (continued)

26. Protein Synthesis (continued)
• Each mRNA contains hundreds of
nucleotides arranged in sequence
determined by the complementary base
pairing with DNA.
• Codon:
▫ Each 3 bases (triplet) is a code word for a
specific amino acid.

27. Protein Synthesis (continued)

28. Transfer RNA
• Translation of the
codons accomplished
by tRNA and enzymes.
▫ tRNA bends on itself,
making an anticodon (3
nucleotides that are
complementary to codon
of mRNA).
• Synthetase enzymes
join specific amino
acids to the ends of
tRNA within a given
codon.

29. Formation of a Polypeptide
• Anticodons of tRNA binds to mRNA codons.
• Each tRNA carries a specific amino acid.
▫ tRNA bring amino acids close together.
▫ Amino acid detaches from tRNA.
 Enzymatically this amino acid is transferred to the amino
acid on the next tRNA.
▫ Polypeptide chain grows.
• Interactions between amino acids cause chain
to twist and fold forming secondary and
tertiary structure.

30. Translation of mRNA

31. Functions of ER and Golgi Complex
• Proteins to be secreted by the cell are
synthesized by mRNA-ribosome complexes
located on granular ER.
▫ Proteins enter the cisternae, and are modified.
▫ Leader sequence of amino acids is attracted to
membranes of ER.
 Once proteins are in cisternae, the leader sequence is
removed.
 Enzymatic removal of regions in protein, alter
structure.

32. Functions of ER and Golgi Complex
(continued)
▫ Secretory proteins are transported to Golgi complex.
 Further modified, packaged in vesicles, and secreted.

33. DNA Replication
• DNA is the only molecule in the body capable of
replication.
• DNA helicases break weak hydrogen bonds to produce 2
free strands of DNA.
• Bases of each of the freed DNA strands can bind to
complementary bases.
• Each copy is composed of one new strand and one strand
from the original DNA molecule.
• Preserves the sequence of bases in DNA.

34. DNA Replication (continued)

35. DNA
• Law of Complementary Base Pairings:
• # of purine bases = # pyrimadine bases.
▫ Adenine only pairs with thymine.
▫ Guanine only pairs with cytosine.
▫ DNA polymerases join the nucleotides together
to form a second polynucleotide chain.

36. Cell Cycle
• Interphase (non-dividing cell phases):
▫ G1:
 Produces mRNA and proteins.
▫ S:
 If cell is going to divide, DNA replicated.
▫ G2:
 Chromosome consists of 2 chromatids joined by centromere.
 Each chromatid contains a complete double-helix DNA
molecule. Each chromatid will become a separate chromosome
once mitotic division completed.
 Completes interphase.

37. CyclinsCyclins promote different phases of the cell cycle.
◦ During G1 phase an increase in cyclin D proteins activates
enzymes to move the cell quickly through the G1 phase.
 Overactivity of a gene that codes for cyclin D might cause
uncontrolled cell division (cancer).
Oncogenes:
◦ Mutated forms of normal genes that contribute to cancer.
Tumor suppressor genes:
◦ Inhibit cancer development.
◦ Suppressor gene p53 indirectly blocks the ability of cyclins to
stimulate cell division.
 Induces the expression of gene p21, which inactivates the cyclin-
dependent kinases.
 Promotes cell differentiation.

38. Mitosis (M Phase)▫ Prophase:
 Chromosomes become visible distinct structures.
▫ Metaphase:
 Chromosomes line up single file along equator.
 Action of spindle fibers attached to kinetochore
▫ Anaphase:
 Centromeres split apart.
 Spindle fibers shorten, pulling the 2 chromatids in each
chromosome to opposite poles.
▫ Telophase:
 Division of cytoplasm, producing 2 daughter cells.

39. Mitosis (continued)

40. Mitosis (continued)

41. Role of Centrosome
• All animal cells have centrosome, located near nucleus in
non-dividing cell.
▫ At center are 2 centrioles.
 Each centriole composed of 9 bundles of microtubules.
 Microtubules grow out of pericentriolar material.
▫ Centrosome replicates itself during interphase (if cell is going to
divide).
▫ Identical centrosomes move away from each other during
prophase.
▫ Take up opposite poles by metaphase.
 Microtubules from both centrosomes form spindle fibers.
 Spindle fibers pull chromosomes to opposite poles during anaphase.

42. Telomeres and Cell Division
• Decreased ability of cells to divide is an indicator of
senescence (aging).
▫ May be related to the loss of DNA sequences at the ends of
chromosomes (regions called telomeres).
 Telomeres serve as caps on the ends of DNA.
 Prevent enzymes from mistaking the normal ends for broken DNA.
 DNA polymerase does not fully copy the DNA at end-regions.
 Each time a chromosome replicates it loses 50-100 base pairs in its
telomeres.
▫ Germinal cells can divide indefinitely due to an enzyme
telomerase.
 Duplicates telomere DNA.

43. Meiosis (Reduction Division)
• Cell division occurring in ovaries and testes to
produce gametes (ova and sperm cells).
• Has 2 divisional sequences:
▫ First division:
 Homologous chromosomes line up side by side
along equator of cell.
 Spindle fibers pull 1 member of the homologous
pair to each pole.
 Each of the daughter cells contains 23 different
chromosomes, consisting of 2 chromatids.

44. Meiosis (Reduction Division)(continued)
▫ Second division:
 Each daughter cell divides, with duplicate
chromatids going to each new daughter cell.
 Testes: produce 4 sperm cells.
 Ovaries: produce one mature egg, polar bodies die.

46. Cell Death
• Pathologically:
▫ Cells deprived of blood supply swell, the membrane
ruptures, and the cell bursts (necrosis).
• Apoptosis:
▫ Cells shrink, membranes become bubbled, nuclei condense.
• Capsases (“executioner enzymes”):
▫ Mitochondria membranes become permeable to proteins
and other products.
• Programmed cell death:
▫ Physiological process responsible for remodeling of tissues
during embryonic development and tissue turnover in the
adult.