Biol 214 - Chromosomes, Mitosis & Meiosis

Exam 2 Biol 214 material. Includes the process of making new body or gamete cells.

Mitosis

(A growth process) divides the replicated DNA equally and precisely, generating daughter cells, which are exact genetic copies of the parent cell.

Meiosis

(A process of sexual reproduction) produces daughter nuclei with half the number of chromosomes of the parental nucleus. The arrangements of genes on chromosomes are different from those in the parent cell.

Gene

A segment of DNA located on chromosomes that carries the instructions for building a specific molecule, which determines an organism's physical traits and helps cells function.

mRNA, or messenger RNA, carries genetic instructions from a gene (DNA) in the cell's nucleus to the ribosomes in the cytoplasm. Ribosomes then follow the DNA instructions.

Histones

Positively-charged proteins that have an affinity for negatively-charged DNA. Histones provide the structural support for the chromosome by acting as spools to condense long DNA molecules so it can fit into the cell nucleus. Not present in prokaryotes, but present in eukaryotes and archaea.

Nucleosomes

The basic structural and functional unit of chromatin in eukaryotes, consisting of a segment of DNA wrapped around a histone octamer (four histones). Can be further condensed into chromosomes.

Chromatin

The loose, "unpacked" form of the genetic material found in a cell's nucleus during interphase, allowing for gene expression.

Chromosomes

The nuclear units of genetic information divided and distributed by mitotic cell division. Composed of one DNA and proteins that stabilize the DNA molecules.

Chromosome Complement

The complete set of chromosomes of a eukaryotic organism.

Karyotype

An image of a metaphase chromosome complement, arranged according to size and shape.

Sister Chromatids

Two identical strands of DNA that make up a replicated chromosome, joined together by a common centromere. Sister chromatids cannot exist independently without a centromere (independent “sister chromatids” are just called chromosomes).

Sister Chromatid Adhesion

The process of physically linking identical sister chromatids to ensure proper chromosome segregation during cell division.

Somatic Cell

Any cell of a living organism other than the reproductive cells.

Centromere

The DNA and protein region on a chromosome where chromatids are joined in sister chromatid adhesion.

Kinetochore

The dynamic protein complex assembled at the centromere that forms the attachment site for the spindle fibers during prometaphase.

Kinetochore Microtubules

A type of spindle microtubule (protein complex) that attaches to the kinetochore and on the centromere of a chromosome.

Nonkinetochore Microtubules/Polar Microtubules

A type of spindle microtubule (protein complex) that are not attached to chromosomes. They are responsible for pushing the spindle poles apart, contributing to cell elongation during anaphase.

Equatorial Plane/Metaphase Plate

An imaginary plane in the middle of the cell where replicated chromosomes align during the metaphase stage.

Interphase

A phase of the eukaryotic cell cycle that contains the G₁, S, and G₂ subphases.

Extended Interphase/G₀ Phase

A temporary or permanent quiescent (inactive) stage outside the active cell cycle (G₁, S, and G₂ phases) where cells are not preparing to divide.

G₁ Phase

The first and longest phase of interphase in the cell cycle, where a cell grows, performs normal functions, and synthesizes necessary proteins and organelles for DNA replication and division.

The only phase of the cell cycle that varies in length – other phases are typically uniform in length. Also the stage in which many cell types stop dividing and are shunted into a G₀ phase – some cells in G₀ reenter G₁, others never resume the cell cycle.

S phase

The phase of interphase where a cell synthesizes or replicates its DNA, preparing for cell division by ensuring each daughter cell receives a complete set of genetic material. During the S phase, the cell also duplicates its centrosomes.

G₂ Phase

The phase of interphase where the cell further grows, synthesizes proteins and organelles needed for cell division (mitosis), and checks the replicated DNA for errors before it enters mitosis.

The substages of M phase (mitosis)

Prophase, Prometaphase, Metaphase, Anaphase, Telophase

Prophase

A stage in mitosis (M phase) where duplicated DNA condenses into visible chromosomes, the mitotic spindle forms to organize and move the chromosomes for separation in subsequent phases. Condensation gives each chromosome a characteristic shape, determined by length and centromere location.

Prometaphase

A stage in mitosis (M phase) where the nuclear envelope breaks down, spindle microtubules grow from centrosomes at opposite spindle poles toward the center of the cell, and a kinetochore forms on each sister chromatid at the centromere.

Metaphase

Spindle microtubules move chromosomes into alignment at the spindle midpoint (metaphase plate).

Anaphase

The spindle separates sister chromatids and pulls them toward opposite spindle poles. Movement continues until the separated chromatids (daughter chromosomes) have reached the two poles. At this point, chromosome segregation is complete.

Telophase

The spindle disassembles and chromosomes at each spindle pole decondense, returning to the extended state typical of interphase. The nucleolus (a dense, non-membrane-bound structure located inside the nucleus responsible for synthesizing ribosomal RNA (rRNA) and assembling ribosomes) reappears, RNA transcription resumes. A new nuclear envelope forms around the chromosomes at each pole producing the two daughter nuclei. At this point, nuclear division is complete – the cell has two nuclei.

Cytokinesis

Produces two daughter cells, each with one of the two daughter nuclei. The plane of cytoplasmic division is determined by the layer of microtubules that persists at the former spindle midpoint.

• In animals, protists, and many fungi, a furrow girdles the cell and deepens until it cuts the cytoplasm into two parts.

• In plants, a cell plate forms between the daughter nuclei and grows laterally until it divides the cytoplasm in two.

The substages of the M phase (meiosis)

Prophase I, Prometaphase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Prometaphase II, Metaphase II, Anaphase II

Synapsis

The precise, gene-by-gene pairing of homologous chromosomes, occurring during Prophase I and forming structures called tetrads.

Crossing-Over

The process in which chromatids of homologous chromosomes physically exchange segments of their DNA, resulting in genetic recombination and increased genetic diversity.

Chiasmata/Crossovers

Sites where crossing-over has occurred. Marked by thickened spots.

Prophase I

Sister chromatids fold and condense in the nucleus. The two chromosomes of each homologous pair undergo synapsis, forming tetrads.

Chromatids of homologous chromosomes cross-over. Sites where crossing-over has occurred are marked by crossovers/chiasmata.

Prometaphase I

The nuclear envelope breaks down in meiosis. The spindle enters the former nuclear area. One chromosome of each pair attaches to the kinetochore microtubules leading to one spindle pole; the other chromosome attaches to the opposite spindle pole's kinetochore microtubules. Sister chromatids remain attached to each other.

Metaphase I

Spindle microtubules align the tetrads on the equatorial plane between the two spindle poles.

Anaphase I

The two chromosomes of each homologous pair separate and move to opposite spindle poles as the spindle microtubules contract. Each chromosome still has two sister chromatids.

Telophase I

A brief stage in which there is little or no change in the chromosomes.

Interkinesis

Follows Telophase I. The single spindle of the first meiotic division disassembles and the microtubules reassemble into two new spindles for the second division.

Prophase II

Chromosomes condense and a spindle forms in meiosis.

Prometaphase II

When the nuclear envelope breaks down in meiosis and the spindle enters the former nuclear area. Spindle microtubules from one pole attach to the kinetochore of one sister chromatid, and spindle microtubules from the opposite pole attach to the kinetochore of the other sister chromatid.

Metaphase II

Spindle microtubules align chromosomes on the metaphase plate in meiosis.

Anaphase II

Spindles separate the two chromatids of each chromosome and pull them to opposite spindle poles in meiosis. Chromatids segregate to the two poles, forming chromosomes.

Telophase II

Chromosomes decondense to an extended interphase (G₀) state. Spindles disassemble and new nuclear envelopes form around the chromatin. The result is four haploid cells.