Dunn M&C - COMPLETE Cell Division/Cycle

homologous chromosomes

two different versions of a chromosome containing different DNA sequence variants (alleles)

haploid

a cell with one complete set of chromosomes

diploid

a cell with two sets of chromosomes (two homologous chromosomes)

sister chromatid

replication is the synthesis of this virtually identical structure

• synthesis of this takes place during S phase

• stays tethered to one another during G2 phase until the end of cell division

encode

contain the sequence for…

alleles

different sequence versions of the same gene

ploidy

number of each chromosome that an organism carries. one set of chromosomes is designated n.

karyotype

the complete set of chromosomes in a cell.

sex chromosomes

special case - they differ in number or type between males and females

autosomes

simply non-sex chromosomes

meiosis

divided into different sections I and II — purpose in mammals in sexual reproduction.

• occurs in germ cells and produces haploid gametes with half the genetic information of that organism (1n in mammals)

• creates a mix up in the genetic pool; which mitigates the negative effect of random recessive mutations and creates a more diverse population

gametes

haploid cells produced during meiosis that are involved in sexual reproduction

• sperm and female eggs

zygotes

fertilized egg — a diploid cell that gives rise to all of the cells of an organism

• divides mitotically

reductive division

A type of cell division that reduces the chromosome number by half, occurring in meiosis: cells go from diploid to haploid in meiosis I

crossing over

homologous recombination where each chromosome is joined at the exact same spot to the paired homolog

• does not occur during mitosis

• genetic mix and variation

• occurs to maternal and paternal alleles during prophase of meiosis I

bivalents/tetrads

paired homologous chromosomes which physically exchange segments of genetic material

Meiosis I

This stage involves two key processes: homologous chromosomes are separated into different cells, and crossing over occurs.

• Prophase I is where homologous chromosomes pair up and exchange genetic material through crossing over, creating genetic diversity.

• Metaphase I, where bivalents (paired homologous chromosomes) line up at the cell's equator.

• Anaphase I separates the homologous chromosomes, pulling them to opposite poles.

• Finally, in telophase I, the cell divides into two haploid cells containing one set of chromosomes, followed by cytokinesis.

Meiosis II

Similar to mitosis and includes several phases resulting in the separation of the sister chromatids

• In prophase II, the nuclear envelope dissolves, and spindle fibers form;

• metaphase II, Replicated chromosomes line up at the middle of the (now haploid) cell in preparation for separating sister chromatids during anaphase.

• Anaphase II, sister chromatids are pulled apart to opposite poles

• Telophase II involves the reformation of nuclear envelopes around the separated chromatids and leads to cytokinesis.

• The result is four haploid cells, each with a unique combination of chromosomes.

meiosis in mammals

meiosis occurs only in germ cells of the gonads to generate highly specialized haploid gametes

nondisjunction

meiotic chromosome segregation errors which can occur in either the first or second meiotic divisions

• can lead to genetic diseases such as down’s syndrome and kleinfelter syndrome

• during meiosis, perfect bipolar attachment and chromosome/chromatid separation must occur during two different metaphase-to-anaphase transitions

aneuploid

a cell with abnormal chromosome number

down’s syndrome/trisomy 21

aneuploidy of chromosome 21 and sex chromosomes are relatively tolerable (non-lethal) disjunction events in humans

cell cycle checkpoints

points in the cell cycle where it can be stopped if conditions are unfavorable

• usually enforced by inhibiting Cyclin-Cdk complexes which drives the cell cycle

• crucial for cancer

start checkpoint

restriction point, determines whether the environment is favorable

• once a cell passes this point, it has committed to completing the cell cycle

G2/M checkpoint

checks if all DNA is replicated and if the environment is favorable to enter mitosis

apoptosis

programmed cell death

• the cell initiates a set of changes that lead to the degradation of its contents and “blebbing” such that contents aren’t released extracellularly

• used frequently during development to remove old and/or damaged cells in adults

metaphase-to-anaphase transition

checks if the chromosomes are connected to the spindle

• when passes, it triggers anaphase and proceeds to cytokinesis

“blebs” in apoptosis

the fragments after broken down are sent through the endocytic pathway to the lysosome for further degradation/recycled (phagocytosis)

chiasma

point where the homologous chromosomes cross over

Cell division

the process by which a single cell becomes two daughter cells

Cell proliferation

refers to when cells are actively growing and dividing to increase in number.

binary fission

a type of reproduction in which a prokaryote cells divides into two cells

eukaryotes division

mitosis and cytokinesis

interphase

the phase of the cell cycle where the cell prepares for division (BUT NON-DIVIDING), including DNA replication and growth.

quiescence phase

reversible, non-dividing state

G0 phase

cells that are actively not dividing; part of interphase

G1 phase

cell grows in size and carries out normal metabolism; organelles duplicate

S phase

DNA replication duplicates (sister chromatid synthesis); centrosomes duplicate

G2 phase

cell grows in size and prepares for mitosis

M phase

mitosis and cytokinesis; close association of chromosome copies is maintained specifically here

chromosome replication

this happens during S (synthesis) phase

chromosome condensation; cell divides

this happens during M (mitosis) phase

unreplicated chromosome

one very long double-stranded DNA

replicated chromosome

two very long double-stranded DNAs

condensed replicated chromosome

two very long double-stranded DNAs

centromere

a specialized region of a chromosome where the two sister chromatids are joined together. It plays a crucial role during cell division as the point at which the spindle fibers attach during mitosis and meiosis

homologous chromosomes

chromosomes that are paired during meiosis, having the same structure and sequence of genes but potentially different alleles

sister chromatid

two identical copies of a single chromosome that are connected by a centromere.

haploid

a cell with one complete set of chromosomes

diploid

a cell with two sets of chromosomes; humans are these

mitosis process

diploid eukaryotic cells; which produces two identical daughter cells

meiosis process

diploid eukaryotic cells which produces four daughter cells having half the genetic content of the parent cell

MPF (mitosis promoting factor)

a heterodimer that is composed of cyclin-dependent kinase (CDK) and cyclin proteins; triggers the progression of the cell cycle from G2 phase to M phase, enabling the onset of mitosis

cyclin-dependent kinase (Cdk)

catalyzes phosphorylation of other proteins to start mitosis; inactive without cyclin

cyclin

regulatory protein that controls the progression of cells through the cell cycle by activating cyclin-dependent kinases; protein levels constantly change over time through the cycle

• when cells are successfully in mitosis, this is degraded to turn off MPF to ensure the cell cycle can only move in the forward direction

kinase

an enzyme that uses ATP to transfer a phosphate to a target, yielding ADP + target-P

• phosphorylation lends usually to a conformational change

CDK process

1. cyclin activates the CDK by binding to the CDK and controls progression throughout the cell cycle.

2. recognizes target proteins that are then phosphorylates by CDK.

3. left with a phosphorylated target protein and cyclin degrades afterwards

mitotic cell divsion stages abbreviation

PPMAT

prophase

chromosomes condense, centrosome radiate microtubules and migrate to opposite poles.

prometaphase

nuclear envelope fully breaks down in order to interact with the microtubules in the cytoplasm; microtubules of the mitotic spindle attach to chromosomes

centrosomes

consist of two centrioles, which consist of modified microtubules

kinetochore

a mitosis-specific protein assembly on centromeres that connects the chromosome to microtubules of the spindle apparatus

• each sister chromatid has a centromere which a kinetochore is assembled.

• each replicated chromosome to undergo bipolar attachment

bipolar attachment

attachment to both poles of the spindle apparatus

spindle apparatus

contains MT motor proteins (not depicted) that push and pull the poles apart, “drag” sister chromatids to their pole

metaphase

chromosomes align in the center of the cell

anaphase

sister chromatids separate and travel to opposite poles by the microtubules

telophase

nuclear envelope re-forms and chromosomes condense; cytokinesis begins during this

cytokinesis

cytoplasmic division to yield cells; different in animal and plant cells

• animal cells: requires a contractile ring

• plant cells: requires a cell plate

contractile ring

a ring of actin filaments that forms against the inner face of the cell membrane at the equator of the cell perpendicular to the axis of what was the spindle

• “acts like a belt constraining against someone’s waist’

cell plate

since plant cells have a cell wall, the division of the cell requires a new cell wall; this forms during late anaphase and telophase and vesicles containing cell wall components fuse to form a new cell wall in the middle of the dividing cell.