1. Cell Structure and Genetic Control
Chapter 03
Cell Structure and Genetic Control
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2. I. Plasma Membrane and Associated Structures

3. Overview Cells are the basic functional units of the body.
They come in a variety of shapes and sizes. This diversity reflects their diverse functions.
Principal parts of cells
Plasma membrane – selectively permeable, gives form, and separates from the external environment
Cytoplasm and organelles – fluid part of cell and little organs that do the functions
Nucleus – contains DNA and directs cell activities

4. Cell Golgi complex Secretory vesicle Nuclear envelope Centriole
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Golgi complex
Secretory
vesicle
Nuclear envelope
Centriole
Mitochondrion
Nucleolus
Lysosome
Chromatin
Plasma membrane
Nucleus
Microtubule
Granular
endoplasmic
reticulum
Cytoplasm
(cytosol)
Agranular
endoplasmic
reticulum
Ribosome

5. Summary of Cell Components

6. Structure of the Plasma Membrane
Phospholipid barrier separates the intracellular and extracellular environments
Hydrophobic center of the double membrane restricts the movement of water, water-soluble molecules, and ions.
Many substances are selectively allowed to pass through protein channels.
Proteins and phospholipids are not trapped in the membrane but constantly move laterally; known as the fluid mosaic model.

7. Plasma Membrane Structure
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Extracellular side
Carbohydrate
Glycoprotein
Glycolipid
Nonpolar
end
Polar
end
Phospholipids
Proteins
Cholesterol
Intracellular side

8. Membrane Proteins Integral proteins are embedded into the membrane.
Peripheral proteins are attached to just one side of the membrane.
Functions:
Structural support
Transport (channels)
Enzymatic control of cell processes
Receptors for hormones and other molecules
“Self” markers for the immune system

9. Other components of the membrane
Carbohydrates – attached to lipids (glycolipids) and to proteins (glycoproteins); serve as antigens and interactions with regulatory molecules
Cholesterol – gives flexibility to the membrane

10. Phagocytosis (cell eating)
Bulk transport of large extracellular substances into the cell
Some cells, like neutrophils and macrophages, can perform amoeboid movement by extending pseudopods to pull the cell forward.
Relies on the bonding of proteins called integrins with extracellular proteins
Pseudopods engulf bacteria, dead cells, or other organic materials and then fuse together to form a food vacuole.
The food vacuole fuses with a lysosome, and the material is digested.
Very important for body defense, inflammation, and apoptosis

11. (both): © Kwang Jeon/Visuals Unlimited, Inc.
Phagocytosis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Pseudopod
Pseudopods
forming food
vacuole
(a)
(b)
(both): © Kwang Jeon/Visuals Unlimited, Inc.

12. Endocytosis Another process for bringing large materials into the cell
The plasma membrane furrows inward rather than extending outward. A small part of the membrane surrounding the substance pinches off and is brought in as a vesicle.
Pinocytosis (nonspecific)
Receptor-mediated endocytosis (specific)
Has receptor proteins in the membrane that will bind to the substance to be brought in
Ex – cholesterol, AIDS virus, hepatitis virus

13. Endocytosis Extracellular Plasma membrane (pit forming) Membrane
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Extracellular
Plasma
membrane
(pit forming)
Membrane
pouching
inward
Cytoplasm
(1)
(2)
Extracellular
Cytoplasm
Vesicle
within
cell
Vesicle
(3)
(4)
(all): From M.M. Perry and A.B. Gilbert. Journal of Cell Science 39: 257–272, Reprinted by permission Company of Biologists, Ltd.

14. Exocytosis
Process of moving large cellular products (proteins) out of the cell.
The Golgi apparatus packages proteins into vesicles that fuse to the plasma membrane, and the contents spill out of the cell.

15. Cilia & Flagella
Cilia - tiny, hairlike structures composed of microtubules that project from the plasma membrane
Primary cilium – most cells have this nonmotile cilium with “9+0” structure; may have a sensory function in some cells
Motile cilia beat in unison to move substances through hollow organs.
Found in respiratory tract and uterine tubes
Have a “9+2” arrangement

16. Cilia
Base of each cilium has a pair of centrioles that are perpendicular to each other and are parts of a structure called the centrosome;
One centriole is parallel to the cilium and called the basal body

17. Cilia Cilia (a) 10 µm 0.15 µm (b)
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Cilia
(a)
10 µm
0.15 µm
(b)
a: © Science Photo Library RF/Getty Images; b: © Biophoto Associates/Photo Researchers, Inc.

18. Flagellum A single whip-like structure that can propel a cell forward
Composed of microtubules with a “9+2” arrangement
The sperm is the only cell in the human body with a flagellum.

19. Microvilli
Folds in the plasma membrane that increase the surface area for chemical reactions and rapid diffusion
Examples: Digestive tracts and kidney tubules
Microvilli
Lumen
Junctional
complexes

20. II. Cytoplasm and Organelles

21. Cytoplasm & Cytoskeleton
Structures within a cell
Includes organelles, a fluid called cytosol, the cytoskeleton, and inclusions.
Inclusions – stored chemical aggregates such as glycogen, melanin, and triglycerides

22. Cytoskeleton
Organized system of microtubules and microfilaments throughout the cytoplasm
Proteins of the cytoskeleton are mobile.
They organize the intracellular environment and allow movement of muscle cells and phagocytic cells.
They form the spindle apparatus that pulls chromosomes apart in mitosis.
They also serve as a “railway” system for vesicles and organelles to move along using molecular motors of myosin, kinesins, and dyneins

23. Cytoskeleton Microtubules Microfilaments Plasma membrane Mitochondrion
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Plasma
membrane
Mitochondrion
Polysome
Microtubules
Endoplasmic
reticulum
Microtubule
Ribosome
Microfilaments
Nuclear
envelope

24. Lysosomes Organelles filled with digestive enzymes
Fuse with vacuoles that contain engulfed extracellular particles (proteins, lipids, polysaccharides, viral proteins, bacterium, etc)
Primary lysosome: only contains digestive enzymes
Secondary lysosome: contains the partially digested contents of the food vacuole or worn-out organelles
Residual body: a lysosome filled with waste, which can be expelled through exocytosis

25. Lysosomes
Besides digesting bacteria, lysosomes are also responsible for:
Autophagy – process of digesting worn-out or damaged organelles and proteins in the cell
Apoptosis – programmed cell death. The lysosome spills its contents, killing the cell.

26. Peroxisomes Contain enzymes specific to certain oxidative reactions
Found in most cells but most numerous in the liver; often oxidize toxic molecules (such as alcohol)
Enzymes used to remove hydrogen from a molecule and transfer it to O2, forming hydrogen peroxide
Also contain the enzyme catalase, which converts hydrogen peroxide into water and O2

27. Mitochondria
Sites of energy production through aerobic cell respiration
Structure
Have an inner membrane and an outer membrane separated by an intermembranous space
Inner membrane is folded into cristae to increase surface area for reactions
Central area is fluid and called the matrix

28. a: Courtesy Keith R. Porter Endowment
Mitochondria
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Inner mitochondrial
membrane
Outer mitochondrial
membrane
Matrix
Cristae
(a)
(b)
a: Courtesy Keith R. Porter Endowment

29. Mitochondria
Most cells have mitochondria, and there can be thousands of mitochondria in a single cell.
Mitochondria can migrate around the cell and can make copies of themselves.
Have their own DNA, all derived from mother
Mutations of mitochondrial DNA may contribute to aging and disease

30. Ribosomes Protein factories of the cell
Messenger RNA takes genetic information to the ribosome so a protein can be assembled.
Very small; made of 2 subunits of ribosomal RNA and protein
Found free in the cytoplasm or associated with the endoplasmic reticulum
Serves as enzymes called ribozymes that are needed for protein synthesis

31. Ribosome

32. Endoplasmic Reticulum (ER)
System of membranous passageways whose membrane are continuous with the nuclear membrane
Rough ER (Granular ER)
Has ribosomes embedded on the outer surface
Functions in protein modification
Smooth ER (Agranular ER)
Has many functions, depending on the cell
Steroid hormone metabolism, detoxification, Ca2+ storage

33. a: Courtesy Keith R. Porter Endowment
Endoplasmic Reticulum
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(a)
Nucleus
Tubule
Membrane
Ribosome
(b)
(c)
a: Courtesy Keith R. Porter Endowment

34. Golgi Complex (Apparatus)
Consists of stacks of hollow, flattened sacs; cavities are called cisternae
One side receives proteins from the ER
These are packaged in vesicles called endosomes, that bud off to (1) fuse with the plasma membrane for exocytosis, (2) fuse and become integral proteins, (3) fuse with lysosomes inside the cell
Proteins are modified within the cisternae based on the type of protein
Retrograde transport – extracellular proteins brought in by endocytosis and then through the Golgi complex to the ER

35. Golgi Complex (Apparatus)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Rough
endoplasmic
reticulum
Protein
Plasma membrane
Secretion
Cisternae
Nucleus
Secretory
storage
Ribosomes
Cytoplasm
Golgi complex
(a)
Lysosome
(b)
a: Courtesy of Mark S. Ladinsky and Kathryn D. Howell, University of Colorado

36. III. Cell Nucleus and Gene Expression

37. Cell Nucleus
The nucleus is enclosed by a double membrane nuclear envelope:
Outer membrane continuous with ER
Inner membrane often fused to outer by nuclear pore complexes, which allow movement of small molecules and RNA into/out of the nucleus
Contains genetic materials stored in form of chromatin, which are highly coiled DNA

38. Cell Nucleus Inner and outer nuclear membranes Nucleus Nucleus
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Inner and outer
nuclear membranes
Nucleus
Nucleus
Chromatin
Nucleolus
Pore
Outer
membrane
Ribosome
Inner
membrane
Pore
complex

39. DNA and Genes
The nucleus contains DNA. A gene is a length of DNA that codes for a specific protein.
Transcription – the gene on the DNA is copied as messenger RNA (mRNA), which then leaves the nucleus to go to the rough ER
Translation – the mRNA is processed at the ribosomes to assemble the proper amino acid sequence to form a protein
These two steps can be called Genetic expression.

40. Nucleolus
The nucleus also has one or more darker regions not surrounded by a membrane; these are called nucleolus.
The nucleolus contain DNA that codes for the production of ribosomal RNA (rRNA).

41. Genome and Proteome
The genome is all the genes in a particular individual or all the genes of a particular species.
Researchers believe humans have ~25,000 different genes.
The proteome is all the proteins that are produced from the genome.
More than 100,000 proteins are produced in the human body.

42. Genome and Proteome How can a gene code for more than one protein?
mRNA is altered after transcription by cutting and splicing different ways
Proteins are made of many polypeptide chains that can associate in different combinations
Post-translational modification by:
Glycosylation
Methylation
Phosphorylation
Cleavage of larger into smaller units

43. Chromatin
DNA in the nucleus is packaged with proteins called histones to form chromatin.
Histones are positively charged and will interact with negatively charged DNA to cause spooling
Two turns of DNA around 8 histone proteins create structure called nucleosomes
Euchromatin: active in transcription, looser; chemical changes in histones (such as acetylation) allow molecules access to the DNA during gene expression.
Heterochromatin: inactive regions, highly condensed; much of the DNA is inactive.

44. Chromatin Chromosome Region of euchromatin with activated genes
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Chromosome
Region of
euchromatin
with activated
genes O O O O
Nucleosome
DNA O O O O

45. Chromatin and Gene Expression
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Condensed chromatin,
where nucleosomes
are compacted
Acetylation
Acetylation of chromatin
produces a more open
structure
Transcription
factor
Transcription factors
attach to chromatin,
activate genes
(producing RNA)
DNA region to be transcribed
Deacetylation
Deacetylation causes
compaction of chromation,
silencing genetic transcription

46. RNA Synthesis Transcription (DNA-directed RNA synthesis)
Occurs inside the nucleus
Involves:
Start and stop regions at the beginning and end of the gene
Promoters, areas of DNA that are not part of the gene but tell enzymes involved where to begin
Transcription factors that bind to the promoter to begin transcription

47. RNA Synthesis C- G- A- U- C- G-C-T-A-G-T-C-G-G
RNA polymerase, breaks the hydrogen bonds between the base pairs of DNA and assembles the appropriate RNA nucleotide
RNA nucleotides pair up to the DNA template
Assembly is complementary.
RNA has uracil instead of thymine.
Forms precursor messenger RNA that detaches from the DNA template
Only 1 freed DNA strand is transcribed C- G- A- U- C-
mRNA-
G-C-T-A-G-T-C-G-G
DNA-

48. Types of RNA
Precursor messenger RNA (pre-mRNA) – made directly by transcription
Messenger RNA (mRNA) – modified pre-mRNA; contains the code to make a specific protein
Transfer RNA (tRNA) – carries amino acids to mRNA for translation
Ribosomal RNA (rRNA) – along with protein, forms ribosomes; site of translation; acts as an enzyme

49. Pre-mRNA modification
Precursor messenger RNA is altered in the nucleus before it leaves as messenger RNA (mRNA).
Introns – portions of the Pre-mRNA that do not actually code for proteins. These must be spliced out.
Introns may regulate the expression of the area that do code for protein
Exons – portions of Pre-mRNA that contain codes for proteins.

50. Pre-mRNA modification
Alternative splicing allows one Pre-mRNA to be used to make multiple proteins.
Exons are joined together by spliceosomes and snRNPs (snurps) which are small nuclear ribonucleoproteins, to form mRNA

51. Messenger RNA Synthesis and Splicing
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A T C G
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. T A
DNA
DNA (gene) G C C A T
RNA C G U
Introns A A C C G G C G A T U
Transcription A U
Pre-mRNA T C G G C G C A U A A
Exon
Intron
Exon
Intron
Exon G G C A U C C G U T C G A T G C G
Exons spliced together
mRNA C G C A G A C T

52. RNA Interference
RNA molecules that don’t code for proteins may prevent some mRNA molecules from being translated.
Two types:
siRNA (short interfering)
Formed from double-stranded RNA and cut by Dicer enzyme into short segments
miRNA (micro interfering)
Formed from introns of Pre-mRNA and cut by Dicer enzyme into short segments

53. RNA interference
One of the strands of siRNA and miRNA enter a protein particles called RNA-induced silencing complex (RISC)
Can bind to mRNA where it is complementary
Prevents tRNA from bringing amino acids to mRNA, so translation is prevented – the gene is silenced
RNA interference may help in cancer treatments as well as other genetic conditions

54. IV. Protein Synthesis and Secretion

55. Protein Synthesis Translation
mRNA attaches to a string of ribosomes to form a polyribosome.
Codon - a group of three bases on mRNA that provide a code for an amino acid
The order of the codons gives the order of amino acids in a polypeptide

56. Newly synthesized protein
Polyribosome
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Ribosomes
Newly synthesized protein
mRNA
© E. Kiselva-D. Fawcett/Visuals Unlimited, Inc.

57. Triplets,Codons, Amino Acids

58. Amino Acids

59. Protein Synthesis T G C G T A C G A C C G T A T G G C C C G DNA double
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. T G C G T A C G A C C G T A T G G C C C G
DNA
double
helix C A G C G G G C
DNA coding strand T A C C C G A G G T A G C C G C G T C G T
Transcription A U G G G C U C C A U C G G C G C A G C A
Messenger RNA
Codon 1
Codon 2
Codon 3
Codon 4
Codon 5
Codon 6
Codon 7
Translation
Methionine
Glycine
Serine
Isoleucine
Glycine
Alanine
Alanine
Protein

60. Transfer RNA (tRNA)
A single strand of RNA bent into a cloverleaf shape
One end has the anticodon, which is three nucleotides that will be complementary to the proper codon.
The other end has the appropriate amino acid bonded by aminoacyl-tRNA synthetase enzyme A
Amino acid-
accepting end C C
Loop 3
Loop 1
Loop 2 U U A
Anticodon
(a) C C A
Loop 3
Amino acid-
accepting end
Loop 1
Loop 2 A U U
Anticodon

61. Formation of a Polypeptide
The mRNA moves through the ribosome, with the proper tRNA attaching at each codon.
Amino acids attached to the tRNAs form peptide bonds to each other and disassociate from the tRNA.
tRNAs disassociate from the mRNA as they lose their amino acids.
This continues until a stop codon is encountered and the whole complex disassociates.
Interactions between amino acids of the growing polypeptide chain cause bends and folds into secondary and tertiary structures
Chaperone proteins aid the formation of correct interactions and folds

62. Translation Codons Anticodons 3 mRNA C Next amino acid 6 U A I E C G
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Codons
Anticodons 3
mRNA C
Next amino acid 6 U A I E C G
tRNA
Codons 1 C G H I
tRNA A U G D U A
tRNA H C G F G G E F
tRNA 5
Next amino acid D 2 E
tRNA
tRNA C 4 D 5 B C 3
Growing
polypeptide
chain 4 A B A 2 3
tRNA 1 2 1
Ribosome

63. Functions of the ER and Golgi Complex
Newly formed proteins destined to leave the cell are made on the rough (granular) ER.
The first ~30 amino acids (leader sequence) are hydrophobic and are attracted to the lipid portion of the membrane of the granular ER.
The growing polypeptide chain enters the cisternae of the ER.
The leader leader sequence is removed and other portions may be removed or added.

64. endoplasmic reticulum
Granular ER Role in Protein Synthesis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cytoplasm
Ribosome
Free ribosome
Granular endoplasmic
reticulum
mRNA
Leader
sequence
Leader
sequence
removed
Protein
Cisterna of
endoplasmic reticulum
Carbohydrate

65. Functions of the ER and Golgi Complex
Secretory proteins are next sent to the Golgi complex.
Proteins may be further modified, including the production of glycoproteins.
Proteins are separated according to destination.
Proteins are packaged and shipped in vesicles to their destinations.

66. Protein Degradation
Regulatory proteins are rapidly degraded, making their effects short-lived. This allows for greater control of cell functions.
In the lysosome, proteins are digested by proteases.
Outside the lysosome, proteins to be destroyed are bonded by a molecule called ubiquitin, which marks them for degradation by a proteasome (large protease complex).
Ubiquitin may also tag membrane proteins and organelles for destruction

67. V. DNA Synthesis and Cell Division

68. DNA Replication
Before cell division, each DNA molecule must replicate itself so that one of each copy can be distributed to the two new cells.
Involves two types of enzymes:
Helicases break hydrogen bonds between the DNA strands. This creates a fork in the double-stranded molecule where nucleotides can be added to both strands.

69. DNA Replication Enzymes
DNA polymerase attaches complementary nucleotides to the exposed strand.
Two new molecules are being made from the original one, each with half old and half new DNA.
This is called semiconservative replication.

70. DNA Replication A T C G Region of parental DNA helix.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A T C G
Region of parental DNA helix.
(Both backbones are light.) A C G G C A T A T G G C
Region of replication. Parental
DNA is unzipped and new
nucleotides are pairing with
those in parental strands. C C G C A G T A T G G C A C T A T C G A T A
Region of completed replication.
Each double helix is composed
of an old parental strand (light
purple) and a new daughter
strand (blue). The two DNA
molecules formed are
identical to the original
DNA helix and to one another. G C C G T C G T A T C G C G G C A T C G G C

71. The Cell Cycle Divided into interphase, mitosis, and cytokinesis
Interphase is divided into G1, S, and G2
Mitosis is divided into Prophase, Metaphase, Anaphase, and Telophase
Cytokinesis overlaps the last parts of mitosis
Mitotic Phase
Anaphase
Metaphase
Telophase
Prophase
Cytokinesis
Mitosis G2 G1
Final growth and
activity before
mitosis
Centrioles
replicate S
DNA replication
Interphase

72. Interphase Lots of RNA synthesis occurring
G1 Phase: The cell is performing the functions characteristic of cells in that tissue.
Cyclin D moves the cell quickly through G1.
Overactivity of the gene for cyclin D has been implicated in some cancers.
p53 is a transcription factor that can stall a gene at the G1/S checkpoint, repair DNA damage or promote apoptosis
If a cell does not divide, it remains in a modified G1 phase its whole life.

73. One (duplicated) chromosome
Interphase
S phase: If a cell is going to divide, it performs DNA replication at this time
G2 Phase: Chromosomes start to condense; consist of two strands called sister chromatids joined by a centromere.
One (duplicated) chromosome
Centromere
Chromatid
DNA
Histone

74. Interphase Interphase
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chromatin
Nucleolus
Centrosomes
Interphase
• The chromosomes are in an extended form and seen as chromatin in the electron microscope.
• The nucleus is visible.
(all): © Ed Reschke

75. Mitosis
Prophase: Chromosomes become visible, nuclear membrane and nucleolus disappear, centrioles move apart, spindle fibers form
Chromatid pairs
Spindle fibers
Prophase
• The chromosomes are seen to consist of two chromatids joined by a centromere.
• The centrioles move apart toward opposite poles of the cell.
• Spindle fibers are produced and extend from each centrosome.
• The nuclear membrane starts to disappear.
• The nucleolus is no longer visible.

76. Mitosis
Metaphase: Chromosomes line up in the center of the cell and have attached to spindle fibers
Spindle
fibers
Metaphase
• The chromosomes are lined up at the equator of the cell.
• The spindle fibers from each centriole are attached to the centromeres of the chromosomes.
• The nuclear membrane has disappeared.

77. Mitosis
Anaphase: Centromeres split as the spindle fibers shorten and pull chromatids to opposite sides.
Anaphase
• The centromeres split, and the sister chromatids separate as each is pulled to an opposite pole.

78. Mitosis
Telophase: Cytoplasm is divided (cytokinesis) and cells separate, new nuclear membrane and nucleolus appears, chromosomes lengthen
Furrowing
Nucleolus
Telophase
• The chromosomes become longer, thinner, and less distinct.
• New nuclear membranes form.
• The nucleolus reappears.
• Cell division is nearly complete.

79. Role of the Centrosome Located near the nucleus of a nondividing cell.
Centrosome contains:
Two centrioles at the center, which replicate in interphase and move away from each other in prophase.
Pericentriolar materials form around the centrioles. Spindle fibers form from this and attach to the centromere of the replicated sister chromatids
Other tubules cause the formation of the cleavage furrow for cytokinesis

80. Role of the Centrosome
In nondividing cells, the centrosome migrates to the plasma membrane and forms the nonmotile primary cilium.
In ciliated cells, hundreds of centrosomes form and become the basal bodies of the cilia
(a)
(b)

81. Telomeres and Cell Division
Telomeres – protective cap on the ends of DNA that prevents DNA degradation.
Each time DNA is replicated, a little more telomere is lost, which will cause the cell to eventually lose its ability to divide.
Damaged telomeres activate p53 which induces cell cycle arrest, senescence, and apoptosis
Cells that can divide indefinitely, such as those in bone marrow, have an enzyme called telomerase that replicates the telomere.

82. Hypertrophy and Hyperplasia
Hyperplasia: growth due to an increase in the number of cells; responsible for the growth of most body regions
Hypertrophy: growth due to an increase in cell size; responsible for increase in skeletal muscle size

83. Cell Death
Necrosis: Cell dies pathologically due to deprivation of blood supply.
Apoptosis: Programmed cell death is performed by enzymes called caspases.
Extrinsic: “Death ligands” attach to the cell and mark it for destruction.
Intrinsic: Intercellular signals trigger death due to DNA damage, cancer, infection, or oxidative stress.
“Knocked-out” mice with their p53 gene removed are used to study cancer and apoptosis

84. Meiosis Homologous Chromosomes
Humans have 23 pairs of chromosomes, one set from each parent. 22 pairs are autosomes and 1 pair are sex chromosomes
Each pair is called homologous chromosomes, and they have the same genes on them (but not identical DNA).
Meiosis – process by which two cell division steps produce gametes (ova and sperm); only occurs in the gonads (ovaries and testes)

85. Homologous Chromosomes

86. Meiosis I – reduction division
Prophase I: Homologous chromosomes pair up; parts are often swapped in a process called crossing-over.
Metaphase I: Homologous chromosomes randomly line up in the center of the cell; maternal and paternal chromosomes are shuffled.

87. Crossing-over (a) First meiotic prophase Chromosomes pairing
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(a) First meiotic prophase
Chromosomes pairing
Chromosomes crossing-over
(b) Crossing-over

88. Meiosis I Anaphase I: Homologous chromosomes are pulled apart.
Telophase I: Homologous chromosomes are separated. This results in two daughter cells with 23 chromosomes each (haploid).
This is reduction division since each cell now has half as many chromosomes.
Necessary for sexual reproduction
Crossing-over and shuffling of chromosomes in metaphase 1 result in genetic recombination and genetic diversity

89. Meiosis II
Proceeds like mitosis with phases prophase II through telophase II.
Sister chromatids line up in the center of the cell. Centromeres are broken and pulled to opposite poles.
Results in 4 cells with 23 chromosomes each.

90. Meiosis Prophase I Tetrad Metaphase I Anaphase I Telophase I Daughter
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Prophase I
Tetrad
Metaphase I
Anaphase I
Telophase I
Daughter
cell
Daughter
cell
Prophase II
Metaphase II
Anaphase II
Telophase II
Daughter cells
Daughter cells

91. Meiosis

92. Epigenetic Inheritance
Silenced genes are passed to daughter cells during mitosis and meiosis without a change in DNA base sequence
Mechanisms of epigenetic inheritance
Posttranslational modifications of histone proteins
Methylation of cytosine bases in DNA that precede guanine
Problems with epigenetic inheritance contribute to cancer, fragile X syndrome, and systemic lupus erythematosus