Big Idea 3 Review: Living systems store, retrieve, transmit, and respond to
information essential to life processes.
Explaining how the structural features of DNA and RNA allow heritable information to be replicated,
stored expressed, and transmitted to future generations.
Explaining how the steps in the cell cycle allow transmission of heritable information between
generations and contribute to genetic diversity.
Meiosis reduces the number of chromosome sets from diploid to haploid
Like mitosis, meiosis is preceded by the replication of chromosomes
Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II
The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis
Each daughter cell has only half as many chromosomes as the parent cell
In the first cell division (meiosis I), homologous chromosomes separate
Meiosis I results in two haploid daughter cells with replicated chromosomes
In the second cell division (meiosis II), sister chromatids separate
Meiosis II results in four haploid daughter cells with unreplicated chromosomes
Three events are unique to meiosis, and all three occur in meiosis l:
Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange
genetic information
At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual
replicated chromosomes
At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate and are
carried to opposite poles of the cell
Evolutionary Significance of Genetic Variation Within Populations
Natural selection results in accumulation of genetic variations favored by the environment
Sexual reproduction contributes to the genetic variation in a population, which ultimately results from
mutations
Genetic variation produced in sexual life cycles contributes to evolution
Mutations (changes in an organism’s DNA) are the original source of genetic diversity
Mutations create different versions of genes
Reshuffling of different versions of genes during sexual reproduction produces genetic variation
Origins of Genetic Variation Among Offspring
The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation
that arises in each generation
Three mechanisms contribute to genetic variation:
Independent assortment of chromosomes
Crossing over
Random fertilization
Independent Assortment of Chromosomes
Homologous pairs of chromosomes orient randomly at metaphase I of meiosis
In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into
daughter cells independently of the other pairs
The number of combinations possible when chromosomes assort independently into gametes is 2n,
where n is the haploid number
For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes
Random Fertilization
Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized
egg)
The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations
Crossing over adds even more variation
Each zygote has a unique genetic identity
Evolutionary Significance of Genetic Variation Within Populations
Natural selection results in accumulation of genetic variations favored by the environment
Sexual reproduction contributes to the genetic variation in a population, which ultimately results from
mutations
Using at least two commonly used technologies, describe how humans manipulate heritable information
and possible consequences.
Determining Gene Function
One way to determine function is to disable the gene and observe the consequences
Using in vitro mutagenesis, mutations are introduced into a cloned gene, altering or destroying its
function
When the mutated gene is returned to the cell, the normal gene’s function might be determined by
examining the mutant’s phenotype
In nonmammalian organisms, a simpler and faster method, RNA interference (RNAi), has been used to
silence expression of selected genes
Identifying mathematical evidence that supports the roles of chromosomes and fertilization in the
passage of traits from parent to offspring. Justify the effects of a change in the cell cycle mitosis and/or
meiosis will have on chromosome structure, gamete viability, genetic diversity, and evolution
Predict possible effects that alterations in the normal process of meiosis will have on the phenotypes of
offspring compared to the normal situation and connect the outcomes to issues surrounding human
genetic diseases.
Down’s syndrome (trisomy 21), Kleinfelter’s syndrome (XXY), Turner’s syndrome (XO)
Non-disjunction
Failure of chromosome pairs to separate during meiosis
Results in gametes with too many or too few chromosomes
-Aneuploidy: abnormal # of a certain chromosome
-Polyploidy: more than 2 complete chromosome sets
An embryo needs at least one X chromosome to survive
Justifying whether a given data set supports Mendelian inheritance. Apply mathematical routines to
determine Mendelian patterns of inheritance provided by data sets, and, using appropriate examples,
explain at the chromosome, cellular, and offspring (organism) levels why certain traits do or do not
follow Mendel’s model of inheritance
Extending Mendelian Genetics for a Single Gene
Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following
situations:
When alleles are not completely dominant or recessive
When a gene has more than two alleles
When a gene produces multiple phenotypes
Pleiotropy
Most genes have multiple phenotypic effects, a property called pleiotropy
For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary
diseases, such as cystic fibrosis and sickle-cell disease
Using appropriate examples, explaining how gene regulation allows for cell specialization and efficient
cell function. Justify how various modes of gene regulation (positive and negative) can explain the
differences seen at the cellular, organismal, and population level. Predict how changes in regulation will
affect cellular functions
Different cell types result from differential gene expression in cells with the same DNA
Differences between cells in a multicellular organism come almost entirely from gene expression, not
differences in the cells’ genomes
These differences arise during development, as regulatory mechanisms turn genes off and on
Differential Gene Expression
Differences between cell types result from differential gene expression, the expression of different
genes by cells within the same genome
In each type of differentiated cell, a unique subset of genes is expressed
Many key stages of gene expression can be regulated in eukaryotic cells
Regulation of Transcription Initiation
Chromatin-modifying enzymes provide initial control of gene expression by making a region of DNA
either more or less able to bind the transcription machinery
Organization of a Typical Eukaryotic Gene
Associated with most eukaryotic genes are control elements, segments of noncoding DNA that help
regulate transcription by binding certain proteins
Control elements and the proteins they bind are critical to the precise regulation of gene expression in
different cell types
The Roles of Transcription Factors
To initiate transcription, eukaryotic RNA polymerase requires the assistance of proteins called
transcription factors
General transcription factors are essential for the transcription of all protein-coding genes
In eukaryotes, high levels of transcription of particular genes depend on control elements interacting
with specific transcription factors
Some transcription factors function as repressors, inhibiting expression of a particular gene
Some activators and repressors act indirectly by influencing chromatin structure
Operons: The Basic Concept
In bacteria, genes are often clustered into operons, composed of
An operator, an “on-off” switch
A promoter
Genes for metabolic enzymes
An operon can be switched off by a protein called a repressor
A corepressor is a small molecule that cooperates with a repressor to switch an operon off
Repressible and Inducible Operons: Two Types of Negative Gene Regulation
A repressible operon is one that is usually on; binding of a repressor to the operator shuts off
transcription
The trp operon is a repressible operon
An inducible operon is one that is usually off; a molecule called an inducer inactivates the repressor and
turns on transcription
The classic example of an inducible operon is the lac operon, which contains genes coding for enzymes
in hydrolysis and metabolism of lactose
Using an appropriate example, describing a signal transduction pathway mechanism that affects protein
expression.
Local and Long-Distance Signaling
Cells in a multicellular organisms communicate by chemical messengers
Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells
In local signaling, animal cells may communicate by direct contact
Local and Long-Distance Signaling
Cells in a multicellular organisms communicate by chemical messengers
Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells
In local signaling, animal cells may communicate by direct contact
Describing the basic processes by which a change in a DNA sequence results in a change in a peptide
sequence.
Point mutations can affect protein structure and function
Mutations are changes in the genetic material of a cell or virus
Point mutations are chemical changes in just one base pair of a gene
The change of a single nucleotide in a DNA template strand leads to production of an abnormal protein
Types of Frame-shift Mutations
mutations within a gene can be divided into two general categories
Base-pair insertions
Base-pair deletions
Describing two processes that increase genetic variation and explaining how genetic variation allows for
natural selection within a population
Mutations, crossing over, independent assortment, sexual reproduction
Describing several mechanisms that result in increased genetic variation and rapid evolution of viruses.
RNA viruses have no “proof-reading” of their nucleotides so mutations go unchanged; proviruses can
pick up DNA from their host and incorporate it into their genome
Describing how both plants and animals use cell-to-cell communication for cellular processes using an
appropriate example from each.
Insulin/glucagon in animals
De-etiolation, phototropism, photoperiodism in plants
Signal transduction pathways link signal reception to response
Plants have cellular receptors that detect changes in their environment
For a stimulus to elicit a response, certain cells must have an appropriate receptor
A potato left growing in darkness produces shoots that look unhealthy and lacks elongated roots
These are morphological adaptations for growing in darkness, collectively called etiolation
After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow
normally
A potato’s response to light is an example of cell-signal processing
The stages are reception, transduction, and response
Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many
developmental responses
Explaining key features of models that illustrate how changes in a signal pathway can alter cellular
responses Construct a model that illustrates how chemical signals can alter cellular responses. Predict
the effects of changes in the signal pathway on cellular responses using appropriate examples.
Type I diabetes mellitus (insulin-dependent) is an autoimmune disorder in which the immune system
destroys pancreatic beta cells
Type II diabetes mellitus (non-insulin-dependent) involves insulin deficiency or reduced response of
target cells due to change in insulin receptors
Invertebrate regulatory systems also involve endocrine and nervous system interactions
Diverse hormones regulate homeostasis in invertebrates
In insects, molting and development are controlled by three main hormones:
Brain hormone stimulates release of ecdysone from the prothoracic glands
Ecdysone promotes molting and development of adult characteristics
Juvenile hormone promotes retention of larval characteristics
Describing how behavior is modified in response to external and internal cues for both animals and
plants using appropriate examples from each.
In male stickleback fish, the stimulus for attack behavior is the red underside of an intruder
When presented with unrealistic models, as long as some red is present, the attack behavior occurs
Dietary Influence on Mate Choice Behavior
An example of environmental influence is the role of diet in mate selection by Drosophila mojavensis
Experiments have demonstrated that food eaten by larvae influences later mate choice in females
It has been proposed that the physiological basis for the observed mate preferences was differences in
hydrocarbons in the exoskeletons of the flies
Chemical Communication
Many animals that communicate through odors emit chemical substances called pheromones
When a minnow or catfish is injured, an alarm substance in the fish’s skin disperses in the water,
inducing a fright response among fish in the area
Gravity
Response to gravity is known as gravitropism
Roots show positive gravitropism
Stems show negative gravitropism
Mechanical Stimuli
The term thigmomorphogenesis refers to changes in form that result from mechanical perturbation
Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls
Thigmotropism is growth in response to touch
It occurs in vines and other climbing plants
Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical
impulses called action potentials
During drought, plants respond to water deficit by reducing transpiration
Deeper roots continue to grow
Heat-shock proteins help plants survive heat stress
Altering lipid composition of membranes is a response to cold stress
Plants defend themselves against herbivores and pathogens
Plants counter external threats with defense systems that deter herbivory and prevent infection or
combat pathogens
Describing how the nervous system detects external and internal stimuli and transmits signals along and
between nerve cells Describe how changes within nerve cells and the nervous system produce
responses to the stimuli
The Resting Potential
Resting potential is the membrane potential of a neuron that is not transmitting signals
Resting potential depends on ionic gradients across the plasma membrane
Concentration of Na+ is higher in the extracellular fluid than in the cytosol
The opposite is true for K+
Production of Action Potentials
Depolarizations are usually graded only up to a certain membrane voltage, called the threshold
A stimulus strong enough to produce depolarization that reaches the threshold triggers a response
called an action potential
An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane
It carries information along axons
Voltage-gated Na+ and K+ channels are involved in producing an action potential
When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell
As the action potential subsides, K+ channels open, and K+ flows out of the cell
During the refractory period after an action potential, a second action potential cannot be initiated
Conduction of Action Potentials
An action potential can travel long distances by regenerating itself along the axon
At the site where the action potential is generated, usually the axon hillock, an electrical current
depolarizes the neighboring region of the axon membrane
Neurons communicate with other cells at synapses
In an electrical synapse, current flows directly from one cell to another via a gap junction
The vast majority of synapses are chemical synapses
In a chemical synapse, a presynaptic neuron releases chemical neurotransmitters stored in the synaptic
terminal
When an action potential reaches a terminal, the final result is release of neurotransmitters into the
synaptic cleft
Direct Synaptic Transmission
Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels
Neurotransmitter binding causes ion channels to open, generating a postsynaptic potential
After release, the neurotransmitter diffuses out of the synaptic cleft
It may be taken up by surrounding cells and degraded by enzymes