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Midterm Final Review
Part II
Somatic Cells
Gametes
• Body cells
• Diploid (2n): 2 of each
type of chromosome
• Divide by mitosis
• Sex cells (sperm/egg)
• Haploid (n): 1 of each
type of chromosome
• Divide by meiosis
• Humans: 2n = 46
• Humans: n = 23
Phases of the Cell Cycle
Phases of the Cell Cycle
 The mitotic phase alternates with interphase:
G1  S  G2  mitosis  cytokinesis
 Interphase (90% of cell cycle)
 G1 Phase: cell grows and carries out normal
functions
 S Phase: duplicates chromosomes
 G2 Phase: prepares for cell division
 M Phase (mitotic)
 Mitosis: nucleus divides
 Cytokinesis: cytoplasm divides
Mitosis: Prophase  Metaphase
 Anaphase  Telophase
Cell Cycle Control System
• Checkpoint = control point where stop/go signals
regulate the cell cycle
Major Checkpoints
1. G1 checkpoint (Most important!)
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–
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Controlled by cell size, growth factors, environment
“Go”  completes whole cell cycle
“Stop”  cell enters nondividing state (G0 Phase)
•
Nerve, muscle cells stay at G0; liver cells called back from G0
2. G2 checkpoint
•
Controlled by DNA replication completion, DNA
mutations, cell size
3. M-spindle (Metaphase) checkpoint
–
Check spindle fiber (microtubule) attachment to
chromosomes at kinetochores (anchor sites)
Internal Regulatory Molecules
• Kinases (cyclin-dependent kinase, Cdk): protein enzyme
controls cell cycle; active when connected to cyclin
• Cyclins: proteins which attach to kinases to activate them;
levels fluctuate in the cell cycle
Internal Regulatory Molecules
MPF = maturation-promoting factor
•
specific cyclin-Cdk complex which allows cells
to pass G2 and go to M phase
External Regulatory Factors
•
•
•
Growth Factor: proteins released by other cells
to stimulate cell division
Density-Dependent Inhibition: crowded cells
normally stop dividing; cell-surface protein binds
to adjoining cell to inhibit growth
Anchorage Dependence: cells must be attached
to another cell or ECM (extracellular matrix) to
divide
Cancer Cells
Cancer: disorder in which cells lose the ability to
control growth by not responding to regulation.
• multistep process of about 5-7 genetic changes
(for a human) for a cell to transform
• loses anchorage dependency and densitydependency regulation
Normal Cells
Cancer Cells
Cancer cells
• Some have abnormal #’s of chromosomes
Karyotype of
Metastatic
Melanoma
Types of Reproduction
ASEXUAL
• Produces clones
(genetically identical)
• Single parent
• Little variation in
population - only
through mutations
• Fast and energy efficient
• Eg. budding, binary
fission
SEXUAL
• Meiosis produces
gametes (sex cells)
• 2 parents: male/female
• Lots of
variation/diversity
• Slower and energy
consumptive
• Eg. humans, trees
Homologous Chromosomes in a Somatic Cell
Meiosis = reduction division
• Cells divide twice
• Result: 4 daughter
cells, each with half
as many
chromosomes as
parent cell
Events Unique to Meiosis I (not in mitosis)
1. Prophase I: Synapsis and crossing
over
2. Metaphase I: pairs of homologous
chromosomes line up on metaphase
plate
3. Anaphase I: homologous pairs
separate  sister chromatids still
attached at centromere
Sources of Genetic Variation:
1. Crossing Over
– Exchange genetic
material
– Recombinant
chromosomes
Sources of Genetic Variation:
2.Independent Assortment of Chromosomes
– Random orientation of homologous pairs in
Metaphase I
Sources of Genetic Variation:
3. Random Fertilization
– Any sperm + Any egg
– 8 million X 8 million = 64 trillion combinations!
Mitosis
Meiosis
Both are divisions of cell nucleus
•
•
•
•
•
•
•
Somatic cells
1 division
2 diploid daughter cells
Clones
From zygote to death
Purpose: growth and repair
No synapsis, crossing over
• Gametes
• 2 divisions
• 4 haploid daughter cells
• Genetically different-less than
1 in 8 million alike
• Females before birth follicles
are formed. Mature ova
released beginning puberty
• Purpose: Reproduction
MENDEL’S PRINCIPLES
1. Alternate version of genes (alleles) cause variations in
inherited characteristics among offspring.
2. For each character, every organism inherits one allele
from each parent.
3. If 2 alleles are different, the dominant allele will be
fully expressed; the recessive allele will have no
noticeable effect on offspring’s appearance.
4. Law of Segregation: the 2 alleles for each character
separate during gamete formation.
dominant (P), recessive (p)
• homozygous = 2 same alleles (PP or pp)
• heterozygous = 2 different alleles (Pp)
– Phenotype: expressed physical traits
– Genotype: genetic make-up
–
Testcross: determine if dominant trait is
homozygous or heterozygous by crossing with
recessive (pp)
Law of Independent Assortment:
• Each pair of alleles segregates (separates) independently
during gamete formation
• Eg. color is separate from shape
Extending Mendelian Genetics
The relationship between genotype and phenotype
is rarely simple
Complete Dominance:
heterozygote and homozygote
for dominant allele are
indistinguishable
• Eg. YY or Yy = yellow seed
Incomplete Dominance: F1
hybrids have appearance that
is between that of 2 parents
• Eg. red x white = pink flowers
Blood Typing
Phenotype
(Blood Group)
Genotype(s)
Type A
IAIA or IAi
Type B
IBIB or IBi
Type AB
IAIB
Type O
ii
Mendelian Inheritance in Humans
Pedigree: diagram that shows the relationship
between parents/offspring across 2+ generations
Woman =
Man =
Trait expressed:
Sex-linked genes
• Sex-linked gene on X or Y
• Females (XX), male (XY)
– Eggs = X, sperm = X or Y
• Fathers pass X-linked genes to daughters, but not
sons
• Males express recessive trait on the single X
(hemizygous)
• Females can be affected or carrier
X-Inactivation
Barr body = inactive X chromosome; regulate gene
dosage in females during embryonic development
•
•
Cats: allele for fur
color is on X
Only female cats can
be tortoiseshell or
calico.
Genetic Recombination: production of offspring
with new combo of genes from parents
• If offspring look like parents  parental types
• If different from parents  recombinants
Linked genes: located on same chromosome and
tend to be inherited together during cell division
Crossing over: explains why some linked genes
get separated during meiosis
• the further apart 2 genes on same chromosome,
the higher the probability of crossing over and
the higher the recombination frequency
Calculating recombination frequency
Linkage Map: genetic map that is based on
% of cross-over events
• 1 map unit = 1% recombination frequency
• Express relative distances along chromosome
• 50% recombination = far apart on same chromosome
or on 2 different chromosomes
Nondisjunction: chromosomes fail to separate
properly in Meiosis I or Meiosis II
Nondisjunction
• Aneuploidy: incorrect # chromosomes
– Monosomy (1 copy) or Trisomy (3 copies)
• Polyploidy: 2+ complete sets of chromosomes;
3n or 4n
– Rare in animals, frequent in plants
A tetraploid mammal. Scientists think this species may have arisen when an
ancestor doubled its chromosome # by errors in mitosis or meiosis.
Chromosomal Mutations
Chromosomal Mutations
Exceptions to Mendelian inheritance
• Genomic imprinting: phenotypic effect of gene
depends on whether from M or F parent
• Silence genes by adding methyl groups to DNA
(methylation)
Exceptions to Mendelian inheritance
• Some genes located in
organelles
– Mitochondria, chloroplasts,
plastids
– Contain small circular DNA
• Mitochondria = maternal
inheritance (eggs)
Variegated (striped or spotted) leaves result from mutations in
pigment genes in plastids, which generally are inherited from
the maternal parent.
Frederick Griffith (1928)
Conclusion: living R bacteria transformed into
deadly S bacteria by unknown, heritable
substance
Avery, McCarty, McLeod (1944)
– Tested DNA, RNA, & proteins in heat-killed
pathogenic bacteria
– Discovered that the transforming agent was
DNA
Hershey and Chase (1952)
• Bacteriophages: virus that infects bacteria; composed of
DNA and protein
Protein = radiolabel S
DNA = radiolabel P
Edwin Chargaff (1947)
Chargaff’s Rules:
• DNA composition varies
between species
• Ratios:
– %A = %T and %G = %C
Rosalind Franklin (1950’s)
• Worked with Maurice Wilkins
• X-ray crystallography = images of DNA
• Provided measurements on chemistry of DNA
James Watson & Francis Crick (1953)
• Discovered the
double helix by
building models to
conform to
Franklin’s X-ray
data and
Chargaff’s Rules.
Structure of DNA
DNA = double helix
– “Backbone” = sugar +
phosphate
– “Rungs” = nitrogenous
bases
Structure of DNA
Nitrogenous Bases
–
–
–
–
Adenine (A)
Guanine (G)
Thymine (T)
Cytosine (C)
purine
pyrimidine
• Pairing:
– purine + pyrimidine
– A=T
– GΞC
Structure of DNA
Hydrogen bonds between base pairs of the two strands
hold the molecule together like a zipper.
Structure of DNA
Antiparallel: one strand (5’ 3’), other strand runs in
opposite, upside-down direction (3’  5’)
DNA Comparison
Prokaryotic DNA
Eukaryotic DNA
•
•
•
•
•
•
•
•
•
•
•
Double-stranded
Circular
One chromosome
In cytoplasm
No histones
Supercoiled DNA
Double-stranded
Linear
Usually 1+ chromosomes
In nucleus
DNA wrapped around histones
(proteins)
• Forms chromatin
Replication is semiconservative
DNA Replication
Leading strand vs. Lagging strand
Proofreading and Repair
• Mismatch repair: special enzymes fix
incorrect pairings
Nucleotide Excision Repair
• DNA polymerases
proofread as bases added
• Nucleotide excision repair:
– Nucleases cut damaged
DNA
– DNA poly and ligase fill in
gaps
Telomeres: repeated units of short nucleotide sequences
(TTAGGG) at ends of DNA
• Telomeres “cap” ends of DNA to postpone erosion of genes
at ends (TTAGGG)
• Telomerase: enzyme that adds to telomeres
– Eukaryotic germ cells, cancer cells
Telomeres stained
orange at the ends of
mouse chromosomes
Telomeres & Telomerase
A Summary
of Protein
Synthesis
1 Gene = 1
polypeptide or
RNA molecule
1 gene = 1 polypeptide or 1 RNA molecule
DNA
• Nucleic acid composed of
nucleotides
• Double-stranded
• Deoxyribose=sugar
• Thymine
• Template for individual
RNA
• Nucleic acid composed of
nucleotides
• Single-stranded
• Ribose=sugar
• Uracil
• Helper in steps from DNA to
protein
• Types: mRNA, pre-mRNA,
tRNA, rRNA, snRNA,
srpRNA, siRNA
The Genetic Code
The Genetic Code
64 different codon
combinations
Reading frame: groups
of 3 must be read in
correct groupings
This code is universal:
all life forms use the
same code.
1. Initiation
Eukaryotes:
TATA box = DNA
sequence (TATAAAA)
upstream from
promoter
Transcription
factors must
recognize TATA box
before RNA
polymerase can
bind to DNA
promoter
2. Elongation
• RNA polymerase
adds RNA nucleotides
to the 3’ end of the
growing chain (A-U, GC)
3. Termination
RNA polymerase
transcribes a terminator
sequence in DNA, then
mRNA and polymerase
detach.
It is now called pre-mRNA
for eukaryotes.
Prokaryotes = mRNA ready
for use
Additions to pre-mRNA:
• 5’ cap (modified guanine) and 3’ poly-A tail (50-520
A’s) are added
• Help export from nucleus, protect from enzyme
degradation, attach to ribosomes
RNA Splicing
• Pre-mRNA has introns (noncoding sequences)
and exons (codes for amino acids)
• Splicing = introns cut out, exons joined
together
RNA Splicing
• Spliceosome = snRNP + Proteins
• Remove introns and join exons
• Ribozyme = RNA acts as enzyme
Why have introns?
• Some regulate gene activity
• Alternative RNA Splicing:
produce different
combinations of exons
– One gene can make more
than one polypeptide!
– 20,000 genes  100,000
polypeptides
Translation:
1. Initiation
• Small subunit binds to start codon (AUG) on mRNA
• tRNA carrying Met attaches to P site
• Large subunit attaches
2. Elongation
3.Termination
• Stop codon reached and translation stops
• Release factor binds to stop codon;
polypeptide is released
• Ribosomal subunits dissociate
Protein Folding
• During synthesis, polypeptide chain coils and
folds spontaneously
• Chaperonin: protein that helps polypeptide
fold correctly
Types of Ribosomes
• Free ribosomes: synthesize proteins that stay
in cytosol and function there
• Bound ribosomes (to ER): make proteins of
endomembrane system (nuclear envelope, ER,
Golgi, lysosomes, vacuoles, plasma
membrane) & proteins for secretion
– Uses signal peptide to target location
Cellular “Zip Codes”
• Signal peptide: 20 AA at leading end of
polypeptide determines destination
• Signal-recognition particle (SRP): brings
ribosome to ER
The Central Dogma
Mutations happen here
Effects play out here
Mutations = changes in the genetic
material of a cell
• Large scale mutations: chromosomal; always cause
disorders or death
– nondisjunction, translocation, inversions,
duplications, large deletions
• Point mutations: alter 1 base pair of a gene
1. Base-pair substitutions – replace 1 with another
• Missense: different amino acid
• Nonsense: stop codon, not amino acid
2. Frameshift – mRNA read incorrectly; nonfunctional
proteins
• Caused by insertions or deletions
Sickle-Cell Disease = Point
Mutation
Prokaryotes vs. Eukaryotes
Prokaryotes
• Transcription and
translation both in
cytoplasm
• DNA/RNA in cytoplasm
• RNA poly binds directly to
promoter
• Transcription makes mRNA
(not processed)
• No introns
Eukaryotes
• Transcription in nucleus;
translation in cytoplasm
• DNA in nucleus, RNA travels
in/out nucleus
• RNA poly binds to TATA box
& transcription factors
• Transcription makes premRNA  RNA processing 
final mRNA
• Exons, introns (cut out)