Textbook Chapter 12

Question 1

Cell division and characteristics of life

Answer 1

The ability of organisms to produce more of their own kind is the one characteristic that best differentiates living and nonliving matter. This capacity to procreate like all biological functions has a basis in cell division.

Virchow quote: “Where a cell exists, there must have been a pre-existing cell, just as the animal arises from only an animal and the plant from only a plant”.

The continuity of life is based on the reproduction of cells or cell division.

Question 2

Several roles of cell division

Answer 2

-When prokaryotic cells divides it is actually reproducing for it is creating another organism
-The same is true for unicellular eukaryotes (ex: amoebas)
-In multicellular eukaryotes cell division enables each of them to develop from a single cell, the fertilized egg
-Cell division is also used in renewal and repair in grown multicellular eukaryotes replacing cells that die from wear and tear or accidents (ex: dividing cells in bone marrow make new blood cells)
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Question 3

Cell division and cell cycle

Answer 3

Cell division is an integral part of the cell cycle from when a cell is first formed through the division of a parent cell until its own division into two daughter cells. Passing identical genetic function is a crucial function of cell division.

Question 4

Cell division and genetically identical daughter cells

Answer 4

In both prokaryotes and eukaryotes most cell division involves the distribution of identical genetic material-DNA- to two daughter cells. (Meiosis is the exception in eukaryotes which can produce sperm and eggs).

Question 5

Cellular organization of genetic material

Answer 5

A cell’s DNA, its genetic information, is called its genome. A prokaryote’s genome is often a single molecule of DNA, eukaryotic genomes usually consist of a number of DNA molecules. Before the cell can divide, all of the DNA needs to be copied and then the two copies separated so that each daughter cell ends up with a complete genome.

Question 6

Chromosomes and their role

Answer 6

DNA replication and distribution is manageable because of the fact that so much DNA are packaged into structures called chromosomes. Each eukaryotic chromosome consists of one very long linear DNA molecule associated with many proteins. The DNA molecule carries several hundred to a few thousand genes, the units of information that specify an organism’s inherited traits. The associated proteins maintain the structure of the chromosome and help control the activity of the genes. The entire complex of DNA and proteins that are the building material of chromosomes are referred to as chromatin.

Question 7

Chromatin definition

Answer 7

The complex of DNA and proteins that makes up eukaryotic chromosomes. When the cell is not dividing, chromatin exists in its dispersed form, as a mass of very long, thing fibres that are not visible under a light microscope.

Question 8

Somatic cells

Answer 8

Any cell in a multicellular organism that is not a sperm or egg, or their precursors.

Question 9

Gametes

Answer 9

A haploid reproductive cell, such as an egg or sperm. Gametes unit during sexual reproduction to produce a diploid zygote.

Question 10

Chromosome numbers in each species

Answer 10

Every eukaryotic species has a specific number of chromosomes in each cell’s nucleus. The nuclei of each somatic cell in the human body contains 46 chromosomes, made up from two sets of 23 chromosomes, one set from each parent. Gametes have half as many chromosomes as somatic cells, with humans having 23. The number of chromosomes in somatic cells varies among species. Ex: 18 in cabbage plants, 48 in chimpanzees and etc

Question 11

Distribution of chromosomes during eukaryotic cell division

Answer 11

When a cell is not dividing or even as it replicates its DNA in preparation for cell division, each chromosome is in the form of a long thin chromatin fibre strand. After DNA replication, the chromosomes condense and become tightly coiled and folded, so that the chromosomes become shorter and thicker and visible under microscopes.

Question 12

Sister chromatids definition

Answer 12

Two copies of a duplicated chromosome attached to each other by proteins at the centromere and sometimes, along the arms. While joined, two sister chromatids make up one chromosome. Eventually separated in metaphase of mitosis and meiosis II.

Question 13

Cohesins

Answer 13

protein complexes that that are initially attached along the sister chromatids, and are responsible for sister chromatid cohesion

Question 14

Centromeres

Answer 14

Each sister chromatid has a centromere, a region made up of repetitive sequences in the chromosomal DNA where the chromatid is most closely attached to its sister. This attachment is mediated by proteins bound to the centromeric DNA; other bound proteins condense the DNA giving it a “slutty” waist. The portion of the chromatid on either side of the centromere is referred to as an arm of the chromatid.

Question 15

Separation of sister chromatids

Answer 15

Once separated in the cell division process sister chromatids are referred to as single chromosomes, doubling the number of chromosomes in a cell. Thus each new nucleus receives a collection of chromosomes identical to that of the parent cell.

Question 16

Mitosis and Cytokinesis

Answer 16

Cytokinesis (the division of the cytoplasm in a cell) immediately follows mitosis (the division of the genetic information in a cell).

Question 17

Walther Flemming

Answer 17

In 1882 developed dyes that allowed him to observe the behaviour chromosomes during mitosis and cytokinesis (he coined the terms mitosis and chromatin).

Question 18

Phases of the cell cycle

Answer 18

Interphase
-G1 Phase
-S phase
-G2 phase
Mitotic Phase

Question 19

Interphase in detail

Answer 19

During G1 a cell grows producing proteins and cytoplasmic organelles such as the mitochondria and the endoplasmic reticulum. Duplication of the chromosomes occurs in the synthesis phase and a cell continues to prepare for cell division in G2.

Question 20

Time of cell cycle

Answer 20

G1- 5-6 hours
S- 10-12 hours
G2- 4-6 hours
M- >1 hour

Question 21

Cells that divide infrequently

Answer 21

These cells spend their time in G1 or in G0 doing their jobs, for example a nerve cell carries impulses.

Question 22

G2 in detail

Answer 22

-A nuclear envelope encloses around the nucleus
-The nucleus contains one or more nucleoli
-Two centrosomes have formed by duplication of a single centrosome. Centrosomes are regions in animal cells that organize the microtubule of the spindle. Each centrosome contains two centrioles.

Question 23

Prophase in detail

Answer 23

-The chromatin fibres become more tightly coiled, condensing into discrete chromosomes observable with a light microscope
-The nucleoli disappear
-Each duplicated chromosome appears as two identical sister chromatids joined at their centromeres and, in some species, all along their arms by cohesion
-The mitotic spindle begins to form. It is composed of the centrosomes and the microtubules that extend from them. The radial arrays of shorter microtubules that extend from the centrosomes are called asters (“stars”)
-The centrosomes move away from each other, propelled partly by the lengthening microtubules between them.

Question 24

Prometaphase in detail

Answer 24

-The nuclear envelope fragments
-The microtubules extending from each centrosome can now invade the nuclear area
-The chromosomes have become even more condensed
-A kinetochore, a specialized protein structure, has now formed at the centromere of each chromatid (two per chromosome)
-Some of the microtubules attach to the
“kinetochore microtubules” which jerks the chromosome back and forth
-Nonkinetochore microtubules interact with those from the opposite pole of the spindle

Question 25

Metaphase in detail

Answer 25

-The centrosomes are now at opposite end poles of the cell -The chromosome have all arrived at the metaphase plate, a plane that is equidistant between the two spindle's poles. The chromosomes' centromeres lie at the metaphase plate. -For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles.

Question 26

Anaphase

Answer 26

-Anaphase is the shortest stage of mitosis, often lasting only a few minutes -Anaphase begins when the cohesin proteins are cleaved, allowing the two sister chromatids of each pair to separate. each chromatid thus becomes a distinct chromosome. -The two liberated daughter chromosomes begin moving towards the opposite ends of the cell as the microtubules shorten. -As the microtubules are attached to the centromeres, they move centromere first. -The cells elongate as the nonkinetochore microtubules lengthen. -By the end of anaphase, the two ends of the cell have equivalent and complete sets of chromosomes

Question 27

Telophase

Answer 27

-Two daughter nuclei form in the cell. Nuclear envelopes arise form the fragments of the parent cell's nuclear envelope and other portions of the endomembrane system -Nucleoli reappear -The chromosomes become less condensed -Any remaining spindle microtubules are depolymerized -Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.

Question 28

Cytokinesis

Answer 28

-The division of the cytoplasm is usually well under way by late telophase, so the two daughter cells appear shortly after mitosis. -In animal cells, cytokinesis involves the formation of a cleavage furrow, which pinches the cell in two. In plant cells a cell plate forms.

Question 29

Mitotic spindle definition

Answer 29

An assemblage of microtubules and associated proteins that are involved in the movement of chromosomes during mitosis.

Question 30

Microtubules definiton

Answer 30

A hollow rod composed of tubulin proteins that makes up part of the cytoskelton in all eukaryotic cells and is found in cilia and flagella.

Question 31

Kinetochore definition

Answer 31

A structure of proteins attached to the centromere that links each sister chromatid to the mitotic spindle.

Question 32

Centriole definition

Answer 32

A structure in the centrosome of an animal cell composed of a cylinder of microtubule triplets arranged in a 9+0 pattern. A centrosome has two centrioles.

Question 33

Centrosome definiton

Answer 33

A structure present in the cytoplasm of animal cells that functions as a microtubule organizing centre and is important during cell division. Has two centrioles.

Question 34

Actin and myosin filaments in cleavage furrow

Answer 34

On the cytoplasmic side of the furrow is a contractile ring of actin filaments associated with molecules of the protein myosin. The actin filaments interact with the myosin molecules causing the ring to contract. The contraction of the dividing cell's ring of microfilaments is like pulling a drawstring. The cleavage furrow deepens and the parent cell is pinched into two.

Question 35

Cytokinesis in plant cells

Answer 35

Due to the cell wall there is no cleavage furrow, instead during telophase vesicles from the Golgi apparatus move along microtubules to the middle of the cell, where they coalesce and produce a cell plate. Cell wall materials inside the vesicles collect inside the plate until its surrounding membrane fuses with the plasma membrane along the perimeter of the cell. Two daughter cells result with their own plasma membrane.

Question 36

Binary fission in bacteria

Answer 36

Prokaryotes (bacteria and archaea) can undergo a type of reproduction in which they double in size firsthand then divide to form two cells. Binary fission means to divide in half, referring to this process and the asexual reproduction of single celled eukaryotes like amoebas.

Question 37

Binary fission process

Answer 37

-Chromosome replication begins, with one copy moving all the way to one side of the cell at the origin while the other is at the opposite end -With one copy each at one end the cell starts to elongate -After replication finishes, a tubulin like protein pinches the membrane of the cell and a new cell wall is deposited -Two daughter cells have formed

Question 38

What does the frequency of cell division depend on?

Answer 38

Depends on the type of cell. Ex: human skin cells divide frequently throughout life but liver cells maintain the ability to divide but save it until the need arises. Some mature cells like muscle or nerve cells do not divide in a mature human. These differences result from regulation at a molecular level.

Question 39

Cell cycle control system definition

Answer 39

A cyclically operating set of molecules in the eukaryotic cell that both triggers and coordinates key events in the cell cycle

Question 40

Checkpoint definition

Answer 40

A control point in the cell cycle where stop and go-ahead signals can regulate the cycle.

Question 41

Regulatory molecules are..?

Answer 41

Proteins of two types: protein kinases and cyclins.

Question 42

When does a kinase become active?

Answer 42

When attached to a cyclin, a protein gets its name from its cyclically fluctuating concentration in the cell., these kinases are called cyclin-dependent kinases or Cdks.

Question 43

What does MPF stand for and its definition?

Answer 43

Maturation-promoting factor; a protein complex required by a cell to progress from late interphase to mitosis. The active form consists of a cyclin and a protein kinase.

Question 44

Steps of the molecular mechanisms that regulate the cell cycle from G1-M

Answer 44

1. Synthesis of cyclin begins in the late S phase and continues through G2. Cyclin is protected from degradation at this stage so it accumulates. 2. Cyclin combines with Cdk, producing MPF. When enough MPF molecules accumulate, the cell passes the G2 checkpoint and begins mitosis. 3. MPF promotes mitosis by phosphorylating various proteins. MPF's activity peaks during metaphase. 4. During anaphase, the cyclin component of MPF degrades, terminating the M phase. The cell enters the G1 phase. 5. During G1, the degradation of cyclin continues and the Cdk component of MPF is recycled.

Question 45

G1 Checkpoint in detail

Answer 45

Called the restriction point, the G1 checkpoint is where the cell needs to be given a go-ahead signal, in the presence of this signal the cell will usually go on to complete the rest of the cell cycle. If there is no signal, the cell exist the cell cycle and enters G0 phase.

Question 46

G0 phase

Answer 46

A non-dividing state occupied by cells that have left the cell cycle, sometimes reversible.

Question 47

Mitosis checkpoint in detail

Answer 47

When all the chromosomes are attached to spindle fibres from both poles, a go-ahead signal allows the cell to continue into anaphase. If any chromosomes are not attached to the spindle fibres the cell receives a stop signal.

Question 48

S phase checkpoint

Answer 48

Stops cells with DNA damage from replication.

Question 49

Growth factor definition

Answer 49

A protein released by certain cells that stimulates other cells to divide. Different cell types respond specifically to different growth factors or combinations of growth factors.

Question 50

Example of growth factor

Answer 50

platelet-derived growth factor, made of blood cell fragments called platelets. Required for the division of cultured fibroblasts, a type of connective tissue.

Question 51

Density-despondent inhibition definition

Answer 51

The phenomenon observed in normal animal cells that causes them to stop dividing when they come into close contact with each other.

Question 52

Anchorage dependence definition

Answer 52

The requirement that a cell must be attached to a substratum (underlying layer or surface) in order to initiate cell division.

Question 53

Do cancer cells display density-despondent inhibition or anchorage dependence?

Answer 53

Cancer cells do not exhibit either of these traits.

Question 54

Why are cancer cells believed to never stop dividing?

Answer 54

Cancer cells do not follow the normal signals that regulate the cell cycle, and do not stop dividing even as growth factors have been depleted. Thus one can logically assume cancer cells have no use for growth factors to divide.

Question 55

Transformation definition

Answer 55

The conversion of a normal animal cell to a cancerous cell.

Question 56

Benign tumour

Answer 56

A mass of abnormal cells with specific genetic and cellular changes such that the cells are not capable of surviving at a new site and generally remain at the site of the tumour's origin.

Question 57

Malignant tumour

Answer 57

A cancerous tumour containing cells that have significant genetic and cellular changes and are capable of invading and surviving at new sites. Malignant tumours can impair the functions of one or more organ.