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AP Genetics Review
Why do cells divide?
• The continuity of life is based on the
reproduction of cells: cell division
• Cells divide to:
– Reproduce
– Renewal
– Repair
– Replacement
– Make new cells
Organization of Genetic Material
• DNA: our genetic
material, our genes
• Chromatin: DNA and
proteins
• Chromosomes: threadlike structures in the
nucleus that are made
of chromatin
• Genome: all of our DNA
Chromosome Duplication
• Before a cell can divide,
chromosomes must
duplicate.
• Each duplicated
chromosome has two
identical sister chromatids,
attached at a centromere.
Phases of the Cell Cycle
• Interphase: 90% of the cell’s life, during which
growth, protein synthesis, and chromosome
duplication occurs. Has 3 sub-phases:
• G1 phase: “first gap” the cell grows
• S phase: “synthesis” chromosomes duplicate
• G2 phase: “second gap” the cell grows some
more and prepares for division
Phases of the Cell Cycle
• The other phase is the Mitotic Phase (M)
during which the cell divides. It has 2 subphases:
• Mitosis: when the nucleus divides
• Cytokinesis: when the cytoplasm and the rest
of the cell divides
The Phases of Mitosis
•
•
•
•
•
•
Prophase
(Prometaphase)
Metaphase
Anaphase
Telophase
Cytokinesis
Binary Fission
• Prokaryotes (bacteria and
Archea) reproduce by binary
fission, meaning “division in
half.”
• Bacteria have one chromosome,
which is a a big circle, which
reproduce starting at the origin
of replication.
• After DNA is duplicated, the
plasma membrane pinches
inward and a new cell wall grows
between the daughter cells.
Kinases and Cyclins
• Kinases are enzymes that
active or inactive proteins
of the cell cycle
• Cyclins are proteins that
must be attached to
kinases to be active; they
are cyclin dependent
kinases (Cdks)
• MPF “maturationpromotion factor”
triggers G2
How cells grow
• Cells don’t grow if they are over-crowded,
which is called density-dependent inhibition
• Most cells are anchorage dependent which
means that they must be attached to
something to grow
• Cancer cells are NEITHER of these things, they
are cells growing out of control wherever they
want.
Inheritance of Genes
• Genes are segments of DNA. Copies of genes
(with some differences due to crossing over)
are passed from parents to children.
• Gametes are sex cells, eggs and sperm, that
carry genes from one generation to the next.
• During fertilization, gametes unite to form a
zygote, which develops into an embryo, then a
fetus, and then a newborn.
Karyotypes
• A karyotype is a picture of all 23 pairs of chromosomes
(duplicated to be 46) arranged in order from 1  23.
• Homologous chromosomes are pairs; one from Mom
and one from Dad
Types of Chromosomes
• 22 pairs of our chromosomes are autosomes,
non-sex chromosomes.
• 1 pair of our chromosomes are called sex
chromosomes, and determine gender.
• Women = XX
• Men = XY
Diploid vs. Haploid
•
•
•
•
•
•
•
Diploid Cells:
2n
46 chromosomes
Somatic cells (body cells)
2 sets of chromosomes
2n = 46
Skin, nerve, muscle...all
body cells
• Zygotes
•
•
•
•
•
•
Haploid Cells:
n
23 chromosomes
Gametes only
Egg and sperm
1 set of unduplicated
chromosomes
• n=23
• Sex cells
Meiosis
• Meiosis is cell division that reduces the number
of sets of chromosomes from two to one,
creating gametes (eggs and sperm).
• It ONLY happens in the ovaries and testes.
The process of meiosis
• Meiosis happens in two steps, Meiosis I and
Meiosis II.
Stages of Meiosis I:
Prophase I
Metaphase I
Anaphase I
Telophase I and
Cytokinesis
Stages of Meiosis II:
Prophase II
Metaphase II
Anaphase II
Telophase II and
Cytokinesis
Prophase I
• 90% of meiosis
• Chromosomes condense
• Crossing over occurs:
DNA in non-sister
chromatids mix and
match; resulting in
genetic variation of
offspring
• Tetrads are held together
at chiasmata
Crossing Over
• Duplicated homologous
chromosomes connect, this
is called synapsis.
• Pieces of DNA swap, called
crossing over.
• All 4 chromatids together
make a tetrad.
• Each tetrad has at least one
site of chiasma, where
crossing over occurs.
Anaphase I
• The chromosomes begin to move to the poles.
• Sister chromatids remain attached!
Anaphase II
• Centromeres of each chromosome separate,
and sister chromatids start moving apart.
Mitosis vs. Meiosis
Mitosis
• Cells divide once
• No crossing over
• Two daughter cells made
• Daughter cells are identical
to each other and parents
• Daughter cells are 2n
• Occurs in somatic cells
Meiosis
• Cells divide twice
• Crossing over (prophase I)
• Four daughter cells made
• Daughter cells are all
different from each other
and parents
• Daughter cells are n
• Occurs in ovaries/testes
• Makes gametes
Why is crossing over important?
• During Prophase I, crossing over occurs and
produces genetic variation.
• This produces recombinant chromosomes, that
carry genes (DNA) from two different parents
• Powers natural selection/evolution: all individuals
are different and the most fit survive to
reproduce
• Makes species more “hardy,” if bad things
happen, at least some will have adaptations that
help them survive.
9.3 Mendel’s principle of segregation describes the
inheritance of a single characteristic
• From his
experimental data,
Mendel deduced
that an organism
has two genes
(alleles) for each
inherited
characteristic
– One characteristic
comes from each
parent!!!
Figure 9.3A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
P GENERATION
(true-breeding
parents)
Purple flowers
White flowers
All plants have
purple flowers
F1
generation
Fertilization
among F1
plants
(F1 x F1)
F2
generation
3/
of plants
have purple flowers
4
1/
4 of plants
have white flowers
GENETIC MAKEUP (ALLELES)
• A sperm or egg
carries only one
allele of each pair
P PLANTS
Gametes
– The 2 alleles for a
gene separate
during gamete
formation, and each
gamete gets a
different one
PP
pp
All P
All p
F1 PLANTS
(hybrids)
Gametes
All Pp
1/
2
1/
P
P
2
p
P
Eggs
Sperm
PP
– This is the law of
segregation
F2 PLANTS
Phenotypic ratio
3 purple : 1 white
p
p
Pp
Pp
pp
Genotypic ratio
1 PP : 2 Pp : 1 pp
Figure 9.3B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Law of Independent Assortment
• Another law Mendel discovered is the Law of
Independent Assortment which says that each
allele segregates independently from another
(traits aren’t linked unless they are on the
same chromosome)
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Genotype and Phenotype
• A genotype is the genetic make-up of an
individual, expressed in letters. (BB, Bb, bb)
• A phenotype is the physical appearance of an
individual, determined by his or her genotype.
(black, brown, short, tall, etc)
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Purple flowers, P, are dominant to white, p.
• Show a Punnett Square crossing a homozygous
purple flower with a heterozygous purple
flower.
• PP x Pp
• What are the genotypic and phenotypic ratios?
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Incomplete Dominance
• In incomplete dominance, the heterozygous
genotype produces a phenotype that is in
between the dominant and recessive ones.
• For example, if RR makes red flowers, and rr
makes white flowers, then Rr makes PINK
flowers (instead of red).
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Co-Dominance
• Co-Dominance- when both alleles are
expressed in the phenotype, an example blood
type AB.
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Dihybrid Crosses
• Instead of crossing just one trait, dihyrbrid
crosses show the crossing of two separate
traits.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Pedigrees
• Pedigrees are used to trace traits through a
family tree.
• Circles are girls, squares are boys.
• Filled in circles and squares represent
individuals affected by a disease.
• A horizontal line connecting the symbols
represents marriage, vertical lines represent
offspring.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Everything that’s left over…
• A testcross is done to figure out an unknown
genotype. The mystery genotype is crossed
with a homozygous recessive individual.
• A carrier is heterozygous for a disease but does
not show symptoms. They CAN pass it on to
offspring.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Other types of inheritance
• Pleiotropy: Genes can affect more than one
phenotype (sickle-cell and malaria)
• Epistasis: One gene affects how a second gene
is expressed
• Polygenic Inheritance: Many genes affect one
phenotype (skin color)
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Sex-linked diseases
• Any gene located on a sex chromosome is
called a sex-linked gene.
• Examples include color blindness, baldness,
hemophilia, and muscular dystrophy.
• These recessive diseases usually affect men
more than women.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
The importance of chromosomes
• In 1902, the chromosomal theory of inheritance
began to take form, stating: genes have specific
locations (loci) on chromosomes, and you
randomly get one chromosome from each parent.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Linked Genes
• Genes on the same
chromosome tend to
be inherited together
“linked genes”
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
So why do
offspring look
different from
parents?
Independent Assortment
• The phenotypes of the parents are called parental
types.
• The offspring, with new and different phenotypes,
are called recombinant types or recombinants.
• This happens because offspring receive one
chromosome from each parent, and end up looking
different.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Linkage Mapping
• Based on a linkage map, one can assume: the
farther apart 2 genes are, the more likely a
crossover will occur between them, therefore the
recombination frequency is higher.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Linkage Mapping
• A linkage map is a genetic map based on recombination
frequencies.
• Units are called map units and show the distance between genes.
• 1 map unit = a 1% chance of recombination.
• If two genes are 50 map units apart, how likely is recombination?
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Abnormal Chromosome Number
• Nondisjunction is when chromosomes do not separate
correctly during meiosis.
• This causes an abnormal chromosome number, called
aneuploidy
• Trisomy is when you have 3 chromosomes instead of 2
(2n + 1)
• Monosomy is when you have 1 chromosome instead
of 2 (2n – 1)
• Polyploidy is having more than one complete set of
chromosomes
• If any of the above organisms survive to birth, it will
have major developmental abnormalities
Alterations of chromosome structure
• Deletion: chromosomal
fragment is deleted
• Duplication: a
chromosomal fragment is
doubled
• Inversion: chromosomal
fragment gets reversed
• Translocation:
chromosomal fragments
get switched around
What is DNA?
• DNA stands for deoxyribonucleic acid.
• DNA is what makes our genes, and along
with protein, makes our chromosomes.
• It encodes our hereditary information.
• It directs the development of our
anatomical, physiological, and behavioral
traits.
The Structure of DNA
 DNA is a double helix.
 It is a polymer made of
monomers called nucleotides.
 Each nucleotide is made of a
nitrogenous base, a pentose
sugar called deoxyribose, and a
phosphate group.
 The backbone of DNA is called
“sugar phosphate” and has
bases attached to it like rungs
of a ladder.
The Structure of DNA
 DNA is “right handed” and




curves to the right.
Hydrogen bonds hold the
bases together
The 5’ end has a phosphate
group
The 3’ end has an OH group
Strands always line up with
one 5’ strand face up
attached to a 3’ strand
Purines
 Purines are nitrogenous bases with 2 organic rings.
 G and A are purines
Pyrimidines
 Pyrimidines are nitrogenous bases with only 1
organic ring
 Cytosine and thymine
DNA Replication
 DNA replicates during the S phase
of interphase, prior to cell division
(mitosis).
 DNA replication is semi-
conservative, meaning that new
DNA strands are made of one new
daughter strand attached to one old
parent strand.
DNA Replication
 DNA polymerases are special enzymes that add
complementary bases to the unzipped DNA.
DNA Replication
 DNA replication can
ONLY go from 5’ to 3’
 So replication is
antiparallel, one strand
elongates normally, called
the leading strand.
 The other is going away
from the replication fork,
called the lagging strand.
DNA Replication
 As the bubble of
replication grows, the
lagging strand is made bit
by bit in fragments,
called Okazaki fragments.
 These are eventually
joined by an enzyme
called DNA ligase.
From Gene to Protein
• The “Central Dogma of Molecular Biology” is
DNA  RNA  protein
• Meaning that our DNA codes our RNA which
provides instructions for making protein
• Proteins (you may remember) do many things:
structure, support, communication,
transportation, enzymes etc.
Transcription and
Translation
• Transcription is the synthesis of RNA from DNA
• Translation is the synthesis of a polypeptide
(protein) from RNA.
Codons
• Proteins are made of amino acids.
• Each amino acid is coded for by a triplet of
nucleotides called a codon.
• For example, AGT = serine
• There are only 20 amino acids, but 64 codons.
Transcription: DNA to
RNA
• First, RNA Polymerase unzips a strand of DNA.
• Transcription can only go from 5’ to 3’
• RNA Polymerase II attaches to DNA at a
promoter
• The portion of DNA being transcribed is called
a transcription unit
Transcription: DNA to
RNA
• RNA is now synthesized, as base pairs
are added to the unzipped DNA
strand.
• RNA is ribonucleic acid. It is a single
helix. Instead of T (thymine) RNA has
U (uracil).
• So every A in DNA now pairs with U
(instead of T).
• The RNA that is made is called mRNA
which stands for messenger RNA.
RNA splicing
• Some of the RNA isn’t needed to code for
proteins, so it is cut out through RNA splicing.
• The non-coding regions that are cut out are
called introns, the coding portions the cell
needs are called exons.
• Little molecules called small nuclear
ribonucleoproteins, snRNA, join with a
molecule called a spliceosome to slice the
RNA.
Translation: RNA to
protein
• The mRNA now leaves the
nucleus and binds to a
ribosome, where protein
synthesis occurs.
• As it passes through the
ribosome, tRNA (transfer
RNA) molecules, each
carrying an amino acid,
begin to form a long chain of
amino acids.
Translation: RNA to
protein
• At one end of tRNA is a triplet code called an
anticodon which matches the mRNA.
• At the other end of the tRNA is an amino acid.
Translation: RNA to
protein
• The ribosome where this all happens has two
pieces, and is made of proteins and RNA called
ribosomal RNA (rRNA)
• The subunits are called “large” and “small”
Operons
• Genes that can be turned on or off as needed.
• The switch that does this is a segment of DNA
called an operator.
• Along with an operator, there is a promoter and
some enzymes that make up the operon.
• Repressors turn off an operon
• Inducers turn on an operon
Trp operon
• Tryptophan is an amino acid that is usually
produced by the body but can be turned off. This
is a “repressible operon”
Lac operon
• The lac operon is usually off but can be stimulated
(induced) and is therefore called an “inducible operon.”
• The lac operon functions in the digestion of lactose, milk
sugar.
Points of control

The control of gene
expression can occur at any
step in the pathway from
gene to functional protein
1. packing/unpacking DNA
2. transcription
3. mRNA processing
4. mRNA transport
5. translation
6. protein processing
7. protein degradation
AP Biology
1. DNA packing
How do you fit all
that DNA into
nucleus?

DNA coiling &
folding





double helix
nucleosomes
chromatin fiber
looped
domains
chromosome
from DNA double helix to
AP Biology chromosome
condensed
Nucleosomes

8 histone
molecules
“Beads on a string”
1st level of DNA packing
 histone proteins




8 protein molecules
positively charged amino acids
bind tightly to negatively charged DNA
AP Biology
DNA
packing movie
DNA packing as gene control

Degree of packing of DNA regulates transcription

tightly wrapped around histones


no transcription
genes turned off
 heterochromatin
darker DNA (H) = tightly packed
 euchromatin
lighter DNA (E) = loosely packed
H
AP Biology
E
DNA methylation

Methylation of DNA blocks transcription factors


no transcription
 genes turned off
attachment of methyl groups (–CH3) to cytosine


nearly permanent inactivation of genes

AP Biology
C = cytosine
ex. inactivated mammalian X chromosome = Barr body
Histone acetylation

Acetylation of histones unwinds DNA

loosely wrapped around histones



attachment of acetyl groups (–COCH3) to histones


AP Biology
enables transcription
genes turned on
conformational change in histone proteins
transcription factors have easier access to genes
RNA interference

Small interfering RNAs (siRNA)

short segments of RNA (21-28 bases)



bind to mRNA
create sections of double-stranded mRNA
“death” tag for mRNA
 triggers degradation of mRNA

cause gene “silencing”


post-transcriptional control
turns off gene = no protein produced
siRNA
AP Biology
6
7
Gene Regulation
protein
processing &
degradation
1 & 2. transcription
- DNA packing
- transcription factors
5
initiation of
translation
4
mRNA
processing
5. translation
- block start of
translation
2
1
initiation of
transcription
AP Biology mRNA splicing
3
3 & 4. post-transcription
- mRNA processing
- splicing
- 5’ cap & poly-A tail
- breakdown by siRNA
6 & 7. post-translation
- protein processing
- protein degradation
mRNA
4 protection