The Principles and Mechanics of Cellular Division Quiz

Answer: DNA replication

DNA is replicated from a point of origin (known as an OriC). In small plasmid DNA (as found in bacteria), one OriC allows for quick DNA replication, but in human chromosomes, there is too much genetic information for just one OriC, and so there are several origins of replication.

Answer: Growth

Egg cells usually contain large amounts of cytoplasm, with stockpiled organelles and mRNA. There is also a supply of nutrients to sustain the egg after fertilisation. The size of the egg varies - goldfish eggs, for example, are several times larger than human eggs, as the egg of a goldfish must survive for longer as an isolated entity. Absent G1 and 2 phases during cell division will therefore duplicate DNA, but cause the cells to become gradually smaller as no growth occurs.

Answer: A protein

As mentioned, chromosomes are only clearly visible under a microscope when in their condensed forms. As a result, it is in this form (the mitotic chromosomal form) that most would recognise a chromosome (an X-shaped body). Even in the less tightly packed form (known as an interphase chromosome), the DNA is condensed by proteins known as histones.

Answer: Phosphorylation

Phosphorylation is ubiquitous in the human body, controlling numerous processes, from smooth muscle contraction to cell signalling to cell division.

Tyrosine, threonine and serine are the three non-essential alcohol amino acids, and therefore contain an -OH group on their R chain. It is to the R chain's -OH group that phosphate groups attach in phosphorylation, bringing about a conformational (and therefore functional) change within a protein.

Answer: The Golgi body

The Golgi body/apparatus (named for Camillo Golgi) is often referred to as the post office of the cell. It receives vesicles from the endoplasmic reticulum and, based on the vesicle's contents, adds or alters a tag on the vesicle which will target it either to a specific organelle or to the extracellular space.

Answer: Centrosomes

Centrosomes establish the poles of the cell in prophase and nucleate the microtubules which form the spindle. It is often necessary to nucleate microtubules, as high concentrations of tubulin (its monomer) are required for its synthesis.
Like centrosomes in the cell cycle, basal bodies nucleate microtubules which grow to form cilia and flagella.

Answer: Cortex microtubules

Microtubules are one group of structures which form the cytoskeleton of a cell. As well as performing a structural role, microtubules are vital for the transport of vesicles throughout the cell. They are synthesised by many tubulin dimers adding together to form protofilaments, and these protofilaments join to form tubular structures (the microtubules).

Answer: Kinetochore

In mitosis, a centromere will possess two kinetichores (one for each sister chromatid). Due to a cell's strict control over the progression of the cell cycle, anaphase can only occur once each kinetichore has attached to a kinetichore microtubule (note: one kinetichore on the centromere will attach to a kinetichore microtubule from one pole and the second kinetichore will attach to a kinetichore microtubule from the other pole).

Answer: The metaphase plate

Once both kinetichores of a centromere have established a connection to the spindle fibres (the microtubules) the chromosome experiences tension. Each chromosome is being pulled equally to opposite poles of the cell, which were established by the centrosomes in late prophase.

As a result, the chromosomes line up along the mid-line of the cell, also called the metaphase plate, or cell equator.

Answer: A kinesin

Both kinesins and dyneins act on microtubules. Microtubules are polarized structures, with the + end at the periphery and the - end at the root (centrosome or basal body). Kinesins "walk" along the microtubules, carrying cargo (usually vesicles) to the + end. Dyneins funtion in a similar way, but carry cargo to the - end.

Answer: Separase

In mitosis, cohesin is cleaved in metaphase, which triggers anaphase. In meiosis, however, the cohesin is cleaved earlier in the cell cycle, leaving only the centromere to hold the chromosomes together. This is important as, in meiosis, the chromosome arms must be free from their sister chromatids to allow crossing over (swapping of genetic information), ensuring genetic variation amongst offspring.

Answer: Catastrophe

M-Cdks, which help regulate the cell cycle, become active at the end of G2 phase, triggering mitosis (M phase). M-Cdk inactivates MAPs (microtubule associated proteins) which otherwise stabilise the microtubules, and activate catastrophin, which increases microtubule catastrophe.

Answer: Intermediate filaments

Intermediate filaments make up the third group of structures which form the cytoskeleton (along with microtubules and microfilaments). Unlike microfilaments (actin) and microtubules, intermediate filaments are fairly stable - most don't polymerise and depolymerise constantly.

Intermediate filaments therefore often perform structural roles in the cell, such as keratin in epithelial cells.

Answer: Myosin

Actin is an example of a microfilament and myosin is a motor protein which act on actin. Myosin II is known as muscle myosin, though it is found in a range of cells. As well as causing muscle contraction, myosin II functions in the contractile ring in the cleavage furrow, causing cytokinesis.

Answer: Integrins

Integrins are proteins found in the cell membrane of many cells. They connect actin within the cell to collagen fibres in the extracellular matrix (via fibronectin, a linker protein).