Cell Biology: Chromosome Structure and Mitosis

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Questions and Answers

In sex-limited inheritance, how is the phenotype expressed in male and female individuals?

The phenotype is expressed differently based on sex (correct)

Different genotypes lead to the same phenotypes

Phenotype expression is independent of the sex of the individual

Both sexes show the same phenotypes regardless of genotype

What causes the black and orange mosaic pattern found in calico cats?

Environmental factors influencing fur color

An X-linked gene with two alleles (correct)

A dominant gene that determines color

A mutation in a specific color gene

What is the phenotype of a male with the genotype Bb in sex-influenced inheritance?

Bald (correct)

Not bald

Cock-feathered

Hen-feathered

Which genotype combination results in a cock-feathered male in sex-limited inheritance?

hh Signup and view all the answers

Which of the following statements is true regarding sex-influenced inheritance?

The same genotype may lead to different phenotypes in males and females Signup and view all the answers

What is the primary role of cyclin-dependent kinases (CDKs) in the cell cycle?

To advance the cell from one stage of the cell cycle to the next Signup and view all the answers

Which checkpoint is responsible for ensuring DNA integrity after replication?

G2 checkpoint Signup and view all the answers

During which phase of meiosis does crossing over occur?

Prophase I Signup and view all the answers

What happens during Anaphase I of meiosis?

Homologs move to opposite sides Signup and view all the answers

What does the G1 checkpoint assess before committing the cell to division?

Cell size, molecular signals, and DNA integrity Signup and view all the answers

What is the main purpose of meiosis I?

To reduce the chromosome number from diploid to haploid Signup and view all the answers

What structure forms during homolog pairing in meiosis I?

Tetrad Signup and view all the answers

What is the result of crossing over during Prophase I?

Exchange of genetic material between homologs Signup and view all the answers

What is the genotype of the hemophilic man described?

Xh Y Signup and view all the answers

Which of the following genotypes must the daughter of the hemophilic man have?

XH Xh Signup and view all the answers

In normal human females, what occurs to one of the X chromosomes?

It remains inactive as a Barr body. Signup and view all the answers

What genetic structure can be observed in the interphase nuclei of somatic cells where X inactivation has occurred?

Barr body Signup and view all the answers

Which syndrome is characterized by the presence of only one X chromosome?

Turner syndrome Signup and view all the answers

What is the expected probability of having a hemophilic son if the daughter's husband is XH Y?

25% Signup and view all the answers

In cases of dosage compensation, what generally happens to additional X chromosomes in mammalian cells?

They become Barr bodies. Signup and view all the answers

What is a common result of X inactivation in females?

Formation of genetic mosaics. Signup and view all the answers

What is the result of monosomy in humans?

Loss of one chromosome Signup and view all the answers

Which trisomy syndrome is most commonly associated with Down syndrome?

Trisomy 21 Signup and view all the answers

Which of the following is a characteristic of sex-linked genes?

They can be recessive or dominant Signup and view all the answers

What is true about Y-linked traits in humans?

They are transmitted from father to son Signup and view all the answers

Which factor significantly contributes to the occurrence of trisomy syndromes?

Nondisjunction during Meiosis Signup and view all the answers

Why are males more likely to be affected by X-linked recessive traits such as hemophilia?

They are hemizygous for the X chromosome Signup and view all the answers

What is the chromosomal configuration in Klinefelter syndrome?

XXY Signup and view all the answers

In the genetic scenario involving hemophilia, which inheritance pattern is observed?

X-linked recessive Signup and view all the answers

What is the primary role of the mitotic spindle during cell division?

To organize and sort chromosomes through mitosis Signup and view all the answers

Which structure provides the attachment points for spindle fibers?

Kinetochore Signup and view all the answers

During which phase do chromosomes align on the equatorial plate?

Metaphase Signup and view all the answers

What happens during anaphase regarding sister chromatids?

Cohesin proteins are removed, allowing separation Signup and view all the answers

What differentiates cytokinesis in plant and animal cells?

Formation of cell plate vs. cleavage furrow Signup and view all the answers

Which type of microtubule is responsible for attaching to the kinetochore?

Kinetochore microtubules Signup and view all the answers

What is the sequence of phases during mitotic cell division starting from Interphase?

Interphase, Prophase, Metaphase, Anaphase, Telophase Signup and view all the answers

What occurs during telophase of mitotic cell division?

Chromosomes uncoil and nuclear envelope reforms Signup and view all the answers

Study Notes

Chromosome Structure

Chromosomes are composed of two sister chromatids joined at the centromere.
The centromere is a highly constricted region of DNA where the sister chromatids are in close contact.
Kinetochores are protein units that attach to spindle fibers and provide the attachment points for microtubules.

Mitotic Spindle

Organizes and sorts chromosomes during mitosis.
Formed from microtubule organizing centers (MTOCs) at centrosomes (two centrioles lying at right angles).
There are three types of microtubules in the mitotic spindle of animal cells:
- Aster microtubules: radiate outward from the centriole to the membrane.
- Polar microtubules: extend between the two poles of the spindle.
- Kinetochore microtubules: attach to the kinetochore.

Phases of Mitotic Cell Division

Prophase:
- Nuclear envelope breaks down.
- Chromosomes become visible.
- Sister chromatids are connected at the centromere.
Prometaphase:
- Chromosomes are clearly visible as double structures.
- Centrioles reach opposite poles of the cell.
- Spindle fibers form.
- Spindle fibers attach to centromeres, specifically the kinetochores.
Metaphase:
- Chromosomes align on the metaphase (equatorial) plate.
Anaphase:
- Removal of cohesin proteins causes centromeres to separate.
- Kinetochore microtubules shorten, pulling separated sister chromatids to opposite poles (characteristic V shape).
Telophase:
- Spindle fibers disappear.
- Centrioles return to normal positioning.
- Chromosomes uncoil.
- Nuclear envelope reforms.
Cytokinesis:
- Division of the cytoplasm (cell division).
- Produces two identical daughter cells.
- Differs in plant and animal cells (cell plate vs. cleavage furrow).
- Does not have to occur in all cells of all organisms (multinucleated tissue).

Cell Cycle Regulation

Checkpoints ensure the cell can transition from one phase to another.
- G1 checkpoint: checks cell size, the presence of molecular signals (e.g., growth factors), and DNA integrity.
- G2 checkpoint: checks DNA integrity and verifies replication.
- M checkpoint (spindle checkpoint): checks if all sister chromatids are correctly attached to spindle microtubules after metaphase.
Cyclin-dependent kinases (CDKs):
- Act as master control molecules, regulating progression through the cell cycle.
- Only function when associated with proteins called cyclins, forming CDK-cyclin complexes.
- Phosphorylate other proteins, activating or inactivating them at specific times in the cell cycle.
- Different CDKs appear at specific times in the cell cycle.

Meiotic Cell Division

More complex than mitosis.
Basic cell cycle is the same (interphase), but the M phase is now meiosis.
Two divisions occur:
- Meiosis I (reductional division): reduces chromosome number from 2n to n.
- Meiosis II (equational division): produces four haploid gametes.
DNA synthesis occurs before the beginning of meiosis I but does not occur again before meiosis II.

Prophase I of Meiosis I

Leptotene (leptonema): Chromatin condenses into very thin chromosomes.
Zygotene (zygonema): Homologs search and recognize each other. Synapsis begins (synaptonemal complex forms).
Pachytene (pachynema): Tetrad formation (homologs pair). Crossing over (recombination nodules appear).
Diplotene (diplonema): Chiasma (chiasmata pl) remains (sections resulting from crossing over). Tetrad begins to pull apart slightly.
Diakinesis: Nuclear envelope fragmentation and synaptonemal complex disappears.

Metaphase I of Meiosis I

Independent assortment of nonhomologs.

Anaphase I of Meiosis I

Homologs move to opposite sides of the cell (one half of each tetrad). Disjunction occurs (centromeres holding sister chromatids do not divide).

Aneuploidy in Humans

Variations in chromosome number (organism gains or loses one or more chromosomes other than an exact multiple of the haploid set).
Monosomy: Loss of a single chromosome (2n - 1). Example: Turner Syndrome (XO).
Trisomy: Gain of a single chromosome (2n + 1).
- Three copies of one chromosome are present, resulting in irregular pairing configurations.
- Often lethal for autosomes, but less dramatic phenotype for sex chromosomes.
- Examples:
  - XXX: Triple X syndrome.
  - XXY: Klinefelter syndrome.
  - XYY: Jacobs syndrome.
- Trisomy 21 (Down syndrome): Most common genetic disorder caused by a chromosomal abnormality. 90% of cases involve an extra chromosome from the mother's egg. Familial Down syndrome is different and runs in families.
- Other trisomies: Patau syndrome (trisomy 13) and Edwards syndrome (trisomy 18) - only 5% chance of survival beyond 1 year.
Trisomies are frequently found in spontaneously aborted fetuses.

Non-disjunction in Meiosis

The cause of aneuploidy.

Sex-linked Inheritance

Genes located on one of the two sex chromosomes, but not both.
- X-linked: Found on the X chromosome (recessive or dominant).
  - Many human disorders, including hemophilia and colorblindness.
  - Males are more likely to be affected due to hemizygosity (only one copy of X chromosome).
- Y-linked (holandric): Found on the Y chromosome.
  - Rare, only a few genes in humans.
  - Transmitted from father to son.

Dosage Compensation

Mammalian cells count their X chromosomes and only allow one to remain active.
In human females, one X is inactivated, becoming highly compacted and forming a Barr body.
- Most genes on the inactivated X cannot be expressed.
- The inactivated X remains compacted and inactive during replication, passed on to all future somatic cells.
Results in genetic mosaics.
- Examples: calico cats and red-green colorblindness in humans.

Number of Barr Bodies

Normal female (XX): 1 Barr body.
Normal male (XY): 0 Barr bodies.
Turner syndrome (female, XO): 0 Barr bodies.
Triple X syndrome (female, XXX): 2 Barr bodies.
Klinefelter syndrome (male, XXY): 1 Barr body.

Sex-limited Inheritance

Expression of a specific phenotype is absolutely limited to one sex.
Example: Hen-feathered and cock-feathered chickens.

Sex-influenced Inheritance

Sex of an individual influences the expression of a phenotype that is not limited to one sex or the other.
Example: Baldness in humans.

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Description

This quiz covers key concepts in cell biology related to chromosome structure and the phases of mitotic cell division. Explore the roles of sister chromatids, centromeres, and the mitotic spindle in the process of mitosis. Test your understanding of how these components interact during cell division.