Biology: Chromosomes, Mitosis and Meiosis

Chromosomes are the major carriers of genetic information in eukaryotes.
They are located within the cell nucleus.
Although often referred to as "colored bodies," chromosomes are typically colorless.
The term reflects their ability to be stained by certain dyes.

Genes are informational units specifying cell functions, made of DNA.
Chromatin is composed of DNA and associated proteins, forming chromosomes in eukaryotes.
Chromosomes facilitate DNA sorting into daughter cells.
In eukaryotes, DNA winds around specific proteins, forming chromatin, which folds and packages to create individual chromosomes.

Nucleosomes: Eight histones plus wrapped DNA.
Histones package DNA into coiled structures (chromosomes).
Histone proteins comprise the bead-like structure in nucleosomes, wrapped in DNA.
These nucleosomes are organized into coiled loops, held together by non-histone scaffolding proteins.
The wrapping of DNA into nucleosomes is the first level of chromosome structure.

The cell cycle describes the series of stages through which a cell progresses.
Interphase: The longest part, with three sub-phases:
- G1 (first gap phase): Cell growth and preparation for DNA synthesis. Enzymes for DNA synthesis increase in activity during this phase. Proteins needed for cell division are also synthesized enabling transition to the S phase.
- S (synthesis phase): DNA replicates and histone proteins are synthesized. Duplicates the chromosomes.
- G2 (second gap phase): Protein synthesis increases further. Organelles, chromosomes, and centrioles (in animal cells) replicate in preparation for cell division.
M (mitotic phase):
- Mitosis (nuclear division): Creates identical nuclei. Begins at the end of G2 phase.
- Cytokinesis (cytoplasm division): Begins before mitosis ends. Divides the cytoplasm, creating two daughter cells.

Sister chromatids are physically linked by a ring-shaped protein complex called cohesin.
Cohesin is distributed along the chromatids' arms, but is concentrated at the centromere.

The division of the cytoplasm.
In animals and fungi, a cleavage furrow forms by action of a contractile ring of actin and myosin filaments. The ring contracts to divide the cytoplasm.
In plants a cell plate forms. This plate grows and divides the cell forming new walls separating the two daughter cells.

Checkpoints monitor the progress of cell division, ensuring events occur correctly before proceeding to the next stage.
Cyclin-dependent kinases (Cdks): Protein kinases that control the cell cycle, only active when bound to cyclins.
Cyclins: Regulatory proteins; levels fluctuate during the cell cycle.
Cyclin–Cdk complexes: Formed through interaction between cyclins and Cdks.

DNA replication begins at a single site on the bacterial DNA.
Replication continues, in both directions, from the initiation site.
When replication is complete the cell begins to divide through the inward growth of the plasma membrane.
Two identical prokaryotic cells result.

Reproduction through the fusion of two haploid sex cells from two parents and results in a single diploid zygote.
Meiosis is the basis for sexual reproduction.

Haploid (n): Contains one member of each homologous chromosome pair.
Diploid (2n): Contains paired homologous chromosomes (similar in length, shape, other features).

A two-stage process that produces four haploid daughter cells from a single diploid cell.
Meiosis has the following primary stages:
- Prophase I: Homologous chromosomes pair up (Synapsis), forming tetrads. Synaptonemal complexes form, and crossing-over between nonsister chromatids occurs.
Metaphase I: Tetrads line up along cell's midplane, held together at chiasmata.
Anaphase I: Homologous chromosomes separate, moving to opposite poles.
Telophase I: Each nucleus contains half the number of original chromosomes (but each chromosome is duplicated).
Prophase II: The duplicated chromosomes condense again.
Metaphase II: Duplicated chromosomes line up on cell's midplane.
Anaphase II: Sister chromatids separate and move to opposite poles.
Telophase II: Nuclei form and cytokinesis occurs, yielding four haploid daughter cells.

Mitosis produces two genetically identical diploid cells from one diploid parent, but meiosis produces four genetically different haploid cells from one diploid parent.
Mitosis doesn't involve homologous chromosome pairing and crossing-over, features unique to meiosis.