D2.1 Cell and Nuclear Division

Distinguish between cytokinesis in plant and animal cells

Animal cells: The cell membrane pinches inward, forming a cleavage furrow

Plant cells: A cell plate forms in the center and develops into a new cell wall

Normal cell division has equal cytokinesis. Why is this beneficial?

It produces daughter cells with approximately equal ammounts of cytoplasm, organelles, and other cell components which allows both cells

What are the two types of unequal cytokinesis

• Oogenesis occurs in the creation of egg gamete cells→ concentrates cytoplasm and nutrients in one egg cell to support early development

• Budding occurs in fungi: splitting unequal→ one daughter cell is much smaller than the other to eventually grow into a new organism

• Both involve cytoplasm dividing asymmetrically

What is the purpose of mitosis?

To create 2 diploid daughter cells that are identical to the mother cell→ for growth and tissue repair

What is the purpose of meiosis?

To make haploid gametes (egg and sperm cells) for reproduction and increase genetic variation

Why is DNA replicated prior to division

So each daughter cell receives a complete copy of the genetic information

When does DNA replication happen?

S phase of interphase

How do the chromosomes look after duplication?

Each chromosome consists of two identical sister chromatids joined at the centromere

What are the mechanisms of DNA condensation?

DNA condenses into chromosones

What are the names of the phases?

• PMAT

• Prophase → Metaphase → Anaphase → Telophase → Cytokinesis

What are the key events of each stage?

• Prophase

  • DNA condenses into chromosomes→ easier to organize

  • Nucleus dissolves

  • Centrioles and spindle fibers form

• Metaphase

  • Chromosomes align in the equator of cell

  • Spindle fibers attach to centromeres in chromosomes

• Anaphase

  • Spindle fibres contract→ pull chromosomes apart

  • Sister chromatids seperate torwards opposite poles

• Telophase

  • Nucleus reforms around each set of chromosones

  • DNA unwinds

  • Cleavage furrow begins

• Cytokinesis: cytoplasm divides, produces two daughter cells

How can you identify stages of mitosis in micrograph photos?

• Prophase: condensed chromosomes visible; not lined up.

• Metaphase: chromosomes lined up across the middle.

• Anaphase: sister chromatids moving toward opposite poles.

• Telophase: two groups of chromosomes at opposite ends; two nuclei forming.

• Cytokinesis: cell is physically dividing into two cells.

Define diploid

A cell with two set sets of chromosomes (2n), one set form each parent

Define haploid

A cell one set chromomes (n)

What is reduced in meiosis?

The chromosome number is reduced from diploid (2n) to haploid (n)

What are the names of the phases in meiosis?

Meiosis I: Prophase I → Metaphase I → Anaphase I → Telophase I

Meiosis II: Prophase II → Metaphase II → Anaphase II → Telophase II

What are the key events of each stage?

• Prophase I:

  • Chromosomes condense

  • Homologous chromosomes pair up

  • Crossing over occurs

  • Nucleus dissolves

• Metaphase I: homologous chromosome pars line up at the equator

• Anaphase I:

  • Homologous chromosomes seperate and move to opposite poles

  • sister chromatids remain together

• Telophase I:

  • Chromosomes reach opposite poles

  • Cytokinesis produces two haploid cells

• Prophase II:

  • Chromosomes condense again

  • Spindle fibers form

• Metaphase II:

  • Chromosomes line up individually at the equators

• Anaphase II:

  • Sister chromatids separate and move to opposite poles

• Telophase II:

  • Nucleus forms

  • Chromosomes decondenses

  • Cytokinesis produces four haploid cells

Define non-disjunction mutation

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate correctly during meiosis

What difference does it make if the mutation happens in Meiosis I or Meiosis II?

• Meiosis I: homologous chromosomes fail to separate. All 4 gametes have an abnormal chromosome number.

• Meiosis II: sister chromatids fail to separate. 2 gametes are normal and 2 have an abnormal chromosome number.

What is Down syndrome from a chromosome abnormality?

Down syndrome is caused by having three copies of chromosome 21 instead of the normal two

Define crossing over

Crossing over is the exchange of DNA between non-sister chromatids of homologous chromosones during prophase I of meiosis

Define random orientation

During metaphase I, homologous pairs arrange randomly on the equator

• Maternal/paternal chromosomes can move to either pole

Define variation in terms of gamete formation

Variation is the production of genetically different gametes containing different combinations of alleles

How does crossin over and random orientation each lead to variation of gametes?

• Crossing over: exchanges DNA between homologous chromosomes, creating new combinations of alleles.

• Random orientation: homologous pairs line up randomly, so maternal and paternal chromosomes are distributed into gametes in different combinations