Chapter 3 Flashcards
Cells
Basic, living, structural, and functional units of the body.
What are the three main parts of a cell?
Plasma membrane, cytoplasm, and nucleus.
Plasma membrane
Form the cell’s flexible outer surface, separating the cell’s internal environment from the external environment. It is a selective barrier that regulates the flow of materials into and out of a cell. Plays a key role in communication among cells and between cells and their external environment.
Fluid mosaic model
Model used to describe plasma membrane. According to this model, the molecular arrangement of the plasma membrane resembles a continually moving sea of fluid lipids that contain a mosaic of many different proteins.
Lipid bilayer
The basic structural framework of the plasma membrane; two back-to-back layers made up of three types of lipid molecules.
What are the three types of lipids molecules that make up the lipid bilayer?
Phospholipids, cholesterol, and glycolipids.
Phospholipids
About 75% of the plasma membrane lipids; lipids that contain phosphorus.
Cholesterol
About 20% of the plasma membrane lipids; a steroid with an attached –OH (hydroxyl) group.
Glycolipids
About 5% of the plasma membrane lipids; lipids with attached carbohydrate group.
The bilayer arrangement occurs because the lipids are ______ molecules, meaning _______.
Amphipathic; they have both polar and nonpolar parts.
How are the phospholipids arranged in the bilayer arrangement?
The polar part of phospholipids is their phosphate containing “head” which is hydrophilic. The nonpolar part are the two fatty acid “tails”, which are hydrophobic. Because “like seeks like”, the phospholipid molecules arrange themselves in the bilayer with their hydrophilic heads facing outwards, towards the watery fluid on either side - cytosol or ECF. The hydrophobic tails in each half of the bilayer point toward one another.
Cholesterol molecules are ______ amphipathic.
Weakly.
What are the two types of membrane proteins?
Integral proteins and peripheral proteins.
Integral proteins
Membrane proteins that are firmly embedded in the membrane, and extend into or through the lipid bilayer. Integral proteins are amphipathic.
Transmembrane proteins
Membrane proteins that span the entire lipid bilayer and protrude into both the cytosol and extracellular fluid. Most integral proteins are transmembrane proteins.
Glycoproteins
Membrane proteins with carbohydrate group attached to the ends that protrude into the extracellular fluid. Many integral proteins are glycoproteins.
Glycocalyx
Sugary coat formed by the carbohydrate portions of glycolipids and glycoproteins. The pattern of carbohydrates in the glycocalyx varies from one cell to another. Therefore, the glycocalyx acts like a molecular “signature” that enables cells to recognize one another. Eg. A white blood cells ability to detect a “foreign” glycocalyx is one basis of the immune response that helps us destroy invading organisms.
Peripheral proteins
Membrane proteins that are not firmly embedded in the membrane. They are attached to the polar heads of membrane lipids or to integral proteins at the membrane’s inner or outer surface.
What are the six membrane functions?
Ion channels, carriers, receptors, enzymes, linkers, cell identity markers.
Ion channels
Pores or holes that specific ions can flow through to get into or out of the cell. Most ion channels are selective, as they allow only a single type of ion to pass through.
Carriers
AKA transporters; selectively move a polar substance or an ion from one side of the membrane to the other.
Receptors
Serve as cellular recognition sites. Each type of receptor recognizes and binds a specific type of molecule. Eg. Insulin receptors bind to the hormone insulin.
A specific molecule that binds to a receptor is called a ______ of that receptor.
Ligand.
Enzymes
Catalyze specific chemical reactions at the inside or outside surface of the cell.
Linkers
Anchor proteins in the plasma membranes of neighbouring cells to one another or to protein filaments inside and outside the cell.
What are the two roles of cell identity markers?
Enable a cell to 1.) recognize other cells of the same kind during tissue formation or 2.) recognize and respond to potentially dangerous foreign cells. Eg. The ABO blood type markers – when you receive a blood transfusion, the blood type must be compatible with your own, or red blood cells may clump together.
Why are membranes fluid structures?
Membrane fluidity is an excellent compromise for the cell; a rigid membrane would lack mobility, and a completely fluid membrane would lack the structural organization and mechanical support required by the cell.
What are the two components of cytoplasm?
Cytosol and organelles
Nucleus
Is a large organelle that houses most of the cell’s DNA. Within the nucleus, each chromosome contains thousands of genes that control most aspects of cellular structure and function.
Selectively permeable
A property in which plasma membranes permit some substances to pass more readily than others. Transmembrane proteins that act as channels and carriers increase the plasma membrane’s permeability to a variety of ions and uncharged polar molecules that cannot cross the lipid bilayer unassisted.
The lipid bilayer portion of the plasma membrane is ______ permeable to nonpolar molecules such as oxygen (O2), carbon dioxide (CO2), and steroids; ______ permeable to small, uncharged polar molecules, such as water and urea (a waste product form the breakdown of amino acids); and _______ to ions and large, uncharged polar molecules, such as glucose.
Highly, moderately, impermeable
Concentration gradient
Is a difference in the concentration of a chemical from one place to another, such as from the inside to the outside of the plasma membrane.
Electrical gradient
A difference in electrical charges between two regions.
Typically, the inner surface of the plasma membrane is more ______ charged and the outer surface is more ______ charged.
Negatively, positively.
Membrane potential
A difference in electrical charges across the plasma membrane.
Electrochemical gradient
The combined influence of concentration gradient and the electrical gradient on movement of a particular ion.
Passive processes
Movement of a substance down a concentration gradient until equilibrium is reached; do not require cellular energy in the form of ATP.
Diffusion
Movement of molecules or ions down a concentration gradient due to their kinetic energy until they reach equilibrium.
What five factors influence the diffusion rate?
Steepness of the concentration gradient, temperature, mass of the diffusing substance, surface area, and diffusion distance.
How does steepness of the concentration gradient influence the diffusion rate?
The greater the difference in concentration between the two sides of the membrane, the higher the rate of diffusion.
How does temperature influence the diffusion rate?
The higher the temperature, the faster the rate of diffusion.
How does mass of the diffusing substance influence the diffusion rate?
The larger the mass of the diffusion particle, the slower its diffusion rate.
How does surface area influence the diffusion rate?
The larger the membranes surface area available for diffusion, the faster the diffusion rate.
How does diffusion distance influence the diffusion rate?
The greater the distance over which diffusion must occur, the longer it takes.
Simple diffusion
Passive movement of a substance down its concentration gradient through the lipid bilayer of the plasma membrane without the help of membrane transporter proteins.
Facilitated diffusion
Passive movement of a substance down its concentration gradient through the lipid bilayer by transmembrane proteins that functions as channels or carriers.
Channel-mediated facilitated diffusion
The process in which a solute moves down the concentration gradient across the lipid bilayer through a membrane channel. Most membrane channels are ion channels. Each ion can diffuse across the membrane at only certain sites.
In typical plasma membranes, the most numerous ion channels are selective for ______ or ______; fewer channels are available for ______ or ______.
K+ (potassium ions), Cl- (chloride ions), Na+ (sodium ions), Ca2+ (calcium ions).
When is a channel said to be gated?
A channel is said to be gated when part of the channel proteins acts as a “plug” or “gate” changing shape in one way to open the pore and in another way to close it.
Carrier-mediated facilitated diffusion
The process in which a carrier (AKA a transporter) moves a solute down its concentration gradient across the plasma membrane. Eg. Glucose, the body’s preferred energy source for making ATP, enters many body cells by carrier mediated facilitated diffusion.
Transport maximum
The number of carriers available in a plasma membrane which places an upper limit.
What happens once all the carriers are occupied?
Once all carriers are occupied, the transport maximum is reached, and a further increase in the concentration gradient doesn’t increase the rate of facilitated diffusion.
Can the transport maximum be elevated?
Yes. Eg. Using the glucose example from before - the hormone insulin, via the action of the insulin receptor, promotes the insertion of many copies of glucose transporters into the plasma membranes of certain cells. Thus, the effect of insulin is to elevate the transporter maximum for facilitated diffusion of glucose into cells.
Osmosis
Passive movement of water molecules across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration.
Aquaporins (AQPs)
Integral membrane proteins that function as water channels. AQPs play a critical role in controlling the water contents of cells. AQPs are responsible for producing cerebrospinal fluid, aqueous humor, tears, sweat, saliva, and urine concentration.
Hydrostatic pressure
The pressure exerted by water in a fluid (Eg. Water molecules in blood) against a neighbouring structure (Eg. Against the walls of blood vessels).
Osmotic pressure
Force exerted by a solution with an impermeable solute. The higher the solute concentration, the higher the solution’s osmotic pressure.
Tonicity
A measure of the solution’s ability to change the volume of cells by altering their water content.
Isotonic solutions
Any solution in which a cell maintains its normal shape and volume
Hypotonic solution
A solution that has a lower concentration of solutes than the cytosol inside a cell.
Hemolysis
The swelling and rupturing of cells, due to water molecules entering the cells faster than they leave. The rupture of cells due to placement in a hypotonic solution is referred to simply as lysis. Pure water is very hypotonic and causes rapid hemolysis.
Hypertonic solutions
A solution that has a higher concentration of solutes than the cytosol inside a cell.
Crenation
The shrinkage of cells, due to water molecules moving out of the cells faster than they enter.
Active processes
Movement of substances against a concentration gradient; requires cellular energy in the form of ATP.
Active transport
An active process in which energy is required for carrier proteins to move solutes across the membrane gradient.
What source is used to drive primary active transport?
Energy obtained from hydrolysis of ATP.
What source is used to drive secondary active transport?
Energy stored in an ionic concentration gradient.
Primary active transport
Active process in which a substance moves across the membrane against its concentration gradient by pumps (carriers) that use energy supplied by hydrolysis of ATP.
Pumps
Carrier proteins that mediate primary active transport.
What is the most prevalent primary active transport mechanism?
The most prevalent primary active transport mechanism expels sodium ions (Na+) from cells and brings potassium ions (K+) in. Because of the specific ion it moves, this carrier is called the sodium potassium pump. Because a part of the sodium-potassium pump acts as ATPase, and enzyme the hydrolyzes ATP, another name for this pump is Na+-K+ pump.
Secondary active transport
In secondary active transport, a carrier protein simultaneously binds to Na+ and another substance and then changes its shape so that both substances cross the membrane at the same time.
Symporters
When the transporters move two substances in the same direction in secondary active transport.
Antiporters
When the transporters move two substances in opposite directions in secondary active transport.
Vesicles
Tiny, spherical membrane sacs that are used as another way for substances to enter and leave cells in an active process. A variety of substances are transported in vesicles from one structure to another within cells.
Endocytosis
Movement of substances into a cell in vesicles.
Exocytosis
Movement of substances out of a cell in secretory vesicles that fuse with the plasma membrane and release their contents into the extracellular fluid.
What are the three types of endocytosis?
Receptor-mediated endocytosis, phagocytosis, bulk-phase endocytosis
Receptor-mediated endocytosis
Ligand–receptor complexes trigger infolding of a clathrin-coated pit that forms a vesicle containing ligands; ligand binds to a receptor to form a ligand-receptor complex. The formation of this complex activates a process in which there is an invagination of the plasma membrane at that location (which has clathrin molecules on the cytoplasmic side of the membrane). The invagination becomes deeper and eventually there is an inward pinching off of that region of the membrane, resulting in the formation of a vesicle which now contains the ligand-receptor complex. This is a mechanism by which the cell is able to import a specific molecule (ligand) which it needs.
What are the steps in receptor-mediated endocytosis?
- Binding
- Vesicle formation
- Uncoating
- Fusion with endosome
- Recycling of receptors to plasma membrane
- Degradation in lysosomes
Phagocytosis
“Cell eating”; movement of a solid particle into a cell after pseudopods engulf it to form a phagosome.
What are phagocytes and what are the two main types?
Body cells that are able to carry out phagocytosis; 2 main types include: 1.) macrophages, which are located in many body tissues, and 2.) neutrophils, a type of white blood cell.
Bulk-phase endocytosis AKA pinocytosis
“Cell drinking”; movement of extracellular fluid into a cell by infolding of plasma membrane to form a vesicle.
Transcytosis
Movement of a substance through a cell as a result of endocytosis on one side and exocytosis on the opposite side.
Cytoplasm
Cellular contents between plasma membrane and nucleus - cytosol and organelles. Site of all intracellular activities except those occurring in the nucleus.
Cytosol
Intracellular fluid (ICF); is the fluid portion of the cytoplasm that surrounds organelles. Constitutes about 55% of total cell volume. The cytosol is the site of many chemical reactions required for a cell’s existence.
Cytoskeleton
A network of protein filaments that extends throughout the cytosol. Three types of filaments contribute to the cytoskeleton’s structure, as well as the structure of the other organelles.
What three types of filaments contribute to the cytoskeleton’s structure, as well as the structure of the other organelles?
Microfilament, intermediate filaments, and microtubules
Microfilaments
The thinnest elements of the cytoskeleton. Have two general functions: they help generate movement and provide mechanical support.
Microvilli
Increase the surface area of a cell.
Intermediate filament
Thicker than microfilaments but thinner than microtubules. Several different proteins can compose intermediate filaments, which are exceptionally strong. They are found in parts of the cell subject to mechanical stress; they stabilize the position of organelles such as the nucleus and help attach cells to one another.
Microtubules
The largest of the cytoskeletal components, are long, unbranched, hallow tubes composed mainly of the protein tubulin. Microtubules help determine cell shape. They also function in the movement of organelles.
Organelles
Specialized structures with characteristic shapes. Each organelle has specific functions.
Centrosomes
AKA microtubule organizing centre; is located near the nucleus and consists of two components - centrioles and pericentriolar matrix; replicates during cell division so that succeeding generations of cells have the capacity for cell division.
Centrioles
Two cylindrical structures, each composed of nine clusters of three microtubules (triplets) arranged in a circular pattern.
Pericentriolar matrix
Surrounds the centrioles. Contains hundreds of ring-shaped complexes composed of the protein tubulin. These tubulin complexes are the organizing centers for growth of the mitotic spindle, which plays a critical role in cell division, and for microtubule formation in nondividing cells.
Cilia
Short, hairlike projections that extend from the surface of the cell. Microtubules are the dominant component. The coordinated movement of many cilia on the surface of a cell causes the steady movement of fluid along the cell’s surface.
Flagella
Are similar in structure to cilia but are usually much longer. Flagella usually move an entire cell.
Ribosomes
Are the sites of protein synthesis; ribosomes that are located within the mitochondria synthesize mitochondrial proteins.
Endoplasmic reticulum (ER)
Membraneous network of flattened sacs or tubules.
Rough endoplasmic reticulum (ER)
Continuous with the nuclear membrane and usually is folded into a series of flattened sacs. The outer surface of the rough ER is studded with ribosomes, the site of protein synthesis.
Smooth endoplasmic reticulum (ER)
Extends from the rough ER to form a network of membrane tubules. Unlike rough ER, smooth ER does not have ribosomes on the outer surface of its membrane. However, smooth ER contains unique enzymes that make it functionally more diverse than rough ER. Because it lacks ribosomes, smooth ER does not synthesize proteins, but it does synthesize fatty acids and steroids, such as estrogen and testosterone.
Golgi complex
Consists of 3-20 flattened membraneous sacs called cisternae; structurally and functionally divided into entry (cis) face, medial cisternae, and exit (trans) face. Entry (cis) accepts proteins from rough ER; medial cisternae form glycoproteins, glycolipids, and lipoprotiens; exit (trans) face modifies molecules further, then sorts and packages them for transport to their destinations.
From the entry face, the cisterns are thought to ______, in turn becoming ______ and then ______.
Mature, medial, exit cisterns.
Transfer vesicles
Bud from the edges of the cisterns and move specific enzymes back towards the entry face and move some partially modified proteins towards the exit face.
Secretory vesicles
Deliver the proteins to the plasma membrane, where they are discharged by exocytosis into the extracellular fluid.
Membrane vesicles
Deliver their contents to the plasma membrane for incorporation into the membrane.
Lysosomes
Can contain as many as 60 kinds of powerful digestive and hydrolytic enzymes that can break down a wide variety of molecules once lysosomes fuse with vesicles formed during endocytosis. Lysosome enzymes help recycle worn-out cell structures.
Autophagy
The process by which entire worn-out organelles are digested. An organ which gets digested is enclosed by a membrane derived from ER to create a vesicle called an autophagosome.
Autolysis
The process by which lysosome enzymes destroy the entire cell that contains them.
Peroxisomes
AKA microbodies; similar in structure to lysosomes but are smaller; contain several oxidases, enzymes that can oxidate (remove hydrogen atoms from) various organic substances. Eg. Enzymes in peroxisomes oxidize toxic substances, such as alcohol. The peroxisomes are very abundant in the liver, where detoxification of alcohol and other damaging substances occurs.
Proteasomes
Tiny barrel-shaped structures consisting of four stacked rings of proteins around a central core; continuously destroy unneeded, damaged, or faulty proteins.
Mitochondria
The powerhouse of the cell. A mitochondrion consists of an external mitochondrial membrane and an internal mitochondrial membrane with a small fluid-filled space between them. Both membranes are similar in structure to the plasma membrane.
The internal mitochondrial membrane contains a series of folds called ______. The central fluid-filled cavity of a mitochondrion, enclosed by the internal mitochondrial membrane is the ______.
Mitochondrial cristae and mitochondrial matrix.
Apoptosis
The orderly, genetically programmed death of a cell. Mitochondria play an important and role in this early on.
Nuclear envelop
A double membrane that separates the nucleus from the cytoplasm. Both layers of the nuclear envelope are lipid bilayers similar to the plasma membrane.
Nuclear pores
Openings that extend through the nuclear envelop. Control the movement of substances between the nucleus and the cytoplasm.
Nucleoli
Spherical bodies inside the nucleus that function to produce ribosomes. Are the sites of synthesis of rRNA and assembly of rRNA and proteins into ribosome subunits.
Genes
Cell’s hereditary units which control cellular structure and direct cellular activities.
Chromatin
The complex of DNA, proteins, and some RNA. Chromatin has a beads-on-a-string structure.
In chromatin, each bead is a ______ that consists of double-stranded DNA wrapped twice around a core of eight proteins called ______, which help organize the colliding and folding of DNA. The string between the beads is called ______, which holds adjacent nucleosomes together. In cells that are not dividing, another histone promotes coiling of nucleosomes into a larger-diameter ______, which then folds into large loops. Just before the cell division takes place, however, the DNA replicates (duplicates) and them loops condense even more forming a pair of ______.
Nucleosome, histones, linker DNA, chromatin fiber, and chromatids.
Genome
The total genetic information carried in a cell or an organism.
Proteome
All of an organism’s proteins.
Gene expression
A process where a genes DNA is used as a template for synthesis of a specific protein. First in transcription and then in translation.
Base triplet
A sequence of three nucleotides in DNA.
Codon
Specifies a particular amino acid.
Each sequence of __ nucleotides codes for __ type of amino acid.
3; 1
Genetic code
The set of rules that relate the base triplet sequence of DNA to the corresponding codons of RNA and the amino acids they specify.
Transcription
A process whereby the genetic information represented by the sequence of base triplets in DNA serves as a template for copying the information into a complementary sequence of codons.
What are the three types of RNA made from the DNA template?
Messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA).
Messenger RNA (mRNA)
Direct the synthesis of a protein.
Ribosomal RNA (rRNA)
Joins with ribosomal proteins to make ribosomes.
Transfer RNA (tRNA)
Binds to an amino acid and holds it in place on a ribosome until it is incorporated into a protein during translation.
One end of the tRNA carries a specific amino acid, and the opposite end consists of a triplet of nucleotides called an ______. By pairing between complementary bases, the ______ attaches to the ______.
Anticodon; tRNA anticodons; mRNA codon
RNA polymerase
An enzyme that catalyzes transcription of DNA. The enzyme must be instructed where to start the transcription process and where to end it.
Promoter
A special nucleotide sequence where transcription begins. It is located near the beginning of a gene. This is where DNA polymerase attaches to the DNA.
Terminator
A special nucleotide sequence where transcription ends. It specifies the end of a gene. When RNA polymerase reaches the terminator, the enzyme detaches from the transcribed RNA molecule and the DNA strand.
Introns
Regions within a gene that do not code for parts of proteins.
Exons
Regions within a gene that do code for parts of proteins.
Translation
A process whereby the base sequence on DNA is copied complementarily onto as molecule of mRNA. Ribosomes in the cytoplasm carry out translation.
Describe the transcription/translation process all together
A segment of DNA is used for transcription. One strand of the double-stranded DNA in that gene is transcribed (copied) to form a strand of RNA. The RNA is often mRNA, and it contains the complementary code sequence of nucleotides that was in the gene. The ,RNA is exported from the nucleus into the surrounding cytosol where ribosomes are located. A ribosome attaches itself near one end of the mRNA and then physically travels along the length of the mRNA. As it does this, it uses the sequence of ribonucleotides in the mRNA to link together the sequence of amino acids to form a protein molecule. Therefore the ribosome has translated the sequence of nucleotides into a sequence of amino acids, i.e., a protein has been synthesized from the information originally stored in the gene.
Polyribosome
Made up of several ribosomes attached to the same mRNA.
Cell division
The process by which cells reproduce themselves.
What are two types of cell division?
Somatic cell division and reproductive cell division
Somatic cell division
In this type of cell division, a cell undergoes a nuclear division called mitosis, and a cytoplasmic division called cytokinesis, to produce two genetically identical cells, each with the same number of chromosomes as the original cell. Somatic cell division replaces dead or injured cells and adds new ones during tissue growth.
Somatic cell
Any cell in the body other than a germ cell.
Germ cell
Is a gamete (sperm or oocyte) or any precursor cell destined to become a gamete.
Cell cycle
Orderly sequence of events in which a somatic cell duplicates its contents and divides it in two.
Homologous chromosomes
The two chromosomes that make up each pair of chromosomes. They contain similar genes arranged in the same (or almost the same) order.
Sex chromosomes
One pair of chromosomes designated X and Y. In females the homologous par of sex chromosomes consists of two X chromosomes. In males the pair consists of an X and a Y chromosome.
Diploid cell
All cells that have homologous pairs. Because somatic cells contain two sets of chromosomes, they are called diploid cells.
Reproductive cell division
Is the mechanism that produces gametes, the cells need to form the next generation of sexually reproducing organisms. This process consists of a special two-step division called meiosis, in which the number of chromosomes in the nucleus is reduced by half.
Interphase
Period between cell divisions; it is during this time that the cell does most of its growing. Consists of three phases: G1 phase, S phase, and G2 – the S stands for synthesis of DNA. Because the G phases are periods where there is no activity related to DNA duplication, they are thought of as gaps or interruptions in DNA duplication.
Mitotic (M) phase
Parent cell produces identical cells with identical chromosomes.
Mitosis
Nuclear division; distribution of two sets of chromosomes into separate nuclei.
What are the four stages of mitosis?
Prophase, metaphase, anaphase, and telophase
Prophase
Nuclear membrane and nucleolus disappear, making room for an organizing spindle which appears shortly. The DNA which to this point was invisible chromatin, suddenly becomes visible as it condenses to form solid chromosomes which move around the cell much easier. Two sister chromatids joined by a centromere make one chromosome. Two centrioles migrate towards the cell poles. As the centrioles move apart, spindle fibers form, stretching between the centrioles to form mitotic spindles.
Centromere
Constricted region that holds the chromatid pair together.
Mitotic spindles
A football-shaped assembly of microtubules that attach to the kinetochore. It is responsible for the separation of chromatids to opposite poles of the cell.
Metaphase
The chromosomes migrate to the metaphase plate and line up along the cell equator.
Metaphase plate
Plane of alignment of the centromeres.
Anaphase
The centromeres divide so that each sister chromatid has its own centromere. The sister chromatids are now their own chromosome. The spindle fibers attach to the centromeres and proceed to pull the sister chromatids apart, forming a cluster of chromosomes at each pole.
Telophase
A nuclear membrane reforms around each of the two clusters of chromosomes. Chromosomes decondence to form chromatin and become invisible again.
Cytokinesis
Cytoplasmic division; contractile ring forms cleavage furrow (slight indentation of the plasma membrane) around center of cell, dividing cytoplasm into separate and equal portions.
What are the three possible destinies of a cell?
1.) To remain alive and functioning without dividing.
2.) To grow and divide.
3.) To die.
Necrosis
The opposite of apoptosis (normal type of cell death); is a pathological type of cell death that results from tissue injury.
Meiosis
The reproductive cell division that occurs in the gonads (ovaries and testes) and produces gametes in which the number of chromosomes is reduced by half.
Haploid cells
Cells that contain a single set of 23 chromosomes. Gametes are haploid cells as they contain a single set of 23 chromosomes.
What are the four stages of meiosis I?
Prophase I, metaphase I, anaphase I, and telophase I.
Prophase I
The nuclear membrane dissolves and mitotic spindles form. When the chromatin condenses, the chromosomes appear as thick X-shaped forms. Each is a tetrad made of a homologous pair and their identical sister chromatids. Crossing over occurs.
Crossing over
Where identical and non-identical chromatids of the tetrad twist around each other, and exchange segments, producing new gene combinations.
Genetic recombination
Crossing-over results in genetic recombination of genes, and accounts for part of the great genetic variation among humans and other organisms that form gametes via meiosis.
Metaphase I
The chromosomes line up along the equator and attach to spindle fibers.
Anaphase I
One homologous chromosome and its identical sister chromatid is torn away from the other homologue and its sister chromatid.
Telophase I
Isolates the two nuclear clusters into new nuclear membranes, and cytokinesis forms the two haploid cells.
What are the four stages of meiosis II?
Prophase II, metaphase II, anaphase II, and telophase II
Prophase II
Chromosomes again condense appearing as single chromosomes which are actually composed of two identical sister chromatids held together by their centromere.
Metaphase II
All chromosomes line up on the equator, centromeres divide and attach to spindle fibers.
Anaphase II
Spindle fibers tear sister chromatids apart, pulling one chromatid toward each pole, ensuring that each daughter cell has an exact copy.
Telophase II
Nuclear membranes form around the new nuclei and when cytokinesis is complete, there are four haploid gametes.
How do cells differ in size and shape?
Cells vary considerably in size. The size of cells is measured in units called micrometers. Additionally, cells vary considerably in shape. They may be round, oval, flat, cube-shaped, column-shaped, elongated, star-shaped, cylindrical, or disc-shaped. A cell’s shape is related to its function in the body.
