AP Bio: chapters 12 - 13

Anchorage dependence

Anchorage dependence

the requirement that a cell must be attached to a substratum in order to initiate cell division

Autosomes

• chromosomes other than sex chromosomes

  • One chromosome from a pair comes from the mother and the other from the father

Sex chromosomes

•  the x and y chromosomes that determine an individual’s sex

  • Xy = male (only small parts are homologous)

  • Xx = female (homologous pair)

Cell cycle

• G0 phase - cells that remain in G0 will not divide as they cannot pass the G1 checkpoint, these cells can leave G0 when necessary

  • Examples include: neurons, liver cells

• G1 phase - first gap of the cell cycle, where the cell is able to grow to reach division, phase is the most variable in length spending on types of cells or other conditions, it is the part of the cycle in which the cell is performing it's typical function

  • G1 checkpoint - the point in the cell cycle where cells reach a point of no return, once they pass the checkpoint they must divide or die off, assesses four things

    • Growth signal - TSH, GH, ECF, assess whether the cell has enough signal that is telling it to divide

    • Size - the cell must be large enough to divide, this means that the cells are older and have not divided recently

    • Density dependence - if the cell does not have enough room to grow the necessary materials for division, it cannot divide

    • Anchorage dependence - checking that the cell is attached to a substratum 

• S phase - synthesis of materials necessary for division in the cell occurs, chromosomes are copied (DNA and chromosomes are continued to be duplicated into the G2 cycle) and the cell continues to grow, occupies ½ of the cell cycle 

• G2 phase - DNA is continued to be copied as well as organelles such as mitochondria or chloroplast which cannot be duplicated through organismal DNA in the nucleus, the cytoskeleton will begin to break down to provide material for the mitotic spindle and for the cell to change shape, the centromeres are duplicated to complete the duplication of DNA, it's usually the shortest of interphase

  • G2 checkpoint - performs checks to decide if the MPF gene has been turned on and thus the cell has grown enough to complete the G2 phase and move on to the M phase to complete division

• M phase - mitosis

Cell plate

structure that forms in the center of dividing plant cells that forms of vesicles containing cellulose that will eventually form a cell wall between the two new nuclei to finally divide a plant cell

Centromere

• a region of the chromosoma

• DNA where the chromatid is attached most closely to its sister chromatid, mediated by proteins bound to the centromeric DNA

Checkpoints (when and what)

• G1 checkpoint - the point in the cell cycle where cells reach a point of no return, once they pass the checkpoint they must divide or die off, assesses four things

  • Growth signal - TSH, GH, ECF, assess whether the cell has enough signal that is telling it to divide

  • Size - the cell must be large enough to divide, this means that the cells are older and have not divided recently

  • Density dependence - if the cell does not have enough room to grow the necessary materials for division, it cannot divide

  • Anchorage dependence - checking that the cell is attached to a substratum 

• G2 checkpoint - performs checks to decide if the MPF gene has been turned on and thus the cell has grown enough to complete the G2 phase and move on to the M phase to complete division

• M checkpoint - The cell examines whether all sister chromatids are correctly attached to the spindle microtubules that separate them. If not, the cell pauses mitosis until all sister chromatids have been attached in the right way.

Chiasmata

x shaped regions where crossovers have occurred in each homologous pair

Chromosome anatomy

Cleavage furrow

• a shallow groove in the cell surface near the old metaphase plate, first sign of cleavage

  • Occurs through a ring of actin filaments that interact with the protein myosin and thus contract to close of the cell membrane, creating two cells eventually

Crossing over

• the DNA molecules of non-sister chromatid are broken and rejoined to each other 

  • Allows for more diversity and the combination of genes that may be more beneficial

Cyclins

a cellular protein that occurs in a cyclically fluctuating concentration, activate commonly inactive kinases that drive the cell cycle

Cytokinesis

final phase of mitosis, the final division of the cytoplasm to create to entirely different cells

Density dependent inhibition

• a phenomenon in which crowded cells stop dividing to prevent an excess

  • An example of the effect of an external physical factor on cell division

Diploid

• any cell with two sets of chromosomes 

  • Has a diploid number of 2n

Haploid

•  cells that contain a single set of chromosomes, gametes

  • 1n

Genes

•  hereditary units with coded information endowed to offspring by there parents

  • Account for familial resemblances

  • Program the specific traits that emerge as we develop from fertilized eggs to adults

Alleles

one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome

Homologous chromosomes

chromosome pairs with the same length, centromere position and staining pattern

Independent assortment

in metaphase of meiosis I, the orientation of pairs of homologs is random, different chromatids will create unique crossovers and combine in meiosis II

Karyotype

a display of the chromosome pairs of a cell arranged by size and shape

What phase is the fish and onion cell in?

metaphase

What phase is the fish and onion cell in?

anaphase

What phase is the fish and onion cell in?

interphase

What phase is the fish and onion cell in?

prophase

What phase is the fish and onion cell in?

telophase

Meiosis I

• called reductional division because it reduces the number of chromosomes from diploid to haploid

• first stage

• Three events unique to meiosis (in meiosis I) - 

  • Synapsis and crossing over - in prophase I, duplicated homologs pair up and crossing over occurs

  • Homologous pairs at the metaphase plate - at metaphase I, chromosomes are positioned at the metaphase plate as pairs of homologs, in mitosis they are paired with chromatids 

  • Separation of homologs - in anaphase I, the duplicated chromosomes of each homologous pairs move toward opposite poles, sister chromatid are still connected, in mitosis sister chromatid will by pulled to opposite poles but they stay together in meiosis due to cohesion that is mediated cohesion proteins

Meiosis II

• occurs as mitosis does, creates a total of four haploid daughter cells from the separation of sister chromatids, called the equational division

• after telophase

Meiosis

• Meiosis I - called reductional division because it reduces the number of chromosomes from diploid to haploid

• Prophase I - 

  • Longest stage, processes in prophase of mitosis occur

  • Duplicated homologous chromosomes pair to create tetras (synapsis) and crossing over occurs 

• Metaphase I - 

  • Tetras or homologous pairs line up on the metaphase plate after being pulled by kinetochore fibers (like in metaphase of mitosis)

  • Chromosomes are attached as a whole to one kinetochore fiber (sister chromatid are attached together and to the same pole)

• Anaphase I - 

  • Each tetra separates and chromosomes move to opposite poles as a unit

• Telophase I and cytokinesis - 

  • Two haploid cells form, each chromosome still consists of two sister chromatids

• Meiosis II - occurs as mitosis does, creates a total of four haploid daughter cells from the separation of sister chromatids, called the equational division

Mitosis

• Prophase - 

  • Chromosomes begin to condense

  • Nuclear membrane dissociates and joins other membranes (plasma, Golgi, etc)

  • Nucleolus dissociates 

  • Sister chromatids are formed

  • Mitotic spindle begins to form and centrosomes move away from each other towards opposite poles through polar fibers pulling them 

• Prometaphase - 

  • Chromosomes continue to condense

  • Kinetochore fibers begin to pull the chromosomes to the metaphase plate

  • Centrosomes are almost at poles

• Metaphase - 

  • Chromosomes are lined on the metaphase plate which each sister chromatid attached to a polar fiber from an opposite pole

  • Centrosomes reach opposite poles and are secured with aster fibers 

  • M checkpoint - required for the cell to reach anaphase, ensures that the cell has duplicated centromeres, chromosomes are the most condensed and all chromosomes are accounted for

• Anaphase - 

  • Kinetochore fibers tug on the sister chromatid, eventually separating them and pulling the sister chromatid to the pole from which the kinetochore extends from 

  • Chromosomes start to unwind slightly

• Telophase - 

  • Nuclei are reassembled with the nuclear envelope and nucleolus 

  • Cytoskeleton is also reformed

• Cytokinesis - 

  • The cells are fully divided once the cytoplasm has split

Meiosis vs. Mitosis

In mitosis bonds between sister chromatids are broken at the end of metaphase by enzymes, where in meiosis, cohesins are released first in anaphase I and again in anaphase II

Property	Mitosis (diploid or haploid)	Meiosis (diploid only)
DNA replication	Occurs during interphase, before the start of mitosis	Occurs during interphase, before meiosis I begins and not before meiosis II
Number of divisions	1, including PPMAT	2, including PMAT
Number of daughter cells and genetic composition	2 daughter cells which are each genetically identical to the parent cell with the same number of chromosomes, haploid if haploid parent cell, diploid if diploid	4 daughter cells which are each genetically different from the parent cell and from other sibling cells, cells will have a haploid number of chromosomes compared to the diploid
Role in the animal or plant body	- Allows multicellular organisms of animal or plant (gametophyte or sporophyte) to arise from a single cell - Produces cells for growth, repair and asexual reproduction in some species - Produces gametes in the gametophyte plant	- Produces gametes in animals or spores (in the sporophyte plant) - Reduces number of chromosome sets by half  - Introduces genetic variability among the gametes or spores

Mitotic spindle (KF, PF, aster)

• structure that begins to form in the cytoplasm during prophase

  • Made up of fibers made out of microtubules and associated proteins

  • Microtubules come from the partial disassembly of the cytoskeleton

  • Spindle microtubules polymerize (elongate) by adding more subunits of the protein tubulin

  • Nonkinetochore/polar fibers (PF) - connect centrosomes and push them toward opposite poles, stretch out the cell to make it larger, part of the mitotic spindle

  • Kinetochore fibers  (KF) - fibers extending from the centrosomes that attach them to a kinetochore protein attached to each sister chromatid to allow the centrosome to move the chromatid/chromosome, part of the mitotic spindle

  • Aster - short microtubules that extend from each centrosome and attach the centrosome to either pole in the cell, part of the mitotic spindle

MPF

• maturation promoting factor, the first discovered cyclin-cdk complex

  • Triggers the cell’s passage to the m-phase past the G2 checkpoint because as pathways are complete in G2 such as duplication of mitochondria, they begin to turn on the gene to produce MPF so, once enough pathways have been complete the MPF gene will be turned on and the cell can proceed into mitosis

Three Life Cycles

All

• only if fertilization occurs, then the diploid is allowed to grow

• common themes are fertilization and meiosis

Most fungi and some protists

• majority of life is in the state of a haploid

Plants in some algae

• half time spent as diploid, while the other half the haploid

Animals

• while they are gametes they are haploid

• majority of life is in the state of a diploid

  • biological benefit of two copies of each gene (so if one is mutated than the other can still function)

Prophase

• The chromatin fibers 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 cohesins (sister chromatid cohesion).

• The mitotic spindle (named for its shape) 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.

Prometaphase

• The nuclear envelope fragments.

• The microtubules extending from each centrosome can now invade the nuclear area.

• The chromosomes have become even more condensed. Each of the two chromatids of each chromosome now has a kinetochore, a specialized protein structure at the centromere.

• Some of the microtubules attach to the kinetochores, becoming "kinetochore microtubules," which jerk the chromosomes back and forth.

• Nonkinetochore microtubules interact with those from the opposite pole of the spindle.

Metaphase

• The centrosomes are now at opposite poles of the cell.

• The chromosomes have all arrived at the metaphase plate, a plane that is equidistant between the spindle's two 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.

Anaphase

• Anaphase is the shortest stage of mitosis, often lasting only a few minutes.

• Anaphase begins when the cohesin proteins are cleaved. This allows the two sister chromatids of each pair to part suddenly. Each chromatid thus becomes a full fledged chromosomes.

• The two liberated daughter chromosomes begin moving toward opposite ends of the cell as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region, the chromosomes move centromere first (at about 1 um/min).

• The cell elongates as the nonkinetochore microtubules lengthen.

• By the end of anaphase, the two ends of the cell have equivalent—and complete collections of chromosomes.

Telophase

• Two daughter nuclei forms in the cell.

• Nuclear envelopes arise from 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.

Random fertilization

any sperm could come to any egg, gametes produce 2²³ possible chromosome combinations which is doubled when fused to another gamete in fertilization

Random mating

any person can chose to mate with anyone else, creates infinite combinations of possibilities to combine genes

Recombination

• produces a huge amount of variation

  • Recombinant chromosomes - products of recombination, individual chromosomes that carry genes derived from two parents

  • Produce chromosomes with new combinations of maternal and paternal alleles which creates versatility

Somatic cell

• all of the cells of the body except gametes and their precursors

  • Diploids (2n)

  • In human somatic cells, there are 46 chromosomes (2 sets of 23)

Germline cell

cells that follow a specialized line to produce gametes; they undergo mitosis to become gametes

Gamete

• reproductive cells in plants and animal cells that transmit genes from one generation to the next (sperm and egg)

  • Haploid (1n)

  • In human gametes, there are 23 chromosomes (1 set of 23)

Syngamy

the process of the union of two gametes for the generation of one zygote (the fusion of their nuclei in reproduction)

Tetrads

• two duplicated homologous chromosomes (4 chromatids) bound together by chiasmata in Prophase I

  • Tetrads form during synapsis and are held in precise positions by a protein lattice/structure called synaptonemal complex

Tumor (malignant/benign)

• Benign tumor - a tumor that remains at the original site because it has too few genetic and cellular changes to survive at another site, they usually do not cause serious problems to the host organism

• Malignant tumor - a tumor in which the cells have undergone enough transformation that they can spread to new tissues and impair function in one or more organs

Sexual reproduction

• 1 sperm + 1 egg = 1 zygote

• meiosis

Asexual reproduction

• organism reproduces with itself

• mitosis and meiosis

Diversity

• good

• mutations are the initial source of all diversity

  • source of all variation, but cause of least variation

• crossing promotes diversity

  • recombination

  • allele / genes that were linked to other genes

• independent assortment

  • changes pair of male and female

  • alignment of homologous

  • recombination

• mate choice

• random fertilization

Problems in cell cycle

• cancer

  • uncontrolled division

Regulation of cell cycle

• checkpoints

2n to 1n ...HOW ....  WHY

• diploid to haploid

• through meiosis

• want to copies of the genes, so if one doesn’t work than than the organism still has functionality

Recombination...  HOW ... WHY

• crossing over

• non sister chromosomes exchange parts

• chiamata where the cross over occurs

• it can be used to help repair broken DNA

• to create more diversity as a result

ID of mitotic/meiotic phases

• meiosis is essentially mitosis double

  • the chromosomes are affected differently

• nuclear membrane reforms in telophase for mitosis, while it reforms in telophase 2 for meiosis