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Chromosomes reside in the _______. They segregate into _______ cells during cell division. Chromosomes are also responsible for sexual ________ in many species including humans.
- nucleus
- daughter cells
- sexual dimorphism
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Chromosomal description for Eukaryotes vs Prokaryotes
- Eukaryotes: Linear with multiple distinct chromosomes
- Prokaryotes: Circular with a single chromosome (possibly with multiple copies)
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Mode of cellular division for prokaryotes
Binary fission: ("division in half") is a kind of asexual reproduction. It is the most common form of reproduction in prokaryotes and occurs in some single-celled eukaryotes like Amoeba and Paramecium. In binary fission, the fully grown parent cell splits into two halves, producing two new cells.
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The steps of binary fission (3)
- DNA replication
- Chromosome segregation
- Cytokinesis
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Mode of cellular division for Eukaryotes
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During Mitosis, ______ ______ separate and _____ ________ _______ form
- sister chromatids
- two daughter nuclei
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The proper order of events of mitosis
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
- Cytokinesis
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Prophase (3-story) *draw and label
- Chromosomes condense and become visible
- Centrosomes move apart toward opposite poles and generate new microtubules
- Nucleoli begin to disappear
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Prometaphase (3-story) *draw and label
- Nuclear envelope breaks down
- Microtubules from the centrosomes invade the nucleus
- Sister chromatids attach to microtubules from opposite centrosomes
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Metaphase *draw and label
- Chromosomes align on the metaphase plate with sister chromatids facing opposite poles
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Anaphase (2-story) *draw and label
- Centromeres divide
- The now separated sister chromatids move to opposite poles
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Telophase (3-story) *draw and label
- Nuclear membranes and nucleoli re-form
- Spindle fibers disappear
- Chromosomes uncoil and become a tangle of chromatin
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Cytokinesis *draw and label
- The cytoplasm divides, splitting the elongated parent cell into two daughter cells with identical nuclei
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________ _______ ensure correct chromosome separation
Regulatory checkpoints
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Two factors that play into a higher organism cell's decision to divide
- Intrinsic factors: such as conditions within the cell that register a sufficient size for division
- Extrinsic factors: environmental signals such as hormonal cues or contacts with neighboring cells that encourage or restrain division
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Checkpoints
Moments at which the cell evaluates the results of previous steps, allowing the sequential coordination of cell-cycle events
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As a result of regulatory checkpoints, under normal circumstances, the chromosomes replicate _____ they condense, and the doubled chromosomes separate to opposite poles only after correct _______ _______ of sister chromatids ensures ______ distribution to the daughter nuclei
- before
- metaphase alignment
- equal
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Breakdown of _______ machinery can produce division mistakes that have crucial consequences for the cell. Improper chromosome segregation for example, can cause serious ________ or even the _____ of daughter cells.
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Where are the three major checkpoints that regulate the cell cycle (mitosis)
- Figure 4.13 (pg 98)
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Mitotically divining and G0 arrested cells are the so called ______ cells whose descendants continue to make up the _______ of each organism's tissues throughout the lifetime of an individual.
- somatic cells
- majority (vast majority)
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Germ cells
Early in the embryonic developments, these cells are fated for a specialized role in the production of gametes
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Germ cells arise later in plants, during floral development instead of during ___________. The germ cells become incorporated i the ________ organs in animals
- embryogenesis
- reproductive
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The union of haploid gametes at fertilization yields _______ offspring that carry the genetic heritage of ____ ______. Sexual reproduction therefore, requires the alternation of _______ and _______ generations of cells.
- diploid
- two parents
- haploid and diploid
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What would happen if gametes were diploid rather than haploid?
Is this bad and if so what prevents this
- The number of chromosomes would double in each successive generation such that in humans for example, the children would have 92 chromosomes per cell, the grandchildren 184, and so on.
- Meiosis prevents this lethal, exponential accumulation of chromosomes
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In Meiosis, the chromosomes replicate ______ but the nucleus divides ______
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What are the two successive nuclear divisions of meiosis called?
- Division I of meiosis (meiosis I)
- Division II of meiosis (meiosis II)
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Each successive round of division in meiosis consists of:
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
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In meiosis I, the parent nucleus divides to form ____ ______ _______; in meiosis II, each of those divides to form _____ _______
- two daughter nuclei
- four nuclei
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The four nuclei are the ______ ______ of meiosis and become partitioned in four separate daughter cells because _________ occurs after both rounds of division
- final products
- cytokinesis
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Why do gametes contain half the number of chromosomes found in other body cells?
This is because the chromosomes duplicate at the start of meiosis I, but they do not duplicate in meiosis II
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A key to understanding meiosis I is the observation that the centromeres joining these chromatids _____ _____ throughout the entire division
remain intact (as opposed to splitting like in mitosis)
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All phases of Meiosis
- Meiosis I: A reductional Division
- Prophpase I
- Metaphase I
- Anaphase I
- Telophase I
- Interkinesis
- Meiosis II: An equational division
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
- Cytokinesis
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5 phases of Prophase I
- Lepotene
- Zygotene
- Pachytene
- Diplotene
- Diakinesis
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Leptotene-Zygotene-Pachytene (2-2-2-story)
- Leptotene:
- 1) Chromosomes thicken and become visible, but the chromatids remain invisible
- 2) Centrosomes begin to move toward opposite poles
- Zygotene:
- 1) Homologous chromosomes enter synapsis*
- 2) The synaptonemal complex forms
- Pachytene:
- 1) Synapsis is complete
- 2) Crossing over, genetic exchange between nonsister chromatids of a homologous pair, occurs
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Diplotene-Diakinesis (4-2-story)
- Diplotene:
- 1) Synaptonemal complex dissolves
- 2) A tetrad of four chromatids is visible
- 3) Crossover points appear as chiasmata holding nonsister chromatids together
- 4) Meiotic arrest occurs at this time in many species
- Diakinesis:
- 1) Chromatids thicken and shorten
- 2) At the end of prophase I, the nuclear membrane (not shown earlier) breaks down, and the spindle begins to form
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Metaphase I-Anaphase I (3-3-story)
- Metaphase I:
- 1) Tetrads line up along the metaphase plate 2) Each chromosome of a homologous pair attaches to fibers from opposite poles
- 3) Sister chromatids attach to fibers from the same pole
- Anaphase I:
- 1) The centromere does not divide
- 2) The chiasmata dissolve
- 3) Homologous chromosomes move to opposite poles
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Telophase I-Interkinesis (2-2-story)
- Telophase:
- 1) The nuclear envelope re-forms
- 2)Resultant cells have half the number of chromosomes, each consisting of two sister chromatids
- Interkinesis:
- 1) This is similar to interphase with one important exception: No chromosomal duplication takes place
- 2) In some species, the chromosomes decondense; in others, they do not.
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Prophase II-Metaphase II-Anaphase II (3-2-1-story)
- Prohpase II:
- 1) Chromosomes condense
- 2) Centrioles move toward the poles
- 3) The nuclear envelope breaks down at the end of prophase II
- Metaphase II:
- 1) Chromosomes align at the metaphase plate
- 2) Sister chromatids attach to spindle fibers from opposite poles
- Anaphase II:
- 1) Centromeres divide and sister chromatids move opposite poles
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Telophase II-Cytokinesis (2-1-story)
- Telophase II:
- 1) Chromosomes begin to uncoil
- 2) Nuclear envelopes and nucleoili (not shown) re-form
- Cytokinesis:
- 1) The cytoplasm divides, forming four new haploid cells
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Zygotene begins as each chromosome seeks out its ________ partner and the matching chromosomes become zipped together in a process known as _________. What is the "zipper" itself? What does it do?
- homologous partner
- synapsis
- The zipper is an elaborate protein structure called the synapotenemal complex
- The complex aligns the homologs with remarkable precision, juxtaposing the corresponding genetic regions of the chromosome pair
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________ begins at the completion of synapsis when homologous chromosomes are united along their length. Each synapsed chromosome pair is known as a ______ (why), or a ______ (why).
- Pachytene
- bivalent, because it encompasses two chromomses
- tetrad, because it contains four chromatids
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On one side of the bivalent is a ______ derived chromosome, on the other side a ______ derived one. Because X and Y chromosomes are not identical, they do not ______ _______
- maternally
- paternally
- synapse completely
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During pachytene, structures called ________ ________ begin to appear along the synaptonemal complex and an exchange of _______ between nonsister (that is, between ______ and ______) chromatids occurs at these ________.
- recombination nodules
- parts
- maternal and paternal
- nodules
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The exchange of parts between nonsister chromatids is called ______ _____; it results in the ________ of genetic material. What is the result? How much genetic info is lost?
- crossing over
- recombination
- As a result of crossing over, chromatids may no longer be of purely maternal or paternal origin
- However, no genetic information is gained or lost, so all chromatids retain their original size
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Diplotene is signaled by the gradual dissolution of the __________ _______ ________ and a slight ________ of regions of the homologous chromosomes. The aligned homologous chromosomes of each bivalent nonetheless remain very tightly merged at intervals along their length called _______. What does this represent?
- synaptonemal zipper complex
- separation
- chiasmata
- It represents the sites where crossing-over occurred
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Why is meiosis I often called a reductional division?
That is because the number of chromosomes is reduced to one-half, the normal diploid number.
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The interphase between meiosis I and meiosis II
Interkinesis
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Why is meiosis II referred to as an equational (equalizer) division?
That is because at the end of meiosis II, each daughter cell (that is, each gamete) has the same number of chromosomes as the parental cell present at the beginning of this division
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During crossover, informational exchange between sex chromosomes is _____. What purpose does it serve?
- low
- It is critically important for proper separation in meiosis
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Two important functions of crossovers
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Structural importance of crossovers (explain)
Informational importance of crossovers (explain)
- Structural: Links homologous chromosomes during meiosis I
- *Prevents nondisjunction and ensures proper segregation of chromosomes
- Informational: Results in greater genetic variety in gametes
- *After crossovers sister chromatids, otherwise identical, will differ as they may carry parts of the homologous chromosome
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Segregational errors during either meitotic division can lead to abberations, such as ________, in the next generation. State an example
- trisomies
- Nondisjunction: when the homologs of a chromosome pair do not segregate during meiosis I (an error), they may travel together to the same pole and eventually become part of the same gamete
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Two types of trisomy
- Autosomal trisomy: lethal in utero
- Nonlethal: like trisomy 21 *genetic basis of Down syndrome (only exception to lethality)
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The wider the variety of gene combinations, the _____ likely some combinations of alleles will be able to survive in a changing environment
more
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Two aspects of meiosis contribute to genetic diversity in a population
- First, because only chance governs which paternal or maternal homologs migrate to the two poles during the first meiotic division, different gametes carry a different mix of maternal and paternal chromosomes
- Second, the reshuffling of genetic info through crossing-over during prophase I, ensures an even greater amount of genetic diversity in gametes
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In ascaris, the chromosome complement, n = 2, compared to humans, how many kinds of gametes can ascaris produce?
- Ascaris: 22 = 4 types of gametes
- Humans: 223 = over 8 million genetically different kinds of gametes
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Why does each chromosome in each different gamete consist of different combinations of maternal and paternal information
That is because, crossing-over recombines maternally and paternally derived genes
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Another not listed but obvious source of producing genetic diversity. (explain)
Sexual reproduction: At fertilization, any one of a vast number of genetically diverse sperm can fertilize an egg with its own distinctive genetic constitution
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With the exception of _______ ______, the 6 billion people in the world are each genetically unique
identical twins
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5 differences between Mitosis and Meiosis
Mitosis:
- Growth in somatic tissue
- Daughter cells are diploid
- Daughter cells genetically identical
- Homologous chromosomes are independent
- Produces two identical cells
Meiosis:
- Takes place in the germline
- Daughter cells are haploid
- Daughter cells undergo independent assortment
- Homologous chromosomes pair in prophase I
- Produces four distinct cells
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