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Why is Meiosis I referred to as a reduction division?

  • It increases the genetic diversity through random mutations.
  • It synthesizes new DNA, ensuring genetic information is doubled.
  • It reduces the chromosome number from diploid to haploid. (correct)
  • It ensures each daughter cell receives a full set of duplicated chromosomes.

During which substage of Prophase I does crossing over occur?

  • Leptotene
  • Zygotene
  • Pachytene (correct)
  • Diplotene

What is the significance of the formation of a 'chiasma' during meiosis?

  • It indicates the point where sister chromatids separate during Anaphase II.
  • It is the visible manifestation of crossing over, where genetic material is exchanged. (correct)
  • It represents the location where chromosomes attach to the spindle fibers.
  • It marks the beginning of chromosome condensation during prophase I.

What is the status of sister chromatids during Anaphase I, and why is this significant?

<p>They remain together, as the centromere does not divide, ensuring each chromosome still consists of two chromatids. (B)</p> Signup and view all the answers

How does genetic variation arise during Meiosis I?

<p>Through the independent assortment of chromosomes and crossing over in Prophase I. (B)</p> Signup and view all the answers

Which of the following best describes the primary function of chromosomes within a cell?

<p>To direct cellular activities and functions through genetic instructions. (A)</p> Signup and view all the answers

If a newly discovered organism has 32 chromosomes in its cells, how would you best describe this number?

<p>Characteristic of that particular species. (D)</p> Signup and view all the answers

During what stage of the cell cycle are chromosomes typically in an extended, uncoiled form referred to as chromatin?

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

A researcher observes a cell undergoing a transformation where its chromatin condenses and thickens. According to the typical cell cycle, in what phase is this cell?

<p>Cell Division Phase (A)</p> Signup and view all the answers

Which process directly contributes to both heredity and variation in sexually reproducing organisms?

<p>Cell division (C)</p> Signup and view all the answers

How does genetics explain the differences seen among siblings from the same parents?

<p>Each sibling inherits a unique combination of traits, leading to variation. (B)</p> Signup and view all the answers

What is the primary role of nutrients in cell reproduction?

<p>To supply the energy required for cell division, growth, and development. (A)</p> Signup and view all the answers

A biologist is studying cells that are not actively dividing. Based on the description of the cell cycle, how should these cells to be classified?

<p>They are not considered to be in the cell cycle. (C)</p> Signup and view all the answers

Which of the following best describes the primary function of p53 in regulating the cell cycle?

<p>Inhibiting cell division when DNA damage is detected, potentially leading to apoptosis if the damage is irreparable. (C)</p> Signup and view all the answers

In which phase of the cell cycle does DNA replication occur?

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

What is the primary function of cyclin-dependent kinases (Cdks) in the cell cycle?

<p>To add phosphate groups to proteins, regulating the cell cycle progression when complexed with cyclins. (A)</p> Signup and view all the answers

A researcher observes that cells in a culture are not entering S phase. Which protein deficiency could explain this observation?

<p>Cdk (C)</p> Signup and view all the answers

What is the role of MPF (Maturation Promoting Factor) in the cell cycle?

<p>Triggering progression through the cell cycle. (B)</p> Signup and view all the answers

Which of the following is NOT a stage of mitosis?

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

A scientist measures high levels of p53 in a cell sample. What is the most likely explanation?

<p>The cells have experienced significant DNA damage. (C)</p> Signup and view all the answers

Which of the following best describes the condition in Li-Fraumeni syndrome?

<p>A genetic defect in p53, leading to a high frequency of cancer. (B)</p> Signup and view all the answers

How does p27 exert its influence on the cell cycle?

<p>By binding to cyclin and Cdk complexes, preventing entry into S phase. (B)</p> Signup and view all the answers

If a drug inhibits cytokinesis, what aspect of cell division will be directly affected?

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

During prophase, what happens to the nuclear membrane?

<p>It breaks down into small vesicles. (A)</p> Signup and view all the answers

What is the role of the centrosomes during prophase?

<p>To organize the production of microtubules that form the mitotic spindle. (D)</p> Signup and view all the answers

What happens to sister chromatids during anaphase?

<p>They are pulled apart and move to opposite poles of the cell as daughter chromosomes. (C)</p> Signup and view all the answers

Which of the following is the key event that defines metaphase?

<p>The alignment of chromosomes along the metaphase plate. (B)</p> Signup and view all the answers

What is the main event occurring during telophase?

<p>The reformation of the nuclear membrane around the chromosomes. (A)</p> Signup and view all the answers

What are 'p' and 'q' arms of a chromosome?

<p>p - petite (short), q - longer (A)</p> Signup and view all the answers

If a cell has 46 chromosomes before mitosis, how many chromosomes will each daughter cell have after mitosis and cytokinesis?

<p>46 (C)</p> Signup and view all the answers

How do heterochromatin and euchromatin differ in terms of structure and activity?

<p>Heterochromatin is tightly packed and transcriptionally inactive, while euchromatin is loosely packed and active. (D)</p> Signup and view all the answers

What is the role of the kinetochore during mitosis?

<p>To attach chromosomes to the spindle fibers. (A)</p> Signup and view all the answers

Why is the alignment and separation of chromosomes in metaphase and anaphase important?

<p>To ensure each daughter cell receives a complete and identical set of chromosomes. (A)</p> Signup and view all the answers

Flashcards

Genetics

The study of heredity and variation in living organisms.

Heredity

The passing of traits from parents to offspring.

Variation

Differences among individuals within a species.

Chromosomes

Genetic material (DNA) organized into structures within cells of Eukaryotes.

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DNA (Deoxyribonucleic Acid)

The molecule that carries genetic instructions.

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Cell Cycle

A repeating sequence of growth and division in a cell's life.

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Chromatin

Long, extended form of chromosomes during interphase.

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Interphase

The period of the cell cycle when the cell grows and prepares for division.

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Cell Cycle Core Concept

Doubling of genome in S phase and halving in mitosis (M phase)

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G1 Stage

Growth and chromosome preparation for replication.

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S Stage

DNA replication occurs.

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G2 Stage

Preparation for mitosis

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M Stage

Nuclear & cytoplasmic division

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Cdk

Adds phosphate to a protein, triggering cell cycle progression.

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MPF

Triggers progression through the cell cycle (includes Cdk and cyclins).

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p53

Blocks cell cycle to repair damaged DNA; can cause apoptosis.

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p27

Blocks entry into S phase by binding to cyclin and Cdk.

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Mitosis

Nuclear division plus cytokinesis, producing two identical daughter cells.

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Meiosis I

The first meiotic division that reduces chromosome number by half and allows genetic recombination via crossing over. Daughter cells are haploid.

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Zygotene

Stage in Prophase I where chromosomes begin to pair off exactly forming homologous chromosomes.

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Pachytene (Crossing Over)

Stage in Prophase I when a segment of sister chromatids from homologous chromosomes are exchanged through a chiasma.

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Anaphase I

Homologous chromosomes separate and move to opposite poles; sister chromatids remain together.

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Prophase

First stage of mitosis where the nuclear membrane breaks down, centrosomes duplicate, and chromosomes condense.

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Centrosome

Structure that duplicates during prophase and migrates to opposite ends of the cell to organize spindle fibers.

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Spindle Fibers

Fibers formed from microtubules that attach to chromosomes to move them during cell division.

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Chromosome (Condensed)

Compact structure of DNA and protein, consisting of two identical sister chromatids during prophase.

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Centromere

Region where sister chromatids are joined together.

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Prometaphase

Later phase of prophase. Chromosomes migrate to the metaphase plate.

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Kinetochore

Structure associated with the centromere where spindle fibers attach.

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Metaphase

Phase where chromosomes align along the metaphase plate.

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Telophase

Final stage of mitosis where nuclear membranes reform and chromosomes decondense.

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Study Notes

  • The chapter introduces genetics and cellular reproduction.
  • Pre-service teachers are expected to explain chromosomes, recall the cell cycle and regulation mechanisms, and differentiate mitosis and meiosis, including the events in each phase.

Introduction

  • Shared features among family members are due to heredity, where traits pass from parents to offspring.
  • Variation demonstrates differences among individuals.
  • Genetics studies heredity and variation to understand trait inheritance and variation.
  • Reproduction at the cellular level, via cell division, underpins reproduction at the organismal level.

The Chromosome

  • Genetic material directs cell activities and functions.
  • Deoxyribonucleic acid, or DNA, ensures life's continuity through generations.
  • In eukaryotic cells, DNA and proteins form chromosomes.

Chromosome Number and the Cell Cycle

  • The number of chromosomes is characteristic of each species.
  • Humans have 46 chromosomes, while rice has 24.
  • The cell cycle includes interphase, where chromosomes are extended and called chromatin, and cell division, where chromosomes condense.
  • Cell Cycle: An ordered set of events that leads to cell growth and division into two daughter cells.
  • Non-dividing cells aren't in the cell cycle.
  • Eukaryotic cells alternate genome duplication, and genome halving in two key processes
  • S phase: DNA/genome doubles (synthesis phase)
  • M phase: Genome halves, during mitosis
  • The four stages of the cell cycle:
  • G1 stage ("GAP 1"): Growth and preparation for chromosome replication
  • S stage (Synthesis): DNA replication occurs
  • G2 stage ("GAP 2"): Preparation for mitosis.
  • M stage (mitosis): Nuclear and cytoplasmic (cytokinesis) division occurs
  • Mitosis includes 4 phases

Cell Cycle Regulation

  • Complex control mechanisms regulate cell division and tissue growth.
  • Cancer is a disease when cell cycle regulation fails.
  • Cdk: A cyclin-dependent kinase that adds phosphate to a protein.
    • Cdk, along with cyclins, controls transitions in the cell cycle (G1 to S or G2 to M).
  • MPF: Maturation Promoting Factor; CdK and cyclins that trigger the cell cycle's progression.
  • p53: A protein that stops the cell cycle if DNA is damaged.
    • Severe damage leads to apoptosis (cell death).
    • p53 levels rise in damaged cells.
    • This protein allows time to repair DNA.
    • p53 mutation is the most frequent mutation leading to cancer.
    • Li Fraumeni syndrome is an extreme case where a genetic defect in p53 leads to a high cancer frequency.
  • p27: A protein that binds to cyclin and cdk, this action blocks entry into S phase.
    • p27 levels determine breast cancer prognosis

Cell Division: Mitosis and Meiosis

  • Mitosis is nuclear division along with cytokinesis producing two identical daughter cells through several stages
  • Interphase is technically not part of mitosis.
    • Interphase includes G1, S, and G2 phases.
    • Mitosis occurs in somatic or body cells, this typically takes about an hour in actively dividing animal cells.
  • Prophase: The nuclear membrane breaks down and the nucleolus disintegrates.
    • The centrosome duplicates, forming two daughter centrosomes.
    • the daughter centrosomes migrate to opposite ends of the cell; they produce the microtubules forming spindle fibres.
    • Chromosomes condense into compact structures.
    • Each replicated chromosome consists of two identical chromatids (or sister chromatids).
    • Chromatids are held together by a structure known as the centromere.
    • The centromere divides the chromosome into shorter p arms and longer q arms.
    • Giemsa staining shows dark and light regions (heterochromatin and euchromatin.)
    • Heterochromatin is more coiled and denser than euchromatin.
  • Prometaphase: Chromosomes migrate to the metaphase plate.
    • The chromosomes are led by their centromeres.
    • This region of the mitotic spindle is on the midline of the cell, at right-angles to the axis.
    • Spindle fibers bind to the kinetochore on each side of the centromere.
    • Chromosomes continue to condense.
  • Metaphase: Chromosomes align along the metaphase plate of the spindle apparatus.
  • Anaphase: The shortest stage of mitosis.
    • Centromeres divide, and sister chromatids separate and move to opposite ends of the cell.
    • Spindle fibers attached to the kinetochore regions pull the sister chromatids.
    • Separated sister chromatids are called daughter chromosomes.
    • This phase of the cycle ensures each receives a copy of every chromosome.
  • Telophase: The final stage of mitosis reverses prophase processes.
    • The nuclear membrane reforms around chromosomes grouped at each cell pole.
    • Chromosomes uncoil, spindle fibers disappear.
  • Cytokinesis: The final cellular division forms two new cells.
    • In plants, a cell plate forms along the metaphase plate line.
    • In animals, the cytoplasm constricts.
    • The cell then enters interphase.
  • Meiosis: A form of eukaryotic cell divisions that produces haploid sex cells or gametes.
    • Gametes contain a single copy of each chromosome.
    • Meiosis occurs in diploid cells, which contain two copies of each chromosome.
    • The process has one DNA duplication followed by two successive divisions (Meiosis I and Meiosis II)
    • Gametogenesis the production of sperm (spermatogenesis) and eggs (oogenesis) happens through meiosis
    • Meiosis is not a cycle
  • Meiosis I serves 2 main purposes
    • It reduces division, reducing the number of chromosomes in half.
    • It makes daughter cells haploid (when the parent cell was diploid).
    • Genetic recombination takes place through crossing over in this stage.
  • Prophase I: This is the longest stage and has 5 sub-stages.
    • Leptotene: Each chromosome is made of two long sister chromatids after replication during the S phase
    • Zygotene: Chromosomes begin to pair up.
    • Pairs of chromosomes are homologous chromosomes
    • This synapsis pairing process is exact.
    • Pachytene: Chromosomes contract via repeated coiling.
      • Crossing over: A segment of one chromosome's sister chromatid exchanges with the same segment of the homologous chromosome's sister chromatid. This happens through a cross-linkage of segments called a chiasma.
      • The resulting sister chromatids may no longer be identical.
      • Diplotene: Chromosomes begin to uncoil.
      • Diakinesis: Paired chromosomes disperse in the nucleus.
  • Metaphase I: Tetrads align at the equator and the spindle is fully formed.
  • Anaphase I: Spindle fibers form and attach to the chromosome centromeres.
    • Homologous chromosomes separate completely and move toward the poles, pulled by spindle fibers.
    • Sister chromatids stay together as the centromere of each chromosome does not divide.
  • Telophase I: Chromosomes with two chromatids decondense, and a nuclear reforms.
    • The resulting nucleus is now haploid.
  • Meiosis II: This serves to reduce the amount of DNA back to normal.
    • It splits the chromosomes so each resultant daughter cell only has one chromatid per chromosome.
  • Phases of meiosis II: This is very similar to mitosis, but each cell only has on ecopy of each chromosome
    • Prophase II: Chromosomes with two chromatids become visible.
      • The nuclear envelope and nucleus disappear, and the spindle forms.
    • Metaphase II: Chromosomes with two chromatids line up at the equator.
      • The spindle is fully formed.
    • Anaphase II: Chromosomes split, such that a chromosome heads toward each pole, with only one chromatid.
    • Telophase II: Chromosomes with only one chromatid decondense and are surrounded by new nuclear envelopes.
      • Four daughter cells are now all haploid
      • They are ready to become sperm or eggs.

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