Meiosis and Sexual Reproduction
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Which cellular process contributes most significantly to the genetic variation observed in offspring resulting from sexual reproduction, assuming no mutation occurs?

  • The fusion of somatic cells from two parents, creating a diploid offspring with a combined genetic makeup.
  • The precise replication of DNA during gamete formation, ensuring each gamete receives an identical copy of the parent's genetic information.
  • The equal division of the cytoplasm during cytokinesis following both mitosis and meiosis.
  • The random alignment and subsequent segregation of homologous chromosomes during meiosis I. (correct)

A researcher is studying a newly discovered organism that reproduces both sexually and asexually. Under what environmental condition would the organism most likely favor sexual reproduction over asexual reproduction?

  • When the population size is small, to efficiently increase the number of individuals without requiring a mate.
  • During periods of rapid environmental change, to increase genetic diversity and adaptability within the population. (correct)
  • In a stable and resource-rich environment, to rapidly colonize available habitats with genetically uniform offspring.
  • When the organism is well-adapted to its current environment, ensuring the continuation of successful traits.

Which statement accurately contrasts asexual and sexual reproduction in terms of genetic diversity and evolutionary potential?

  • Both asexual and sexual reproduction lead to similar levels of genetic diversity, providing equal opportunities for adaptation and evolution.
  • Asexual reproduction results in clones, which are genetically uniform and have limited adaptive potential, while sexual reproduction generates diverse offspring with greater evolutionary potential. (correct)
  • Asexual reproduction generates high genetic diversity, enabling rapid adaptation to changing environments, while sexual reproduction produces genetically uniform populations.
  • Sexual reproduction produces offspring with identical genetic information to the parent, limiting adaptation, while asexual reproduction allows for greater evolutionary potential.

How does meiosis contribute to maintaining the normal chromosome number across generations in sexually reproducing organisms?

<p>By reducing the chromosome number in gametes from diploid to haploid, which, upon fertilization, restores the diploid number in the zygote. (B)</p> Signup and view all the answers

If a somatic cell in an organism contains 50 chromosomes, how many chromosomes would be present in a gamete of that same organism, assuming normal meiosis?

<p>25 (B)</p> Signup and view all the answers

During meiosis I, what is the significance of the tetrad formation in prophase I?

<p>It precisely aligns homologous chromosomes gene by gene, enabling crossing over to occur. (D)</p> Signup and view all the answers

How does the outcome of meiosis differ fundamentally from the outcome of mitosis regarding chromosome number and genetic diversity?

<p>Meiosis produces four haploid cells with increased genetic variation, while mitosis produces two diploid cells identical to the parent cell. (B)</p> Signup and view all the answers

What cellular process occurs during interphase before both meiosis and mitosis, and how does it impact the genetic material?

<p>DNA replication doubles the amount of DNA, creating identical sister chromatids. (B)</p> Signup and view all the answers

If a diploid cell with 20 chromosomes undergoes meiosis, how many chromosomes will be present in each daughter cell after meiosis II, assuming no errors occur?

<p>10 chromosomes (B)</p> Signup and view all the answers

During crossing over, a segment of DNA is exchanged between two non-sister chromatids. What is the most significant consequence of this exchange?

<p>Increasing genetic variation by creating new combinations of alleles. (B)</p> Signup and view all the answers

What distinguishes prophase I of meiosis from prophase in mitosis?

<p>Synapsis and crossing over occur in prophase I of meiosis but not in prophase of mitosis. (B)</p> Signup and view all the answers

A researcher discovers a chemical that prevents synapsis during meiosis. What is the most likely consequence of this disruption?

<p>Improper alignment of homologous chromosomes, leading to errors in crossing over and segregation. (D)</p> Signup and view all the answers

How does the arrangement of chromosomes during metaphase I contribute to genetic diversity?

<p>It allows for the random alignment of homologous chromosomes, leading to different combinations of maternal and paternal chromosomes in each daughter cell. (A)</p> Signup and view all the answers

What is the most significant outcome regarding chromosome behavior that differentiates anaphase I of meiosis from anaphase of mitosis?

<p>Homologous pairs separate, but sister chromatids remain joined in anaphase I, unlike in anaphase of mitosis where sister chromatids separate. (D)</p> Signup and view all the answers

If a researcher is studying genetic diversity, why might they focus on prophase I of meiosis?

<p>Because homologous chromosomes undergo synapsis and crossing over, increasing genetic diversity. (B)</p> Signup and view all the answers

What would be the most likely consequence if cytokinesis did not occur at the end of meiosis II?

<p>The cell would contain one nucleus with twice the haploid number of chromosomes. (D)</p> Signup and view all the answers

In what way does the alignment of chromosomes at the metaphase plate during metaphase I of meiosis contribute to genetic diversity?

<p>It allows for the random and independent assortment of maternal and paternal chromosomes. (C)</p> Signup and view all the answers

A cell entering meiosis has 32 chromosomes. How many chromosomes and chromatids will each daughter cell have after telophase II?

<p>16 chromosomes and 16 chromatids (B)</p> Signup and view all the answers

Which of the following is a direct result of the genetic variation produced during meiosis?

<p>Enhanced potential for evolutionary adaptation in sexually reproducing organisms. (B)</p> Signup and view all the answers

During meiosis II, what is the state of the sister chromatids, and how are they attached to the spindle apparatus during metaphase II?

<p>Genetically different and attached to microtubules from opposite poles. (B)</p> Signup and view all the answers

How does the outcome of meiosis correlate with the need for maintaining a stable chromosome number across generations in sexually reproducing organisms?

<p>Meiosis produces haploid gametes, which fuse during fertilization to restore the diploid number. (B)</p> Signup and view all the answers

During DNA replication, what would be the most likely consequence if topoisomerase were non-functional?

<p>The replication fork would stall due to excessive torsional strain, preventing further DNA synthesis. (B)</p> Signup and view all the answers

Why is the semiconservative nature of DNA replication crucial for maintaining genetic information across generations?

<p>It ensures that each new DNA molecule contains one original strand, which serves as a template for accurate replication. (C)</p> Signup and view all the answers

How would the function of helicase be directly affected if a cell were depleted of ATP?

<p>Helicase would be unable to unwind the DNA double helix, halting DNA replication. (B)</p> Signup and view all the answers

Considering the antiparallel nature of DNA strands, what challenge does this present during replication, and how is it overcome?

<p>It necessitates continuous synthesis on one strand and discontinuous synthesis on the other, addressed by Okazaki fragments. (C)</p> Signup and view all the answers

Given that DNA replication starts at the origins of replication, what would likely happen if a eukaryotic chromosome had only one origin of replication?

<p>Replication would take an extremely long time, and the cell cycle could be significantly delayed. (D)</p> Signup and view all the answers

If a mutation occurred that disabled the 3' to 5' exonuclease activity of DNA polymerase, what would be the most direct consequence?

<p>Increased mutation rate due to the accumulation of uncorrected errors. (D)</p> Signup and view all the answers

If a scientist introduces a non-hydrolyzable analog of ATP into a cell undergoing DNA replication, what immediate effect would you expect to observe?

<p>Inhibition of helicase, preventing the unwinding of the DNA double helix. (A)</p> Signup and view all the answers

During DNA replication, short RNA sequences are used as primers. What would be the most likely outcome if these RNA primers were not removed and replaced with DNA?

<p>The integrity of the genome would be compromised due to the presence of RNA within DNA. (B)</p> Signup and view all the answers

In what way does the arrangement of chromosomes during metaphase I of meiosis contribute to genetic diversity?

<p>It allows for the independent assortment of maternal and paternal chromosomes. (A)</p> Signup and view all the answers

Which of the following events uniquely characterizes anaphase I of meiosis, distinguishing it from anaphase of mitosis?

<p>The halving of the chromosome number in the resulting cells. (C)</p> Signup and view all the answers

How would the disruption of chiasmata formation during prophase I affect meiosis?

<p>It would prevent genetic recombination between homologous chromosomes and potentially lead to improper chromosome segregation. (D)</p> Signup and view all the answers

What is the immediate consequence if a cell skips prophase I during meiosis?

<p>Homologous chromosomes will not pair or exchange genetic material, potentially leading to segregation errors and reduced genetic diversity. (C)</p> Signup and view all the answers

If a diploid cell with 2n = 8 chromosomes undergoes meiosis, how many chromosomes will each daughter cell have after telophase I?

<p>4 (D)</p> Signup and view all the answers

How does the behavior of sister chromatids in meiosis I differ from their behavior in meiosis II?

<p>Sister chromatids separate during anaphase II but remain together during anaphase I. (C)</p> Signup and view all the answers

In a cell undergoing meiosis, what might be the result if the kinetochores of sister chromatids attach to microtubules from the same pole during metaphase I?

<p>One daughter cell would receive both homologous chromosomes, while the other would receive none, leading to aneuploidy. (D)</p> Signup and view all the answers

What evolutionary advantage is most directly derived from the events of meiosis I, specifically crossing over and independent assortment?

<p>Greater genetic diversity within a population. (B)</p> Signup and view all the answers

Why are telomeres essential for maintaining the integrity of linear eukaryotic chromosomes?

<p>They compensate for the shortening of DNA during replication by providing non-coding repetitive sequences, preventing loss of essential genes. (B)</p> Signup and view all the answers

Considering the antiparallel nature of DNA strands, what would be the most likely consequence if DNA ligase were non-functional during DNA replication?

<p>Okazaki fragments on the lagging strand would not be joined, resulting in fragmented DNA. (D)</p> Signup and view all the answers

How does the packaging of DNA into chromatin affect gene expression in eukaryotic cells?

<p>More highly packaged DNA reduces gene expression by limiting the access of transcription enzymes to the DNA. (C)</p> Signup and view all the answers

What is the fundamental difference in chromosome structure between bacterial and eukaryotic cells?

<p>Bacterial chromosomes are circular DNA molecules with little protein, whereas eukaryotic chromosomes are linear DNA molecules associated with a large amount of protein. (A)</p> Signup and view all the answers

Considering the unidirectional activity of DNA polymerase, why is the lagging strand synthesized in Okazaki fragments?

<p>Because DNA polymerase can only add nucleotides to the 3' end of an existing strand, and the lagging strand runs in the 3' to 5' direction. (D)</p> Signup and view all the answers

Which evolutionary constraint necessitates the use of RNA primers by DNA polymerase during DNA replication?

<p>DNA polymerase can only add nucleotides to an existing 3'-OH group, which is initially provided by the RNA primer. (C)</p> Signup and view all the answers

How might telomerase activity contribute to the development of cancer?

<p>By preventing telomere shortening, which allows cancer cells to bypass normal cellular senescence and continue dividing indefinitely. (A)</p> Signup and view all the answers

What would be the direct consequence if a mutation rendered DNA polymerase unable to differentiate between ribonucleotides and deoxyribonucleotides?

<p>New DNA strands would incorporate RNA nucleotides, leading to instability and degradation of the DNA. (B)</p> Signup and view all the answers

How would the suppression of shugoshin protein function during meiosis II most likely affect the resulting daughter cells?

<p>Sister chromatids would separate prematurely, leading to an uneven distribution of genetic material in daughter cells. (A)</p> Signup and view all the answers

If a researcher discovers a mutant cell line where independent assortment does not occur during meiosis I, what is the most likely direct consequence?

<p>The genetic variation among the resulting gametes will be reduced. (D)</p> Signup and view all the answers

A cell with a diploid number of 2n=6 undergoes meiosis. At the end of telophase I, how many chromosomes and chromatids are present in each daughter cell?

<p>3 chromosomes, 6 chromatids (A)</p> Signup and view all the answers

What is the functional significance of the reduction in chromosome number from diploid to haploid during meiosis?

<p>It prevents the doubling of chromosome number after each round of fertilization. (B)</p> Signup and view all the answers

If a drug prevents cytokinesis from occurring after telophase II, what would be the most likely outcome for the affected cell?

<p>A single cell with four nuclei, each containing a haploid set of chromosomes, would form. (C)</p> Signup and view all the answers

In a human karyotype, chromosomes are arranged in homologous pairs. What is the primary criterion used to classify chromosomes into these pairs?

<p>The similarity of their banding patterns, gene loci, and centromere positions. (D)</p> Signup and view all the answers

During meiosis in a diploid organism, when do homologous chromosomes separate, and what is the significance of this separation?

<p>They separate during anaphase I, reducing the chromosome number from diploid to haploid. (C)</p> Signup and view all the answers

A researcher is analyzing a karyotype and observes an individual has 47 chromosomes, including two X chromosomes and one Y chromosome (XXY). Which condition does this karyotype indicate?

<p>Klinefelter syndrome, characterized by male infertility and the presence of female secondary sexual characteristics. (C)</p> Signup and view all the answers

Considering a gene located on an autosome, what is the probability that a specific allele from the paternal grandfather will be present in a sperm cell produced by his grandson?

<p>25%, Assuming no crossing over, the allele has a predictable chance of inheritance. (C)</p> Signup and view all the answers

During what stage of meiosis does crossing over typically occur, and what is its primary effect on the genetic makeup of the resulting gametes?

<p>Prophase I; it increases genetic variation by creating new combinations of alleles. (C)</p> Signup and view all the answers

If a scientist discovers a new species with a diploid number of 2n = 16, but some individuals exhibit a trisomy for one chromosome. How many chromosomes would be present in the somatic cells of a trisomic individual of this species?

<p>17 (B)</p> Signup and view all the answers

A researcher is comparing karyotypes from different cells of the same organism. Which of the following variations would most likely indicate a significant genetic abnormality rather than normal variation?

<p>The presence of an extra chromosome or the absence of one in some cells. (D)</p> Signup and view all the answers

How does the arrangement of chromosomes in a karyotype directly facilitate the diagnosis of genetic disorders, such as translocations or aneuploidies, at the chromosomal level?

<p>By visualizing the size, shape, and number of chromosomes in a standardized format. (B)</p> Signup and view all the answers

A plant species exists in two varieties: one with a dominant allele for disease resistance and another with a recessive allele making it susceptible. If a farmer cultivates both varieties in adjacent fields, what evolutionary outcome is least likely to occur over many generations, assuming no other evolutionary forces are acting on the populations?

<p>The two populations will undergo reproductive isolation, eventually leading to speciation. (A)</p> Signup and view all the answers

A population of birds colonizes a new island. Initially, the population exhibits a wide range of beak sizes. Over several generations, a severe drought occurs, resulting in a scarcity of small, soft seeds. What evolutionary change is most likely to occur in the bird population's beak size distribution?

<p>The average beak size will increase, and the range of beak sizes will narrow, exhibiting directional selection. (A)</p> Signup and view all the answers

Consider a scenario where a population of insects is exposed to a novel pesticide. Initially, a small fraction of the population possesses a gene conferring resistance. What is the most likely long-term outcome if the pesticide is continuously applied, and the resistant insects have a slightly lower reproductive rate than susceptible insects in the absence of the pesticide?

<p>The frequency of the resistance gene will increase, but stabilize at an intermediate equilibrium due to the fitness cost of resistance. (B)</p> Signup and view all the answers

In a population of plants, flower color is determined by a single gene with two alleles: R (red) and r (white). Red flowers (RR or Rr) are pollinated more frequently by bees than white flowers (rr). However, red flowers are also more susceptible to a fungal disease. What evolutionary outcome is most likely for the flower color alleles in this population?

<p>The <em>R</em> and <em>r</em> alleles will reach a stable equilibrium frequency due to the opposing selective pressures of pollination and disease. (B)</p> Signup and view all the answers

A researcher is studying a population of fish in a freshwater lake that is gradually becoming more saline due to climate change. Fish with a particular enzyme variant are better able to tolerate the increasing salinity. However, this variant also makes them more susceptible to a common parasite in the lake. What is the most likely evolutionary outcome for this fish population?

<p>The frequency of the enzyme variant will reach an equilibrium, balancing the benefits of salinity tolerance with the cost of parasite susceptibility. (C)</p> Signup and view all the answers

Flashcards

Genes

Segments of DNA that code for the basic units of heredity and are transmitted from one generation to the next.

Locus

The specific location of a gene on a chromosome.

Asexual Reproduction

Reproduction involving a single parent passing exact copies of all its genes to offspring, resulting in a clone.

Sexual Reproduction

Reproduction involving two parents contributing genes to offspring, resulting in greater genetic variation.

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Somatic Cells

Any biological cell forming the body of a multicellular organism other than a gamete, germ cell, gametocyte or undifferentiated stem cell.

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DNA Replication

The process of creating new DNA strands from existing ones.

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Semiconservative Replication

Each new DNA molecule contains one original strand and one newly synthesized strand.

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Origins of Replication

Specific locations on DNA where replication begins.

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Helicase

Enzymes that unwind the DNA double helix.

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Topoisomerase

Enzymes that relieve strain ahead of the replication fork by breaking and rejoining DNA strands.

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Antiparallel DNA Strands

The two DNA strands run in opposite directions.

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5' End

The end of a DNA strand that has a phosphate group attached to the 5' carbon of deoxyribose.

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3' End

The end of a DNA strand that has a hydroxyl group attached to the 3' carbon of deoxyribose.

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Synapsis & Crossing Over

Synapsis and crossing over occur during meiosis I, increasing genetic diversity, but not during mitosis.

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Metaphase Plate Alignment

Homologous pairs align in meiosis I, while individual replicated chromosomes align in mitosis. This allows independent assortment.

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Chromosome Separation in Anaphase

Homologous pairs separate in anaphase I of meiosis, sister chromatids remain attached. In mitosis, sister chromatids separate.

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Meiosis II

The second cellular division in meiosis, starting with a haploid cell.

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Prophase II

A spindle forms and sister chromatids move to the metaphase plate.

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Metaphase II

Haploid number of chromosomes align on the metaphase plate. Sister chromatids are genetically varied due to crossing over.

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Anaphase II

Centromeres of sister chromatids separate, and individual chromosomes move to opposite poles.

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Telophase II & Cytokinesis

Chromosomes reach poles, nuclei reappear, and cytokinesis occurs, resulting in four haploid, genetically different cells.

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Meiosis

Cell division with two stages (meiosis I and II) resulting in four daughter cells with half the number of chromosomes as the parent cell.

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Mitosis

Cell division with one stage, producing two identical daughter cells with the same number of chromosomes as the parent cell.

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Synapsis

The pairing and joining of homologous chromosomes along their length during prophase I, forming a tetrad.

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Tetrad

The structure formed during synapsis, consisting of two homologous chromosomes (four chromatids) aligned gene by gene.

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Crossing Over

The exchange of DNA segments between homologous chromosomes during prophase I, increasing genetic variation.

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Chiasmata

The point where homologous chromosomes crossover and remain connected during meiosis I.

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Homologous Chromosomes

Duplicated chromosomes (pairs) that carry the same genes

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Telophase I

Two haploid cells form; each chromosome still consists of two sister chromatids.

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DNA Polymerases

Enzymes that catalyze the elongation of new DNA at the replication fork.

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RNA Primers

Short sequences needed by DNA polymerase to start DNA replication.

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Leading Strand

DNA replication occurs continuously along the 5' to 3' strand.

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Lagging Strand

The strand copied in segments, requiring discontinuous replication.

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Okazaki Fragments

Short DNA fragments synthesized on the lagging strand.

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DNA Ligase

Enzyme that seals Okazaki fragments together.

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Telomeres

Repetitive nucleotide sequences at the ends of eukaryotic chromosomes that do not contain genes.

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Chromatin

The complex of DNA and proteins that makes up eukaryotic chromosomes.

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Diploid

Cells with two sets of chromosomes (2n) in somatic cells.

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Haploid

Cells with one set of Chromosomes (n).

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Zygote

The resulting cell from the fusion of two haploid gametes during fertilization; contains a diploid number of chromosomes.

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Independent Assortment

Random alignment of homologous chromosome pairs during metaphase I leading to increased genetic variation.

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Karyotype

A picture of an organism's complete set of chromosomes, arranged in homologous pairs.

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Sex Chromosomes

Chromosomes that determine sex; X and Y in humans.

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Autosomes

Non-sex chromosomes; all chromosomes except X and Y.

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Gametes

Sperm and egg cells, containing half the number of chromosomes as somatic cells.

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Haploid Cells

Cells containing half the number of chromosomes (n) as somatic cells.

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Fertilization

The fusion of gametes, restoring the full chromosome number.

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Study Notes

Meiosis I

  • During fertilization, the chromosome number is reduced by half; each gamete contains one chromosome of every homologous pair.
  • Fertilization is the combination of a sperm cell and an egg cell; a sperm cell fertilizes an egg cell, forming a zygote.
  • A zygote is a fertilized egg and is diploid, symbolized by 2n, with two sets of chromosomes.
  • Meiosis and mitosis share some features but have very different outcomes.
  • Both are preceded by the replication of the cell’s DNA.
  • In meiosis, this replication is followed by two stages of cell division, meiosis I and meiosis II; mitosis has only one division.
  • The final result of meiosis is four daughter cells, each of which has half as many chromosomes as the parent cell–one chromosome from each homologous pair.
  • The final result of mitosis is two identical daughter cells with the same number of chromosomes as the parent cell.

Interphase

  • Replication occurs, making a copy of each chromosome.
  • The replicated chromosome has two identical sister chromatids which roughly doubles the amount of DNA in the cell.

Meiosis I

  • This is the first cellular division in meiosis and begins with a diploid cell.

Prophase I

  • Understanding prophase I is critical to understanding meiosis, so study the unique events of prophase I carefully.
  • The chromosomes condense, and sister chromatids are attached at their centromeres.
  • Synapsis takes place–the joining of homologous chromosomes along their length.
  • The newly formed structure is called a tetrad and precisely aligns the homologous chromosomes gene by gene.
  • This perfect alignment is necessary for the next step–crossing over.
  • During crossing over, the DNA from one homolog is cut and exchanged with an exact portion of DNA from the other homolog.
  • Each chromosome is now a mix of maternal and paternal genes because a small part of the DNA from one parent is exchanged with the DNA from the other parent, resulting in increased genetic variation.

Three events occur during meiosis I that do not occur during mitosis.

  • Synapsis and crossing over do not occur during mitosis; crossing over increases genetic diversity.
  • At metaphase I, paired homologous chromosomes (tetrads) are positioned on the metaphase plate, rather than individual replicated chromosomes, as in mitosis.
  • This allows the independent assortment of maternal and paternal chromosomes, which increases genetic diversity.
  • At anaphase I, duplicated chromosomes of each homologous pair separate, but the sister chromatids of each duplicated chromosome stay attached while in anaphase of mitosis, sister chromatids separate.

Meiosis II

  • This is the second cellular division in meiosis and begins with a haploid cell.

Prophase II

  • A spindle apparatus forms, and sister chromatids move toward the metaphase plate.

Metaphase II

  • The haploid number of chromosomes is now arrayed on the metaphase plate.
  • Because of crossing over the sister chromatids are not genetically identical.
  • The kinetochores of each sister chromatid are attached to microtubules from opposite poles.

Anaphase II

  • The centromeres of the sister chromatids separate, and individual chromosomes move to opposite ends of the cell.

Telophase II and cytokinesis

  • The chromosomes have moved all the way to opposite ends of the cell, nuclei reappear, and cytokinesis occurs.
  • Each of the four daughter calls has the haploid number of chromosomes and is genetically different from the other daughter cells and from the parent cell.

Telophase I and Cytokinesis

  • Homologous chromosomes move to opposite poles and cytokinesis (the division of the cytoplasm) occurs.
  • Each daughter cell contains a haploid set of chromosomes with each chromosome still consisting of two sister chromatids.
  • Even though the sister chromatids are still attached to each other, the homologous pairs have separated.
  • The chromosomes numbers have been cut in half, and the daughter cells are now haploid as they now have only one chromosome of each homologous pair.
  • Homologous pairs line up on the metaphase plate at metaphase I and are separated and pulled toward the poles in anaphase I, sorting maternal and paternal chromosomes.
  • The result of independent assortment is an increase in genetic variation because maternal and paternal chromosomes of each pair sort randomly.

Unit 5 - Heredity

  • Genes are segments of DNA that code for the basic units of heredity and are transmitted from one generation to the next.
  • In animals and plants, reproductive cells that transmit genes from one generation to the next are called gametes.
  • The locus (plural, loci) is the location of a gene on a chromosome.
  • In asexual reproduction, a single parent passes copies of all its genes to its offspring.
  • The new offspring arise by mitosis and have virtually exact copies of the parent's genome.
  • An individual that reproduces asexually gives rise to a clone, a group of genetically identical individuals.
  • In sexual reproduction, two individuals (parents) contribute genes to the offspring.
  • This form of reproduction results in greater genetic variation in the offspring than asexual reproduction.

Fertilization and meiosis alternate in sexual life cycles.

  • Somatic cells are all cells in the body that are not gametes.
  • Each somatic cell in humans has 46 chromosomes.
  • Liver cells and neurons are examples of somatic cells.

Unit 6 - Gene Expression and Regulation

  • DNA is a double helix which can be described as a twisted ladder with rigid rungs.
  • The side, or backbone, is made up of sugar-phosphate components, whereas the rungs are made up of pairs of nitrogenous bases. A nucleotide is composed of a sugar (deoxyribose) attached to a phosphate and a nitrogen base.
  • The nitrogenous bases of DNA are adenine (A), thymine (T), guanine (G), and cytosine (C).
  • In DNA, adenine pairs only with thymine, and guanine pairs only with cytosine.
  • Adenine and guanine are two-ring structures and are referred to as purines.
  • Thymine and cytosine are single-ring structures are referred as pyrimidines.
  • In forming the DNA double helix, a purine is always paired with a pyrimidine.
  • The specificity of base pairing (A = T and G = C) is a key factor contributing to the accuracy of DNA replication.
  • Because DNA polymerase can only add nucleotides only to the 3' end of a molecule, it has no ability to complete the 5' end of the DNA molecule at the end of the chromosome. Everytime a chromosome is replicated a small portion of the tip of the chromosome is removed.
  • Telomeres–short, repetitive nucleotide sequences capped on the linear ends of eukaryotic chromosomes that do not contain genes to avoid losing the terminal genes.
  • Any given cell can only divide a finite number of times before essential information is lost.
  • In tumor cells, such as HeLa cells, a mutation activates an enzyme called telomerase, and the cells become "immortal". Bacterial chromosome is one double-stranded, circular DNA molecule with a small amount of protein.
  • Eukaryotic chromosomes are linear DNA molecules associated with large amounts of protein.
  • In eukaryotic cells, DNA and proteins are packed together as chromatin.
  • As DNA becomes more highly packaged as chromatin, it becomes less accessible to transcription enzymes, which reduces gene expression.
  • In interphase cells, chromatin is in the highly extended form and is available for transcription.
  • During mitotic division, the chromatin condenses to chromosomes, and chromatin is no longer available for transcription.
  • The two strands of DNA are termed antiparallel.
  • The strand on the right side runs in one direction, whereas the strand on the opposite, upside-down direction.
  • The left side runs 5' to 3', and the opposite strand runs 3' to 5'.
  • Recall that carbons are numbered, and you will see that the number 5 carbon and number 3 carbon, and the resultant nucleotides, are flipped relative to each other.
  • Nucleic acid strands are always antiparallel, whether they are DNA/DNA or DNA/RNA or RNA/RNA interactions.
  • Replication is the making of DNA from an existing DNA strand.
  • DNA replication is semiconservative.
  • At the end of the first replication, each of the daughter molecules has one old strand, derived from the parent strand of DNA, and one newly synthesized strand.

Steps involved in DNA replication:

  • The replication begins at sites called the origins of replication.
  • Helicase enzymes unwind the parental double helix, exposing the nucleotides to be replicated.
  • The unwinding of the double helix causes tighter twisting ahead of the replication fork.
  • Topoisimerase is an enzyme that helps relieve the strain by breaking and reforming DNA strands.
  • A group of enzymes called DNA polymerases catalyze the elongation of new DNA at the replication fork.
  • DNA polymerase requires RNA primers to start DNA replication.
  • DNA polymerase adds nucleotides to the growing chain one by one, working in a 5' to 3' direction, matching adenine nucleotides with thymine nucleotides and guanine nucleotides with cytosine nucleotides.
  • DNA polymerase ican only ass new nucleotides to the 3' end.
  • DNA strands are antiparallel which means DNA replication occurs continuously along the 5' to 3' strand that is the leading strand.
  • The strand that runs 3' to 5' is copied in a series of segments called Okazaki fragments that are sealed together by DNA ligase, to form the lagging strand.

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Explore the mechanisms of genetic variation focusing on meiosis and sexual reproduction. Understand how meiosis maintains chromosome number across generations and the significance of tetrad formation. Contrast asexual and sexual reproduction based on genetic diversity and evolutionary potential.

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