Cell Biology Flashcards
1
Q
What name is given to the basic functional and structural unit of all living organisms?
A
cell
2
Q
The cell theory, originally composed in 1838, includes three primary tenets. Name them.
A
- The cell is the basic unit of life.
- All living things are composed of cells, whether one or many.
- All cells arise from other cells.
3
Q
What main features characterize eukaryotic cells?
A
They have membrane-bound organelles, including nuclei, and linear chromosomes. They are also larger than prokaryotic cells and differ in specific aspects like flagellum structure.
Eukaryotic cells can comprise either unicellular or multicellular organisms.
4
Q
Define:
organelle
A
A separate, specialized structure within a cell.
Many organelles are enclosed by lipid bilayers, but some, including ribosomes, are not membrane-bound.
5
Q
Which organelles are membrane-bound?
A
- nucleus
- mitochondria
- Golgi apparatus
- endoplasmic reticulum
- peroxisomes
- lysosomes
In plants, chloroplasts and vacuoles fall into this category as well.
Ribosomes, on the other hand, are not membrane-bound; while some are attached to the ER, they are not surrounded by membranes of their own.
6
Q
Which organelles contain DNA?
A
The nucleus is the location of most genetic material, but the mitochondria also contain DNA.
In plants, chloroplasts have DNA as well.
7
Q
What main cellular function is performed by the mitochondria?
A
They are involved in cellular metabolism, specifically the production of energy via aerobic respiration.
In the mitochondria, the Krebs cycle produces electron carriers, while the electron transport chain facilitates the formation of a proton gradient. This gradient is used to produce ATP.
8
Q
Which organelle found in animal cells may have arisen as a result of mutualism?
A
Mitochondria may have evolved from a symbiotic relationship between small bacteria and larger cells. This is known as the endosymbiotic theory.
Like all instances of mutualism, this situation is thought to have provided benefits to both organisms. The smaller bacterium was given a livable environment while providing energy for the larger host.
9
Q
Within the cell, what is the role of the endoplasmic reticulum (ER)?
A
The ER is involved in a variety of processes, with smooth ER and rough ER performing different functions. The smooth ER is involved in lipid anabolism and detoxification, while the rough ER, with its many ribosomes, is the site of protein translation.
Both types of ER help synthesize macromolecules and shuttle them to the Golgi apparatus to be secreted from the cell.
10
Q
What biological products are synthesized in the rough endoplasmic reticulum?
A
proteins
These can include enzymes and peptide hormones, among other examples.
11
Q
What biological products are synthesized in the smooth endoplasmic reticulum?
A
lipids
These include steroid hormones and phospholipids, among other examples.
12
Q
What are peroxisomes, and what cellular function do they perform?
A
They are small membrane-bound organelles that contain enzymes. They function in fatty acid breakdown, detoxification, and facilitation of the pentose phosphate pathway.
Peroxisomes are named for hydrogen peroxide (H2O2), which can be both formed and broken down within the organelle. This is important because H2O2 is a poisonous radical initiator.
13
Q
Which eukaryotic cell types are encased in a cell wall?
A
Fungal and plant cells include cell walls, while animal cells do not. Bacterial cells, though not eukaryotic, also have cell walls.
Fungal walls are made of chitin, plant walls are made of cellulose, and bacterial cell walls are composed of peptidoglycan.
14
Q
Define:
cytoskeleton
A
A structural web of protein that can change shape to perform a variety of cellular processes.
The prefix “cyto-“ means “cell,” so the cytoskeleton is the “cell’s skeleton.”
15
Q
What main functions are executed by the cytoskeleton?
A
- Mainly functions to provide shape to the cell.
- Also involved in cell movement, endocytosis, and the formation of the spindle apparatus during cell division.
Cytoskeletal proteins have the unusual ability to self-assemble into larger units, and can thus change shape based on the cell’s needs.
16
Q
Name the three primary, protein-based components of the cytoskeleton.
A
- microtubules
- intermediate filaments
- microfilaments
17
Q
What factor plays the biggest role in limiting the size of a cell?
A
The cell must have a sufficiently large surface area-to-volume ratio. As the cell becomes larger, volume increases faster than surface area, causing this ratio to decrease.
A large surface area gives the cell a greater ability to obtain nutrients and eliminate waste through its cell membrane.
18
Q
What is the cellular role of the plasma membrane?
A
Also called the cell membrane, it protects the interior of the cell from its environment. It also limits the movement of specific materials into and out of the cell.
19
Q
Explain the fluid mosaic model.
A
This is used to describe the plasma membrane. It is composed of lipids with a “mosaic” of embedded proteins and other components, and its “fluidity” allows these macromolecule components to move laterally within the membrane.
20
Q
What traits distinguish molecules that can easily pass through the plasma membrane?
A
Molecules can easily travel through the membrane if they are small and nonpolar. To move passively, they also must be traveling down their concentration gradient.
Large molecules and ions must enter cells through special protein channels or via endocytosis.
21
Q
What is the difference between passive and active transport?
A
- Passive transport involves the movement of a substance down its concentration gradient without the use of energy.
- Active transport requires energy, usually in the form of ATP, and moves a substance against its gradient.
22
Q
What term describes the passive movement of water or another solvent down its concentration gradient?
A
osmosis
Water will always move from areas of low solute to areas with a higher solute concentration.
Osmosis is generally tested in cases where two compartments are separated by a semipermeable membrane. The membrane allows water, but not solute, to pass through; osmosis is thus required to promote similar solute concentrations on both sides.
23
Q
What is the meaning of the term “isotonic?”
A
Itis one with the same solute concentration as a solution to which it is compared.
For example, fluids that are administered in an IV should be isotonic with human cells. In other words, they should have the same osmolality, or solute concentration.
24
Q
What is the difference between a hypertonic and a hypotonic solution?
A
In comparison to a reference solution, a hypertonic solution has a greater solute concentration, while a hypotonic solution has a smaller solute concentration.
For example, say that a certain compartment contains 4 grams of NaCl per liter of solution. A solution of 2 g NaCl per L solution would be hypotonic, while a solution of 8 g NaCl per L solution would be hypertonic.
25
The hammerhead shark lives in a tropical ocean environment. Are its cells likely to be hypertonic or hypotonic compared to human cells?
Shark cells are likely **hypertonic** to human cells.
## Footnote
To avoid excessive water loss or gain, shark cells must be specialized to be isotonic, or at least close, to their surroundings. Since the ocean is composed of salt water, shark cells likely contain more solute than the cells of land-dwelling species.
26
What traits distinguish molecules that require membrane channels to enter a cell?
Substances that require membrane channels include large molecules, polar molecules, and ions.
## Footnote
When these materials move down their concentration gradient, the process is known as **facilitated diffusion**. Small, nonpolar molecules do not require membrane channels and can travel via **simple diffusion**.
27
Name the three distinct junctions that separate the cells of vertebrates.
1. tight junctions
2. desmosomes
3. gap junctions
28
Which type of junction is especially important in the heart?
Gap junctions
## Footnote
The electrical impulse that triggers contraction must travel throughout the entire heart muscle, beginning at the SA node. Gap junctions facilitate the movement of ions, allowing the cells to act together as one unit.
29
What main features characterize **prokaryotic cells**?
They lack membrane-bound organelles. They generally contain one circular chromosome within a nucleoid region, but can also possess circular plasmids outside the genome.
## Footnote
Prokaryotic cells always comprise unicellular organisms.
30
All cells can be categorized into which two broad groups?
1. Prokaryotic cells
2. Eukaryotic cells
## Footnote
Prokaryotic cells are generally simpler and include bacterial species. Eukaryotes can range from single-celled organisms (like yeast) to complex animals (like humans).
31
# Define:
cytosol
The fluid contained within a cell.
## Footnote
In contrast, the cytoplasm includes both the intracellular fluid and all of the extranuclear organelles.
32
# Define:
cytoplasm
Includes both the intracellular fluid, or cytosol, and the organelles.
## Footnote
The only organelle that is not included in the cytoplasm is the nucleus.
33
What structural features are present in the **nucleus**?
The nucleus is encased in a double membrane, known as the nuclear envelope. This membrane is marked by channels called nuclear pores. Inside, a fluid (the nucleoplasm) surrounds linear chromosomes.
34
In what part of the cell is the **nucleolus** located, and what function does it serve?
It is located within the nucleus. It serves as the site of ribosomal RNA transcription and synthesis of ribosomal subunits.
35
What membranes and spaces are present in a mitochondrion?
It contains both an outer and an inner membrane. The intermembrane space is located between the two, while the mitochondrial matrix is the innermost space, bounded by the inner membrane.
## Footnote
Both membranes are phospholipid bilayers.
36
Binary fission is most relevant to the production of which organelle?
mitochondria
## Footnote
Just like prokaryotic asexual reproduction, which produces identical daughter cells, this method of division yields identical organelles.
37
What is the cellular role of **lysosomes**?
They break down engulfed pathogens, nutrient molecules, and components of the cell itself that are no longer functional.
## Footnote
Like the stomach, a lysosome contains enzymes and an acidic interior.
38
Describe the structural characteristics of the **endoplasmic reticulum (ER)**.
The ER is a folded membrane divided into two regions: **rough ER** and **smooth ER**.
## Footnote
Rough ER contains ribosomes bound to its surface, while the smooth ER does not.
39
What is the cellular role of the **Golgi apparatus**?
It modifies molecules that arrive from the ER. It has the ability to break off into vesicles and can thus facilitate the exocytosis of these modified products.
40
Which two membrane-bound organelles are unique to plant cells?
* chloroplasts
* vacuoles
## Footnote
Chloroplasts resemble mitochondria and perform the reactions of photosynthesis. Vacuoles are involved in storage.
41
Describe the structure and role of **microfilaments**.
They are linear, thin filaments composed of many actin monomers. Microfilaments are thinner than either microtubules or intermediate filaments.
## Footnote
As actin polymers, microfilaments play a crucial role in muscle contraction and also facilitate cytokinesis.
42
Describe the structure and role of **intermediate filaments**.
They are fibers composed of various cell-specific proteins; many are composed of keratin. Intermediate filaments are thicker than microfilaments but thinner than microtubules.
## Footnote
These fibers contribute to the rigidity of the cell and also form desmosomes, a type of cellular junction.
43
Describe the structural role of **microtubules**.
They are thick, hollow tubulin polymers. Specifically, units of alpha- and beta-tubulin dimerize, and many of these dimers come together to form a microtubule.
## Footnote
Microtubules form the spindle apparatus, an essential component of cell division. They also comprise cilia and flagella and perform various other functions.
44
What **structural and functional differences** exist between cilia and flagella?
* **Cilia** are short, found in large numbers, and beat rhythmically in a back-and-forth pattern.
* **Flagella** are longer, with only a few found per cell at the most, and beat more slowly but powerfully to propel the cell in a certain direction.
## Footnote
In eukaryotes, both cilia and flagella have a 9+2 method of organization. This consists of 9 microtubule pairs arranged in a circle surrounding 2 single microtubules.
45
Describe the composition of the **plasma membrane**.
It consists of a **phospholipid bilayer**, with polar heads on the exterior (pointing toward the extracellular fluid and cytoplasm) and nonpolar tails on the interior.
## Footnote
The membrane also contains cholesterol, associated large proteins, and sphingolipids, among other components.
46
What will happen to a human cell when it is placed in a hypertonic solution?
Water will exit the cell, causing it to **shrivel**.
## Footnote
A hypertonic solution is one that contains a comparatively high amount of solute (salt). Due to osmosis, water will flow from a low-solute to a high-solute environment in an attempt to equalize the solute concentrations.
47
What is a **concentration gradient**, and what is its biological significance?
It is a difference in the amount of solute molecules per unit volume between one region and another.
## Footnote
Molecules are prone to moving down their concentration gradient. For example, if the outside of a cell contains a much higher concentration of glucose than the inside, glucose will tend to move into the cell (if possible). If the membrane is impermeable to glucose, water will tend to move out instead.
48
If a normal human cell is placed in a container of pure water, what will result?
The cell will **swell**, possibly to the point of lysing (rupturing).
## Footnote
Since pure water contains no solute, it is hypotonic in comparison to the cell. Water will travel down its concentration gradient from the exterior of the cell to the interior.
49
What is **facilitated diffusion**, and is it an active or passive process?
* It is the movement of a substance down its concentration gradient with the help of transmembrane protein channels.
* Since facilitated diffusion does not require energy, it is a form of passive transport.
50
# Define:
endocytosis
It is a form of transport in which a material is engulfed by the cell membrane, then enters the cell in a vesicle. Endocytosis requires the expenditure of energy.
## Footnote
Phagocytosis and pinocytosis are both subtypes of endocytosis.
51
# Define:
exocytosis
It is a form of transport in which a material is packaged into a vesicle which then fuses with the cell membrane. This process, which requires energy, allows the material to be exported from the cell.
## Footnote
Waste products and secreted hormones often leave a cell via exocytosis.
52
# Define:
pinocytosis
It is a type of endocytosis. In this process, the cell membrane engulfs the extracellular fluid, as well as the small particles it contains, in a vesicle.
## Footnote
Other types of endocytosis include phagocytosis and receptor-mediated endocytosis.
53
What features characterize **tight junctions**?
They are composed of multiple proteins, including claudins, and form an impermeable seal between adjacent cells. This prevents fluid and solutes from going "around" the cell to enter a cavity.
## Footnote
Tight junctions are found between cells in the same epithelial layer.
54
What features characterize **desmosomes**?
They are composed of intermediate filaments and are found at localized regions throughout a cell's membrane. They generally attach one epithelial or cell layer to another.
## Footnote
Desmosomes are found in epithelial and muscle tissue.
55
What features characterize **gap junctions**?
They are small channels formed from connexin proteins. They allow small solutes and fluid to pass from one cell to another.
## Footnote
While gap junctions are found between most cells, they are generally associated with cells that must communicate or function together, like neurons.
56
How do eukaryotic and prokaryotic cells differ with respect to **organelles**?
Unlike eukaryotes, prokaryotes lack a nucleus, as well as all membrane-bound organelles.
## Footnote
Note that membrane-bound organelles include mitochondria, lysosomes, the ER, and the Golgi apparatus, but not ribosomes. Prokaryotes do contain ribosomes, a fact that may appear on the AP Biology exam.
57
How do eukaryotic and prokaryotic organisms differ in their **cellular organization**?
Prokaryotes are always unicellular, while eukaryotes can be either unicellular or multicellular.
## Footnote
One common example of a unicellular eukaryote is yeast, a fungus. Most other single-celled eukaryotes are classified as protists.
58
Determine if an organism with the following traits is a prokaryote or a eukaryote:
* composed of a single cell
* linear DNA
* has mitochondria
eukaryote
## Footnote
Only a eukaryote would possess mitochondria, since prokaryotes lack membrane-bound organelles. Eukaryotes also have linear, not circular, chromosomes. Note that both eukaryotes and prokaryotes can be unicellular.
59
In animal cells, which organelle serves as the location for DNA in the form of linear chromosomes?
The **nucleus**.
## Footnote
It is also the site of DNA replication and transcription.
While the mitochondria also include DNA, mitochondrial DNA is found in small circular chromosomes, not linear ones.
60
Which organelle has two subunits and serves as the location for protein synthesis?
The ribosome.
## Footnote
Ribosomes are small organelles found in both eukaryotic and prokaryotic cells. At these organelles, proteins are synthesized (translated). A typical ribosome includes a small and a large subunit, although the sizes of these subunits vary depending on the type of cell.
61
How do eukaryotic and prokaryotic flagella differ in their protein composition?
* Eukaryotic flagella are composed of **tubulin**.
* Prokaryotic flagella are made up of **flagellin**.
## Footnote
Tubulin is the protein subunit of microtubules. Specifically, each eukaryotic flagellum consists of nine pairs of microtubules surrounding two individual ones.
62
What term refers to the tubular structure used for locomotion in some bacteria?
flagellum
63
How do eukaryotic and prokaryotic organisms differ in the **composition of their ribosomes**?
* Eukaryotic ribosomes are slightly larger, with a 40S and a 60S subunit combining to yield 80S.
* Prokaryotic ribosomes have a 30S and a 50S subunit, which combine to form 70S.
## Footnote
The abbreviation "S" refers to the rate at which a molecule settles in a centrifuge.
64
In animal cells, where is most **cholesterol** found?
plasma membrane
## Footnote
Specifically, cholesterol (shown here) is a lipid that confers rigidity to the membrane. Cholesterol can either be acquired through the diet or synthesized by cells.
65
What is the **difference** between peripheral and integral membrane proteins?
* **Peripheral** proteins are loosely attached to either the inner or outer face of the plasma membrane. These proteins can be easily removed from the lipid bilayer.
* **Integral** proteins are firmly attached to the lipid bilayer and are very difficult to remove. These proteins extend at least partially into the hydrophobic interior region of the bilayer.
## Footnote
A transmembrane protein is a specific type of integral protein that spans, or extends through, the entire lipid bilayer.
66
What are four functions that can be served by plasma membrane proteins?
* function in cell-to-cell **adhesion**
* function in cell-to-cell **communication and recognition**
* serve as **receptors** for hormones and other ligands
* assist in **transport** across the membrane
67
# Define:
solute
In a solution, it is the substance whose phase is lost after solvation (or the substance of which less is present). For example, dissolving a small amount of solid NaCl into liquid water produces a liquid solution. In this case, NaCl is the solute.
68
# Define:
bulk flow
It is the movement of molecules (typically fluid molecules) in one direction in response to pressure.
## Footnote
A classic example is the movement of blood through blood vessels. Blood, like typical fluids, tends to travel from areas of high pressure to areas of low pressure.
69
What term is given to the separation of molecules based on their differing abilities to diffuse through a semipermeable membrane?
Dialysis
## Footnote
In a dialysis procedure, large particles are unable to diffuse through the dialysis membrane, so small particles are left to diffuse in an attempt to reduce the overall concentration gradient between the compartments. This technique is often used to remove toxins from the blood in cases of kidney failure.
70
What term describes diffusion between two regions in which fluids are flowing in opposite directions?
Countercurrent exchange
## Footnote
This form of transport allows for a fairly consistent difference in concentration gradient between the two regions, maximizing diffusion. A classic example of this mechanism occurs in the loop of Henle, a region of the nephron (the structural unit of the kidney).
71
Name the four phases of the cell cycle.
1. G1
2. S (synthesis)
3. G2
4. M (mitosis)
## Footnote
An additional phase is G0, in which the cell is not immediately preparing for cell division. G0 can be either temporary or permanent.
72
What is the **difference** between germ cells and somatic cells?
* **Germ cells**, including sperm and ova, are haploid and participate in reproduction. They are formed via meiosis.
* **Somatic cells** are diploid and include the rest of the cells in the body. Somatic cells only undergo mitosis.
73
Name the two types of eukaryotic cell division.
1. mitosis
2. meiosis
## Footnote
Mitosis produces somatic cells that are identical to the parent, while meiosis produces germ cells with only half of the parental genetic material.
74
Explain the **difference** between a haploid and a diploid cell.
* **Haploid cells** have only one full set of chromosomes. In humans, the only haploid cells are germ cells (sperm and eggs) produced by meiosis.
* **Diploid cells** have two distinct sets of chromosomes. In humans, all somatic cells are diploid and are produced by mitosis.
75
What is the significance of the G0 phase?
It is a stage of prolonged inactivity. In this phase, the cell is not preparing for mitosis.
## Footnote
Mature neurons and cardiac muscle are in a permanent G0 phase, since they do not undergo mitosis.
76
During which phase of the cell cycle would DNA helicase be especially active?
DNA helicase, as well as the other enzymes involved in DNA replication, would be most active during the **S phase**.
## Footnote
In the S (synthesis) phase, DNA is replicated to prepare for cell division.
77
List the four phases of mitosis in the order that they occur in a dividing cell.
1. prophase
2. metaphase
3. anaphase
4. telophase and cytokinesis
78
What features **distinguish** homologous chromosomes from sister chromatids?
* **Homologous chromosomes** can be genetically different; each homologous chromosome has the same loci, but can have different genes than its partner. Homologous chromosomes separate during meiosis I.
* **Sister chromatids** are genetically identical, as they result from the replication of a single chromosome. Sister chromatids separate during meiosis II and mitosis.
79
What cellular events characterize **metaphase**?
* At this point, spindle fibers are bound to the kinetochore of each chromosome.
* The chromosomes line up along the center of the cell, a region called the metaphase plate.
80
# Define:
cytokinesis
It is the actual formation of two distinct cells, characterized by the distribution of cytosol and organelles and the pinching of the cell membrane between the two nuclei.
## Footnote
Cytokinesis happens simultaneously with the later phases of mitosis and meiosis, but is often grouped with telophase.
81
An individual develops a disease in which certain cells undergo mitosis rapidly, with interphase dramatically shortened. What name is commonly used for this condition?
Cancer
## Footnote
Cancer can be promoted by many factors, including carcinogens, DNA damage, failure of tumor suppressor genes, and failure of apoptosis, or a combination of these events.
82
Name four broad differences between **mitosis** and **meiosis**.
* Mitosis involves a single division, while meiosis involves two.
* Mitosis produces two identical diploid cells, while meiosis produces up to four haploid cells that are not identical.
* Mitosis is used to form somatic cells, while meiosis produces germ cells (gametes).
* Crossing over occurs in meiosis, but does not happen in mitosis.
83
During which stage of meiosis do daughter cells become haploid?
After meiosis I.
## Footnote
Meiosis I is known as "reductional division," since homologous chromosomes separate during this stage. This leaves two daughter cells, each with a single chromosome from each pair and its replicated copy. Meiosis II, or "equational division," separates these copies.
84
Mitosis is broadly similar, but not identical, to meiosis I. Name three differences between the two processes.
* Mitosis involves the separation of sister chromatids, while meiosis I involves the separation of homologous chromosomes.
* Mitosis produces diploid cells, while the products of meiosis I are haploid.
* Crossing over occurs in prophase of meiosis I, but does not occur at all during mitosis.
85
Which phases of the cell cycle are part of interphase?
* G1
* S
* G2
## Footnote
In these phases, the cell grows, replicates its DNA, and prepares for mitosis.
The M phase (mitosis itself) is not a part of interphase. G0 is generally thought to be distinct when it is permanent (as in neurons, which do not divide).
86
What does the cell assess at the G1 checkpoint?
The DNA, to ensure that it is in proper condition to be replicated.
## Footnote
The **G1 checkpoint** is also known as the G1/S checkpoint or the restriction checkpoint.
Passage through this checkpoint is required to enter the S phase of the cell cycle.
87
What does the cell assess at the G2 checkpoint?
It makes sure that sufficient cell growth has occurred and that the DNA was replicated properly.
## Footnote
The **G2 checkpoint** is also known as the G2/M checkpoint.
Passage through this checkpoint is required to enter the M phase of the cell cycle.
88
Which type of cytoskeletal fiber is associated with **MTOCs**?
**Microtubules** are anchored at MTOCs, or microtubule-organizing centers. The fibers attach to the MTOC at one end and radiate toward other parts of the cell.
## Footnote
Centrosomes are a specific type of MTOC involved in cell division. Spindle fibers attach to the centrosomes, each of which is composed of two tubulin-based centrioles.
89
Describe the role of the **centrioles** during cell division.
They are cylinder-shaped microtubular structures, generally found in pairs as part of the centrosome, which act to organize the spindle apparatus along which the chromosomes align.
90
Describe the role of the **centromere** during cell division.
At the centromere, sister chromatids attach in pairs. This region also contains the kinetochore, which binds to a spindle fiber and allows the chromatids to be pulled to opposite poles of the cell.
## Footnote
The centromere is the location on the chromosome that appears "pinched."
91
Describe the role of the **kinetochore** during cell division.
It is a complex arrangement of various proteins that forms on the centromere. Here, spindle fibers bind, allowing sister chromatids to be separated to opposite poles of the cell.
92
Name the microtubule-based structure that separates chromosomes during cell division.
The spindle apparatus
## Footnote
Spindle fibers radiate from the centrosome, which contains centrioles, and bind at the kinetochore region of the centromere.
93
What cellular events characterize **prophase**?
* Loose chromatin condenses into visible chromosomes.
* The nuclear membrane disintegrates.
* The nucleolus disappears.
* The centrioles and spindle apparatus form.
94
What cellular events characterize **anaphase**?
* Sister chromatids are cleaved apart.
* Spindle fibers shorten, causing each sister chromatid to move to the opposite pole as its partner.
95
What cellular events characterize **telophase**?
* The cell elongates.
* Each daughter nucleus forms a new nuclear membrane and nucleolus.
* The chromosomes start to become less condensed.
96
Label the following structures on the diagram below: centrosome, centriole, spindle fiber, aster.
97
Which two processes ensure that none of the four daughter cells produced during meiosis are genetically identical?
1. crossing over
2. independent assortment
## Footnote
Crossing over is a method of genetic recombination that occurs during prophase I of meiosis. In this process, homologous chromosomes exchange genetic material, increasing total variation.
The principle of independent assortment, or Mendel's second law, states that genes for different traits separate into gametes independently of each other (as long as those traits are not linked).
98
Which process of genetic recombination occurs during prophase I of meiosis?
**Crossing over**, or the trading of DNA segments between homologous chromosomes, occurs when these chromosomes are paired together in prophase I.
99
What process most notably occurs in the cell during **S phase**?
DNA replication
## Footnote
More specifically, the S, or "synthesis," phase of the cell cycle involves replication of all chromosomes to form identical copies. Together, a chromosome and its copy are termed "sister chromatids."
100
What processes most notably occur during the **G1** and **G2** phases of the cell cycle?
The G1 and G2 phases are marked by growth and the synthesis of proteins, including enzymes.
## Footnote
These phases help the cell prepare for cell division, or mitosis (the M phase).
101
In which situations might a cell undergo **apoptosis**?
Also known as programmed cell death, this generally occurs in damaged cells. However, it can also serve as a normal part of development.
## Footnote
Viral infection, UV-induced damage, and hypoxia can all induce apoptosis.
102
In animal cells, the first marker of cytokinesis is an indentation between the two newly-forming daughter cells. What term is given to this indentation?
The cleavage furrow.
## Footnote
This indentation acts as a contractile ring, gradually closing due to the presence of actin filaments which pinch it shut. At the end of cytokinesis, the two daughter cells are fully separated.
103
Name the eight stages of meiosis in order.
1. prophase I
2. metaphase I
3. anaphase I
4. telophase I
5. prophase II
6. metaphase II
7. anaphase II
8. telophase II
104
The pairing of homologous chromosomes that occurs during meiosis is known as:
Synapsis
## Footnote
This process occurs during prophase I of meiosis and allows for crossing over, or genetic exchange between the homologous chromosomes, to occur.
105
# Define:
tetrad
A structure that forms during prophase I of meiosis.
## Footnote
This structure consists of both sets of homologous chromosomes, each of which includes two sister chromatids, all in close proximity to each other.
106
In animal cells, another term for meiosis is:
Gametogenesis
## Footnote
Gametes, or sex cells, are produced via meiosis in animal cells. These gametes are haploid, meaning that they have half of the chromosomes held by a typical somatic (body) cell. When two gametes join during the process of fertilization, they create a zygote, or diploid cell that can grow into a new organism.
107
In animal cells, two types of gametogenesis can occur. What are these two types?
* Spermatogenesis
* Oogenesis
## Footnote
Biologically speaking, spermatogenesis occurs in male organisms and oogenesis occurs in females.
108
Which two types of molecules determine when a cell will enter the next phase of the cell cycle?
* cyclins
* cyclin-dependent kinases
## Footnote
Cyclins are proteins that change in concentration depending on a cell's location in the cell cycle. These proteins can bind to cyclin-dependent kinases (Cdks), which are enzymes that phosphorylate other proteins. The behavior of cyclins and Cdks is key in the regulation of the cell cycle.
