AP Biology
Chapter 7
AP Biology – Chapter 7
7.1 Different Life Cycles Use Different Modes of Cell Reproduction
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The lifespan of an organism is linked to cell reproduction – usually called cell division
Organisms have to basic strategies for reproducing themselves
o Asexual Reproduction
o Sexual Reproduction
Cell division is also important in growth and repair of tissues
In asexual reproduction, the offspring are clones – genetically identical to the parent
Any genetic variations are due to mutations
A unicellular prokaryote may reproduce itself by binary fission
Single-celled eukaryotes can reproduce by mitosis
Other eukaryotes are also able to reproduce through asexual or sexual means
Sexual reproduction requires gametes – 2 parents each contribute one gamete to an offspring
Gametes form by meiosis – a process of cell division
Gametes and offspring differ genetically from each other and from the parents
DNA in eukaryotic cells is organized into chromosomes
A chromosome consists of a single molecule of DNA and proteins
Somatic Cells – body cells not specialized for reproduction
Each somatic cell contains 2 sets of chromosomes (homologs) that occur in homologous pairs
Gametes contain only one set of chromosomes – one homolog from each pair
Haploid cell – number of chromosomes = n
Fertilization – 2 haploid gametes (female egg and male sperm) fuse to form a zygote
Chromosome number in zygote = 2n and cells are diploid
All kinds of sexual life cycles involve meiosis
o Haplontic life cycle – in protists, fungi, and some algae; the zygote is only diploid stage
 After zygote forms it undergoes meiosis to form haploid spores, which
germinate to form a new organism
 Organism is haploid and produces gametes by mitosis – cells fuse to form
diploid zygote
o Alternation of Generations – most plants, some protists; meiosis gives rise to haploid
spores
 Spores divide by meiosis to form the haploid generation (gametophyte)
 Gametophyte forms gametes by mitosis
 Gametes then fuse to form diploid zygote (sporophyte), which in turn produces
haploid spores by meiosis
o Diplontic Life cycle – animals and some plants; gametes are the only haploid stage
 A mature organisms is diploid and produces gametes by meiosis
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AP Biology
Chapter 7
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Gametes fuse to form diploid zygote; zygote divides by mitosis to form mature
organism
The essence of sexual reproduction is that it allows the random selection of half the diploid
chromosome set
This forms a haploid gamete that fuses with another to make a diploid cell
Thus no two individuals have exactly the same genetic makeup
7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
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Four events must occur for cell division
o Reproductive signal – to initiate cell division
o Replication of DNA
o Segregation – distribution of DNA into the 2 new cells
o Cytokinesis – division of the cytoplasm and separation of the 2 new cells
In prokaryotes, cell division results in reproduction of entire organism
Most prokaryotes have one chromosome, a single molecule of DNA (usually circular)
o 2 important regions in the reproduction
 Ori – where replications starts
 Ter – where replication ends
Eukaryotic cells divide by mitosis followed by cytokinesis
Replication of DNA occurs as long as strands are threaded through replication complexes
DNA replication only occurs during a specific stage of the cell cycle (s phase)
In segregation of DNA after cell division, one copy of each chromosome ends up in each of the 2
new cells
In eukaryotes, the chromosomes become highly condensed
Mitosis segregates them into 2 new nuclei – the cytoskeleton is involved in the process
Cytokinesis follows mitosis
The process in plant cells (which have cell walls) is different than in animal cells
The cell cycle – period between cell divisions
In eukaryotes it is divided into mitosis and cytokinesis – called the M phase – and a long
interphase
During interphase, the cell nucleus is visible and cell functions including replication occur
Interphase begins after cytokinesis and ends when mitosis starts
Interphase has 3 subphases: G1, S, G2
o G1 (Gap 1) – variable, a cell may spend a long time in this phase carrying out its
functions
o S phase (synthesis) – DNA is replicated
o G2 (Gap2) – the cell prepares for mitosis, synthesizes microtubules for segregating
chromosomes
During interphase, only the nuclear envelope and the nucleolus are visible
o The chromatin (DNA) is not yet condensed
3 structures appear in prophase
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AP Biology
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Chapter 7
o The condensed chromosomes
o Centrosome
o Spindle
Condensed chromosomes appear during prophase
Sister chromatids – 2 DNA molecules on each chromosome after replication
Centromere – region where chromatids are joined
Kinetochores are protein structures on the centromeres and are important for chromosome
movement
The karyotype of an organism reflects the number and sizes of its condensed chromosomes
Karyotype analysis can be used to identify organisms, but DNA sequence is more commonly
used
Segregation is aided by other structures:
o The centrosome determines the orientation of the spindle apparatus
o Each centrosome can consist of 2 centrioles – hollow tubes formed by microtubules
o Centrosome is duplicated during S phase and each moves towards opposite sides of the
nucleus
Centrosomes serve as mitotic centers or poles, the spindle forms btwn the poles from 2 types of
microtubules
o Polar microtubules form a spindle and overlap in the center
o Kinetochore microtubules – attach to kinetochores on chromatids. Sister chromatids
attach to opposite halves of the spindle
Chromosome separation and movement is highly organized
During prometaphase, the nuclear envelope breaks down
o Chromosomes consisting of 2 chromatids attach to the kinetochore microtubules
During metaphase, chromosomes line up at the midline of the cell
During Anaphase, the separation of sister chromatids is controlled by M phase cyclinCdk,
cohesin is hydrolyzed by separase
After separation, they move to opposite ends of the spindle and are referred to as daughter
chromosomes
Telophase occurs after chromosomes have separated
o Spindle breaks down
o Chromosomes uncoil
o Nuclear envelope and nucleoli appear
o 2 daughter nuclei are formed with identical genetic info
Cytokinesis
o Division of cytoplasm differs in plant and animal cells
o In animal cells, the plasma membrane pinches btwn the nuclei because of a contractile
ring of microfilaments of actin and myosin
After Cytokinesis
o Each daughter cell contains all of the components of a complete cell
o Chromosomes are precisely distributed
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AP Biology
Chapter 7
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The orientation of cell division is important to development, but organelles are not
always evenly distributed
The reproductive rates of most prokaryotes respond to environmental conditions
In eukaryotes, cell division is related to the needs of the entire organism
Cells divide in response to extracellular signals, like growth factors
7.3 Cell Reproduction is Under Precise Control
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The eukaryotic cell cycle has 4 stages: G1, S, G2, and M
Progression is tightly regulated – the G1 –> S transition is called R, the restriction point
Passing this point usually means the cell will proceed with the cell cycle and divide
Specific signals trigger the transition from one phase to another
Evidence for substances as triggers come from cell fusion experiments
o Nuclei in cells at different stages are fused together by polyethylene glycol, and both
entered the phase of DNA replication(s)
Transitions also depend on activation of Cylclin-dependent kinases (Cdk’s)
o A protein kinase is an enzyme that catalyzes phosphorylation from ATP to a protein
o Phosphorylation changes the shape and function of a protein by changing it charges
o Cdk is activated by binding to Cyclin (by allosteric regulation); this alters its shape and
exposes its active site
The G1 – S cyclin-Cdk complex acts as a protein kinase and triggers transition from G1 to S
Other cyclin-Cdk’s act at different stages of the cell cycle, called cell cycle checkpoints
Example of G1-S cyclin-Cdk regulation:
o Progress past the restriction point in G1 depends on retinoblastoma protein (RB)
o RB normally inhibit the cell cycle, but when phosphorylated by G1-S cyclin-Cdk, RB
becomes inactive and no longer blocks the cell cycle
7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity
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Meiosis consists of 2 nuclear divisions but DNA is replicated only once. The function of Meiosis is
to:
o Reduce the chromosome number from diploid to haploid
o Ensure that each haploid has a complete set of chromosomes
o Generate diversity among the products
Meiotic division reduces the chromosome number
o In Meiosis I, homologous pairs of chromosomes come together and line up along their
entire lengths
o After metaphase I, the homologous chromosome pairs separate, but individual
chromosomes made up of 2 sister chromatids remain together
Sister chromatids separate during meiosis II, which is NOT proceeded by DNA replication
The products of meiosis I and II are four cells with a haploid number of chromosomes
These four cells are NOT genetically identical
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AP Biology
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Chapter 7
2 processes may occur
o Crossing over
o Independent assortment
In prophase of Meiosis I homologous chromosomes pair by Synapsis
The four chromatids of each pair of chromosomes form a tetrad, or bivalent
The homologs seem to repel each other but are held together at Chiasmata
Crossing over is an exchange of genetic material that occurs at the chiasma
Crossing over results in recombinant chromatids and increases genetic variability of the
products
7.4
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Meiotic errors
o Nondisjunction: homologous pairs fail to separate at Anaphase I – sister chromatids fail
to separate, or homologous chromosomes may not remain together
o Either results in aneuploidy, which is when there are too many or not enough
chromosomes
o Organisms with triploid (3n), tetraploid (4n), and even higher levels are polyploid
 This can occur through an extra round of DNA duplication before meiosis, or the
lack of spindle formation in meiosis II
 Polyploidy occurs naturally in some species, and can be desirable in plants
o If crossing over happens btwn non-homologous chromosomes, the result is a
translocation
o A piece of chromosome may rejoin another chromosome, and its location can have
profound effects on the expression of other genes
 Example: Leukemia
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