A FACT
•  The great minority of individuals live long
enough to die of old age.
THE CELL THEORY
WHERE IS THE CONTROL?
16
A FACT
•  The number of chromosomes varies
greatly across species ( 1 , > 50)
64
12
24
TWO NON-TRIVAL
PROBLEMS
•  ACCESS & TRANSMISSION
•  FAITHFUL COPIES
WHY NON- TRIVIAL?
•  Amount of information stored is
tremendous. Living cells are the most
information-dense structures known.
•  Many copies must be made. Errors
could accumulate.
A FACT
•  Eukaryote DNA carries at least a
magnitude more genes than does
prokaryote DNA
PROKARYOTE FISSION
ANOTHER FACT
•  Eukaryotes have chromosomes with lots
of structure maintained by protein
molecules. These proteins control shape
of chromosomes
and activity of genes!
YET ANOTHER FACT
•  Most gene products are not needed
most of the time. Thus, some form of
gene regulation is required.
•  Some ‘housekeeping’ genes are
needed all the time and others rarely.
GENE REGULATION PROKARYOTES
•  Primarily at the (initiation of)
transcription phase GENE REGULATION EUKARYOTES
•  Primarily at the (initiation)
transcription phase
•  Rate of rna processing (polymerase)
•  Rate of rna degradation
•  Rate of rna translation (cytoplasm)
•  Protein transport
A TYPICAL CELL CYCLE
A FACT
•  In g2 phase apparently essential
division proteins are made. EARLY PROPHASE
•  DNA is tightly packed to facilitate
transport
PROPHASE
•  Note identical paired
chromatids
•  Note microtubules
METAPHASE
•  Why is alignment important?
ANAPHASE
•  Why tug on the
centromeres?
TELOPHASE
NOTE: Nuclear envelope
reforms
SEXUAL REPRODUCTION
•  MEIOSIS
A FACT
•  Many organisms reproduce asexually
From The World Book (TM) Multimedia Encyclopedia (c) 1999 World Book, Inc., 525 W. Monroe, Chicago, IL 60661.
All rights reserved. "World Book illustration by John D. Dawson
IT’S ALSO TRUE
•  Many organisms reproduce both sexually
and asexually
From The World Book (TM) Multimedia Encyclopedia
(c) 1999 World Book, Inc., 525 W. Monroe, Chicago, IL
60661. All rights reserved. "World Book illustration
From The World Book (TM) Multimedia Encyclopedia (c) 1999 World Book, Inc., 525 W. Monroe, Chicag
All rights reserved. "World Book illustration by John D. Dawson
SEXUAL REPRODUCTION
•  3 BIG EVENTS:
- Meiosis
- Gamete formation
- Fertilization SOME MEIOSIS FACTS
•  Meiosis is a nuclear division mechanism
•  Meiosis divides the chromosome number
in half
•  During meiosis, chromosomes may swap
hereditary material
•  Chromosomes are randomly assigned to
gametes!
REMEMBER
•  Sexual reproduction is performed by
organisms with 2 or more sets of
chromosomes and through meiosis, they
produce gametes with just one set of
those chromosomes
HOMOLOGOUS
CHROMOSOMES
HOMOLOGOUS
CHROMOSOMES
Each pair harbours the same set of
software
•  Each gene found at a position on one
chromosome is found at the same locus
on the other
•  Different molecular forms (alleles)
may be found at a given locus
•
IMPORTANT FEATURES
OF MEIOSIS
•  Assemblage •  Transport
•  Plus halving
PROPHASE 1
•  NOTE: Chromosomes are
much more threadlike
than when in mitosis
•  Membrane breakdown
•  Microtubules
WHY ALLOW CROSSOVER?
•  Facilitation of novel genotypes
•  e.g.
A
B
C
d
e
f
a
b
c
D
E
F
METAPHASE 1
•  NOTE: Tetrad alignment
•  Unbiased positioning
•  Microtubule positioning
ANAPHASE 1
•  NOTE: Identical sisters are
still connected
•  Homologues move toward
opposite poles TELOPHASE 1 AND
CYTOKINESIS
•  NOTE: Sister chromatids
•  Each cells still has a
double set of chromatids
MEIOSIS II
IMPLICATIONS
•  Start with 2 sets of
software from 2 different
donors
•  End with 4 cells each
harboring one complete
set of software from
some combination of
parental donors
HOW MANY
COMBINATIONS?
•  2N where N = number of chromosomes
•  In humans 223 = 8388608 million possible
combinations
THE BIG QUESTION
•  Why reproduce sexually?
THE COST OF SEX
•  Special structures
•  Mating process
•  Loss of 1/2 of genes
THE PRINCIPLE OF
ALLOCATION
•  Resources allocated
to one function
cannot be allocated to
other functions
REMINDER
•  Sexual reproduction generates great
variation but at a cost to the individual
•  Genetic variation could help a
population or species persist BUT group
level selection is normally very weak
AN EXAMPLE
•  Many birds appear to produce clutches
of offspring that is smaller than what
they could successfully rear. Why?
•  The old answer: By individuals setting
low clutches, that ensures that the there
are sufficient resources in poor years to
allow the population to persist
LOGIC PROBLEM
•  Thought experiment: Suppose every
bird produced two young even though it
could produce four.
•  Now, introduce a selfish mutant that
produces 4 young while everyone else
holds back with 2
•  The selfish mutant will proliferate
ANOTHER EXAMPLE
•  Prudent predation:
Predators often take the
young and infirm prey.
Why? That leaves the good
quality prey to breed and
produce lots of food for the
future?
•  What should the logical
mutant do?
SO…
•  Just like the clutch size example, we
need to find an explanation for sex that
benefits the individual
TWO BEST EXPLANATIONS
FOR SEX
•  Tangled Bank
•  Red Queen
TANGLED BANK
•  Tangled Bank – “… It is interesting to
contemplate a tangled bank, clothed with
many plants of many kinds, with birds
singing on bushes…” Darwin, Origin of
Species
•  The world is spatially heterogeneous and
competition for resources is great TANGLED BANK II
•  Rather than just churn out
more of the same, parents
benefit by producing
variants that might be able
to exploit other resources
in the environment
(remember that it is
heterogeneous)
THE RED QUEEN
•  From Through The
Looking Glass: I’m doing
all the running I can do to
keep in the same place.
•  One must keep adapting
just to survive. Identical
descendents will suffer
greatly in a changing
world.
WHAT MAKES THE WORLD
SO DANGEROUS?
•  Parasites, both macro and
micro
A KEY ELEMENT
•  Parasites frequently evolve at much faster
rates than their hosts due to significantly
shorter generation times
A SOLUTION
•  Combine your genes with
someone else to create novel
genotype descendents.
•  Be careful who you choose.