Bk3 Ch1 Life Cycle of The Cell

Question 1

Growth curve

Answer 1

Graphical representation of the size of a cell population over time. Applicable not just to bacteria growing in a laboratory flask, but to any population of organisms growing in limiting conditions.

Question 2

Lag phase

Answer 2

Phase of cell population growth in which the cells are adapting to new conditions, and growth is slow.

Question 3

Exponential phase

Answer 3

Stage of a population’s growth curve in which the rate of reproduction is maximal for the environmental conditions, and in which numbers increase exponentially, which is characterised by doubling in cell numbers at equal time intervals; that is, one cell becomes two, two become four, four become eight, and so on.
For bacterial culture Nt=No x 2^n. No= initial number cells; Nt= no bacteria at time t; n=no divisions during time.

Question 4

Stationary phase

Answer 4

The part of a cell population growth curve where the number of new cells is balanced by the number of deaths, so the size of the population remains constant. Growth rate is slow because nutrient availability is depleted and inhibitory waste products have accumulated.

Question 5

Death phase

Answer 5

Part of a cell population growth curve in which all the nutrients are used up and the culture contains mainly dead and dying cells and their waste products, and the number of cells begins to decline.

Question 6

Cell cycle checkpoints

Answer 6

The ordered sequence of events that occurs in cells, which ensure that each phase of the cell cycle has been accurately completed before the cell progresses to the next phase. These checkpoints ensure that damaged or abnormal cells are prevented from proliferating.

Question 7

Cyclin–dependent protein kinases (Cdpks)

Answer 7

Family of protein kinases and key regulators of progression through the cell cycle in all eukaryotic cells. At the appropriate points in the cell cycle, these Cdks phosphorylate and thereby activate particular target proteins that are necessary to complete each stage of the cell

Question 8

Cyclin

Answer 8

Type of protein having no enzyme activity of its own, but its expression levels in the cell rise and falls during the cell cycle. When their concentration is high, cyclins bind to and activate their partner Cdk (hence the name, cyclin-dependent protein kinases). Thus, the oscillating levels of cyclins ensure that Cdks are only activated at the appropriate time in the cell cycle to trigger events by phosphorylating and activating their target proteins.

Question 9

Restriction point

Answer 9

The point in G1 phase of the eukaryotic cell cycle at which a cell becomes committed to the cell cycle. Once a cell passes the restriction point, it is committed to completing the rest of the cell cycle. The restriction point is therefore the first of several important checkpoints in the cycle and it monitors several factors, including nutrient availability, cell size and the presence of growth factors.

Question 10

Retinoblastoma (RB) protein

Answer 10

Product of the tumour suppressor gene, Rb, present in the cell nucleus. In its unphosphorylated state, it acts as a brake on the cell cycle by binding to the transcription factors needed for the expression of the molecular machinery for cell proliferation, including those required for DNA replication. When Rb is phosphorylated by G1 cyclin-Cdks, it releases the transcription factors and the cell can progress through the cell cycle.

Question 11

Tumour suppressor

Answer 11

Genes whose products act to suppress cell proliferation and appear to prevent the formation of a cancer. An example is the Rb gene and its product the Rb protein.

Question 12

DNA replication checkpoint

Answer 12

A key factor in determining whether the a cell will complete the cycle is the state of its DNA. If stopped or ‘stalled’ DNA replication forks are detected and a DNA replication checkpoint is activated. This prevents the activation of M cyclin–Cdk complexes so that the cell is halted at the transition from G2 to M, unless it can resolve the problem.

Question 13

DNA damage checkpoints

Answer 13

Damage to DNA is monitored throughout the cell cycle by DNA damage checkpoints that can halt the cycle at the transitions from G1 to S or G2 to M by inhibiting cyclin–Cdk complexes. A protein called p53, often referred to as the ‘guardian of the genome’, has a key role in these two DNA damage checkpoints.

Question 14

Microtubule associated proteins

Answer 14

Phosphorylated and activated by M cyclin–Cdks, microtubule associated proteins bind to the plus ends of microtubules and regulate their stability. The microtubule associated proteins cross-link microtubules as they overlap in the centre of the cell, thereby stabilising the plus ends and preventing depolymerisation.

Question 15

Kinetochore

Answer 15

Protein complex assembled at the centromere to which spindle microtubules attach during mitosis and meiosis and which forms on each chromosome during prophase. In pairs of sister chromatids, the kinetochores face in opposite directions and bind to microtubules projecting from opposite poles of the cell, thus ensuring that the two chromatids will be segregated to opposite ends of the cell.

Question 16

Kinetochore microtubules

Answer 16

Microtubules that have captured a kinetochore

Question 17

Metaphase plate

Answer 17

An imaginary plane, which is located at the equator (centre) of the spindle, perpendicular to the long axis of the spindle during metaphase and to which the chromosomes, with their two chromatids attached to microtubules from opposite poles, have lined up on.

Question 18

The mitotic spindle checkpoint

Answer 18

At metaphase, the mitotic spindle checkpoint is in place to prevent chromosome segregation from occurring before all the chromosomes have correctly lined up at the metaphase plate. If, for example, a kinetochore has become inappropriately attached to a microtubule extending from the wrong pole, the kinetochore will detach and reattach to another microtubule. Mitosis cannot continue until every chromosome is correctly oriented.

Question 19

Aneuploidy

Answer 19

An abnormal number of chromosomes in a cell. A common cause of miscarriages and birth defects and is also a characteristic of most cancerous cells.

Question 20

Anaphase promoting complex (APC)

Answer 20

A ubiquitin ligase that carries out the ubiquitination of M cyclins, which is itself activated by the M cyclin–Cdk complex, so the cyclins in effect trigger their own destruction once their job is done. The APC tags M cyclin for destruction and breaks down proteins that hold the duplicated chromatids together, triggering chromatid separation.

Question 21

Mitogens

Answer 21

Growth factors, which stimulate cell division by overcoming the inhibitory controls, such as the Rb protein that prevent progression through the cell cycle. Mitogens promote proliferation by activating intracellular signalling pathways that increase the synthesis of G1 cyclins.

Question 22

Oncogenes

Answer 22

Mutated genes, so called from the Greek onkos, meaning protuberance or swelling. The normal gene that, when mutated, can give rise to an oncogene, is known as a proto-oncogene, of which there are many types.

Question 23

Proto– oncogenes

Answer 23

Normal gene that encodes a protein involved in cell proliferation. When mutated it can form an oncogene, which can cause abnormal cell proliferation and hence promote cancer.

Question 24

Neurodegenerative

Answer 24

Inherited diseases in which neurons degenerate in the brain, leading to significant loss of muscle coordination and cognitive function. Huntington’s disease and Parkinson’s disease are examples of neurodegenerative conditions.

Question 25

Necrosis

Answer 25

Typical response of cells to injury in the form of harmful reagents, infections or wounds. The damaged cell swells because the cell membrane fails to control the passage of ions and water, and finally lyses (bursts), releasing its contents, which in vertebrates can stimulate a potentially damaging inflammatory response.

Question 26

Chromatin

Answer 26

Complex of DNA and histone proteins, which makes up eukaryotic chromosomes.

Question 27

Programmed cell death

Answer 27

Also known as apoptosis. Type of cell death where particular cell populations die in a reproducible manner in every individual. Because of its predictable nature, this form of death was believed to occur as the result of a death ‘programme’, and so was named programmed cell death. Well-known examples are the loss of the cells between the digits (e.g. during the development of fingers), and in the tail of the tadpole, when it metamorphoses into a frog. In adult tissues, cell death usually balances cell division, ensuring that tissues and organs retain the same size and structure as old cells are replaced.

Question 28

Apoptosis

Answer 28

Type of cell death where particular cell populations die in a reproducible manner in every individual. Because of its predictable nature, this form of death was believed to occur as the result of a death ‘programme’, and so was named programmed cell death. Well-known examples are the loss of the cells between the digits (e.g. during the development of fingers), and in the tail of the tadpole, when it metamorphoses into a frog. In adult tissues, cell death usually balances cell division, ensuring that tissues and organs retain the same size and structure as old cells are replaced.

Question 29

Caspases

Answer 29

A family of proteases, so named because they are cysteine aspartases, involved in apoptosis by cleaving proteins between adjacent cysteine and aspartate amino acid residues. They are considered to be the cell executioners. Caspases of different sorts are present in cells all the time in an inactive form called a procaspase.

Question 30

Procaspase

Answer 30

An inactive form of caspase present in cells all the time. In order to become an active enzyme, a procaspase requires proteolytic cleavage to reveal the enzyme’s active site. Involved in apoptosis.

Question 31

Apoptosome

Answer 31

A complex formed by the aggregation of cytochrome c and molecules of a protein called Apaf-1, that cleaves and activates the initiator caspase, caspase 9, thus triggering the caspase activation cascade in the intrinsic pathway of apoptosis.

Question 32

Bad

Answer 32

Pro apoptotic gene

Question 33

Differentiation

Answer 33

The changes that occur in a cell as it becomes specialised. Differentiation occurs as a result of differential gene expression.

Question 34

Morphology

Answer 34

The form or structure of an organism or a cell.

Question 35

Cell fate

Answer 35

Describes what a particular cell at a given stage of development will normally give rise to.

Question 36

Totipotent

Answer 36

A cell which has the potential to form an entire organism.

Question 37

Blastula

Answer 37

A hollow sphere of cells surrounding a fluid-filled cavity (a blastocyst in mammals), formed when the zygote initially undergoes a series of mitotic divisions known as cleavages (because no cell growth occurs between them). A group of cells (called the inner cell mass) inside the blastula or blastocyst will ultimately form the embryo.

Question 38

Pluripotent

Answer 38

Cells which can no longer individually give rise to a whole organism, but which can still form virtually any type of cell in the body. For example, a group of cells (called the inner cell mass) inside the blastula or blastocyst that will ultimately form the embryo.

Question 39

Germ layers

Answer 39

In all animal embryos, three germ layers are formed that will give rise to the different tissues of the mature animal. In vertebrates, the inner layer, or endoderm, gives rise to the gut, liver and lungs. The middle layer, or mesoderm, gives rise to the muscle, heart, kidney, blood and skeleton. The outer layer, the ectoderm, gives rise to the epidermis of the skin and the nervous system. Similar tissues derive from similar germ layers in many kinds of invertebrates.

Question 40

Multipotent progenitor cells

Answer 40

Cells that have the potential to give rise to a limited range of cell lineages. For example, a haematopoietic cell is a multipotent blood cell progenitor that can develop into one of several types of blood cell, but cannot develop into other cell types.

Question 41

Meristems

Answer 41

Clusters of undifferentiated cells in plant tissues, which are found in particular zones where growth takes place. The meristem can also regenerate damaged parts (or in some species, an entire plant) from individual cells.

Question 42

Cell fate determinants

Answer 42

Intrinsic cytoplasmic molecules, including transcription factors and other types of regulatory molecules, that determine cell fate.

Question 43

Asymmetric cell division

Answer 43

Cell division, which generates two daughter cells that differ in their ability to develop into particular cell types.

Question 44

Combinatorial control

Answer 44

The way a group of transcription factors work together to determine level of expression of a single gene.

Question 45

Cell fate

Answer 45

Describes what a particular cell at a given stage of development will normally give rise to.