D2.1 Cell and nuclear division

Why do organisms need to produce new cells?

For growth, maintenance, and reproduction.

What is cell division?

One cell divides into two, producing daughter cells.

What happens to the mother cell during cell division?

It disappears as an entity.

What is the theory about new cell production?

New cells are only produced by division of pre-existing cells.

What does the cell theory imply about our bodies?

All cells in our bodies come from a pre-existing cell.

Where does the origin of all cells trace back to?

The original zygote produced by sperm and egg fusion.

How far back can the origin of cells be traced?

To the earliest cells on Earth, hundreds of millions of years ago.

What is cytokinesis?

The division of cytoplasm between two daughter cells.

What processes occur with cytokinesis?

Nuclear division by mitosis or meiosis.

When does cytokinesis begin?

When chromosomes are far enough apart to avoid errors.

What happens during cytokinesis in animal cells?

The plasma membrane forms a cleavage furrow and pinches apart.

What proteins are involved in animal cell cytokinesis?

Actin and myosin.

What is the role of microtubules in plant cell cytokinesis?

They create a scaffold for vesicle assembly.

What forms the plasma membranes of daughter plant cells?

Vesicles fusing together across the equator.

What forms the middle lamella in plant cells?

Pectins and substances deposited by exocytosis.

What does each plant daughter cell deposit across the equator?

Cellulose for building its own cell wall.

How is cytoplasm divided in growing root tips?

Equally, to support columnar cell differentiation.

What are examples of unequal cytoplasm division?

Budding in yeast and oogenesis in humans.

How does budding occur in yeast?

A small outgrowth receives a nucleus and cytoplasm, then splits.

What is a polar body in oogenesis?

A small cell that does not develop further.

Why is cytoplasm unequally divided during oogenesis?

To ensure one large egg cell with stored nutrients forms.

What happens to anucleate cells?

They cannot synthesize polypeptides and have limited lifespans.

What happens to red blood cells without a nucleus?

They survive about 120 days.

What is the role of mitosis and meiosis?

To produce nuclei before cell division.

What is the chromosome number in mitosis-produced cells?

The same as the parent cell.

Why does mitosis maintain the genome?

It ensures all daughter cells have the same genes as the parent cell.

What does meiosis produce?

Haploid gametes from diploid germ cells.

Why does meiosis generate genetic diversity?

Genes are randomly dealt to daughter cells.

What must happen before nuclear division?

DNA is replicated.

What holds chromatids together?

Cohesin loops until anaphase.

What are sister chromatids?

Identical DNA molecules from replication.

What are non-sister chromatids?

Strands on different chromosomes, usually not identical.

What happens to chromosomes during mitosis?

They are moved to opposite poles of the cell.

How are chromosomes condensed?

By wrapping DNA around histone proteins into nucleosomes.

What do microtubules do during mitosis?

They form a spindle array linking the poles of the cell.

What structures are assembled on chromatids?

Kinetochores on the centromere.

What happens when cohesin is cut?

Sister chromatids move to opposite poles.

What holds homologous chromosomes in meiosis?

Chiasmata, until they slide to the ends.

What are the four phases of mitosis?

Prophase, metaphase, anaphase, and telophase.

What happens during interphase?

DNA is replicated, forming elongated chromatids.

What happens during prophase?

Chromosomes condense into thicker, shorter structures.

What forms at the poles during prophase?

Microtubule organizing centers (MTOCs).

What happens at the end of prophase?

The nuclear membrane breaks down.

What is metaphase?

The phase when chromosomes are released from the nucleus.

What happens during anaphase?

Chromosomes are moved to poles from the equator.

What happens during telophase?

Nuclei reform, and chromosomes decondense.

Why must DNA be replicated before division?

So each daughter cell receives a full complement of genes.

What is the spindle-shaped array?

A structure of microtubules linking poles during mitosis.

What is the role of kinetochores?

To act as microtubule motors for chromosome movement.

What do spindle microtubules do during anaphase?

Shorten, pulling chromatids to poles.

What allows chromosomes to move without tangles?

Condensation into shorter structures.

Why do chromosomes become visible in mitosis?

They shorten and thicken, making them observable.

What is the role of histones in chromosomes?

To form nucleosomes and condense DNA.

What does cytokinesis ensure?

That all cytoplasm and organelles are shared between daughter cells.

What happens to chromosomes after mitosis?

They decondense and are no longer visible.

What happens to vesicles in plant cell division?

They fuse to form membrane layers across the cell equator.

How do daughter cells build cell walls?

By exocytosis of cellulose and other materials.

What happens during budding in yeast?

A small cell splits from the mother cell, leaving a scar.

Why are four sperm cells produced in spermatogenesis?

Cytoplasm is equally divided in both cell divisions.

What attaches microtubules to chromatids in metaphase?

Microtubules attach to the centromere of each chromatid.

What ensures chromosomes are aligned correctly in metaphase?

The spindle microtubules are put under tension to test attachment correctness.

Where are chromosomes aligned at the end of metaphase?

At the equator of the cell.

What happens to cohesin loops during anaphase?

Cohesin loops are cut, allowing chromatids to become separate chromosomes.

How are chromosomes pulled to the poles in anaphase?

A kinetochore shortens the microtubules, pulling the chromosome to the pole.

What occurs at the end of anaphase?

All chromosomes have reached the poles but have not decondensed.

What happens to chromosomes during telophase?

Chromosomes decondense and spread out to form chromatin inside the nucleus.

What reforms around chromosomes during telophase?

A nuclear membrane reforms around the chromosomes.

What occurs to the cytoplasm during telophase?

The cell begins dividing its cytoplasm, producing two daughter cells.

What happens to genes in chromosomes during interphase?

Genes in decondensed chromosomes are transcribed, and mRNA is translated to synthesize proteins.

What happens to cell size during interphase?

The cell grows, usually doubling in size before the next mitosis.

What does a chromosome consist of?

A very long DNA molecule with some associated proteins.

What does a gene sequence in chromosomes provide?

Continuity through generations in a species.

What are homologous chromosomes?

Chromosomes with the same sequence of genes as each other.

What process explains homologous chromosomes being not identical?

Recombination and gene mutation.

What is the haploid number and its symbol?

The characteristic number of homologous chromosome types, symbolized as n.

What is the diploid number?

Two homologous chromosomes of each type, represented as 2n.

What is a haploid nucleus?

A nucleus with a single set of non-homologous chromosomes.

What is a diploid nucleus?

A nucleus with two sets of chromosomes forming homologous pairs.

How are diploid cells formed in sexual reproduction?

By the fusion of haploid gametes (e.g., sperm and egg).

What prevents doubling of chromosome number in sexual reproduction?

Reduction division (meiosis) halving the chromosome number from diploid to haploid.

What happens in Anaphase I of meiosis?

Homologous chromosomes separate and move to opposite poles, halving the chromosome number.

What happens in Anaphase II of meiosis?

Chromatids of each chromosome separate and move to opposite poles.

What is non-disjunction?

Failure of chromosomes or chromatids to separate properly during meiosis.

What condition can result from non-disjunction of chromosome 21?

Down syndrome, causing three copies of chromosome 21.

How does meiosis generate genetic diversity?

Through random orientation of bivalents and crossing over.

What is crossing over?

Exchange of DNA and genes between non-sister chromatids in homologous chromosomes.

What forms a bivalent?

A pair of homologous chromosomes during meiosis.

What is synapsis?

The pairing process of homologous chromosomes during meiosis.

How is genetic material exchanged in crossing over?

Non-sister chromatids exchange DNA at random points.

What determines allele inheritance in meiosis?

Random orientation of bivalents during first metaphase.

What is the number of combinations generated by random orientation of bivalents in humans?

2^23, or over 8 million combinations.

What happens during Prophase I of meiosis?

Homologous chromosomes pair up, and crossing over occurs.

What happens during Metaphase I of meiosis?

Homologous chromosomes attach to spindle microtubules and align at the equator.

What happens during Anaphase I of meiosis?

Homologous chromosomes separate and move to opposite poles.

What happens during Telophase I of meiosis?

A nuclear membrane forms around chromosomes, and two haploid cells are produced.

What happens during Prophase II of meiosis?

Chromatids condense, and crossing over ensures non-identical chromatids.

What happens during Metaphase II of meiosis?

Chromatids attach to spindle microtubules and align at the equator.

What happens during Anaphase II of meiosis?

Chromatids separate and move to opposite poles, becoming chromosomes.

What happens during Telophase II of meiosis?

Four haploid cells are produced with nuclear membranes reassembled.