Exam IV Flashcards
- In order for DNA was to be accepted as the genetic material, scientists needed to show DNA
o Was present in the cell nucleus and in condensed chromosomes
o Doubled during S phase of the cell cycle
o Was twice as abundant in the diploid cells as in the haploid cells of a given organism
o Showed the same patterns of transmission as the genetic information it was supposed to car
- DNA was first isolated in 1868 by
Fredrich Miescher.
Miescher isolated cell nuclei rom white blood cells in
a fibrous substance came out of solution. He called it nuclein and found it contained the elements C, H, O, N, P.
- People stained cells and confirmed two predictions of DNA
o Virtually all nondivding somatic cells of a particular organism have the same amount of nuclear DNA
o Similar experiments show that after meiosis, gametes have half the amount of nuclear DNA as somatic cells
- Chromosomes in eukaryotic cells contain DNA, but they also contain
proteins that are bound to DNA. Therefore, it was difficult for scientists to rule out that genetic information might be carrier on proteins.
- Many viruses, including bacteriophage, are composed of DNA and only one
or a few kinds of proteins. When a bacteriophage infects a bacterium, it takes about 20 minutes for the virus to hijack the bacterium’s metabolic capabilities and turn the bacterium into a virus factory. Minutes later, the bacterium is dead and hundreds of viruses are released
- The transition from bacterium to virus producer is
a change in the genetic program of the bacterial cell.
- Careful chemical analyses and observations by electron microscope showed that only
the viral DNA is injected into the cell during infection. This was further evidence that DNA and not protein was the genetic material
- Scientists rely on experiments to provide
proof of a cause and effect relationship.
- In order to confirm that DNA was the genetic material, biologists used
model organisms such as bacteria in transformation experiments. They found that the addition of DNA from one strain of bacterium could genetically transform another strain of bacterium
o Bacterium strain A + strain B DNA -> bacterium strain B
- The transformation of mammalian cells carrying genetic mutations provided another model system for showing that
that DNA is the genetic material. For example, certain cells were found to lack the gene for thymidine kinase, an enzyme that catalyzes the first step in a pathway that converts thymidine into dTTP. Such cells cannot grow in a medium that contains thymidine as the only source for dTTP synthesis. However when the cells were incubated with DNA containing the gene for thymidine kinase, some of the cells became transformed with the TK gene and were able two grow.
- For successful genetic transformation,
DNA must pass through the cell membrane into the cytoplasm and get incorporated into a host cell chromosome.
- In early transformation experiments, a major stumbling block was the first step,
because DNA is negatively charged and so are the surfaces of cell membranes. Since like charges repel, DNA does not tend to bind to cell membranes.
- Bios found they could circumvent this by
by incubating the DNA and cells in a solution.
- Transgenic
new genetically transformed organism
- The most crucial evidence for the structure of DNA was obtained using
XRAY crystallography
- Some chemical substances, when they are isolated and purified, can be made to form crystals.
The positions of atoms in a crystalized substance can be inferred from the diffraction pattern of X rays passing through the substance. The structure of DNA would not have been characterized without the crystallography prepared in the early 1950s by the English chemist Rosalind Franklin.
- Franklin’s work, in turn, depended on the success of the English biophysicist
Maurice Wilkins, who prepared samples containing very uniformly oriented DNA fibers. These fibers and the crystallographs Franklin prepared from them suggested a spiral or helical molecule
- DNA is a polymer of
nucleotides
- Each of these nucleotides consist of
a molecule of the sugar deoxyribose, a phosphate group, and a nitrogen base. The only differences between the four nucleotides of DNA are their nitrogenous bases: the purines adenine, guanine, and the pyrimidines cytosine and thymine
- Erwin Chargaff found the rule
that the amount of adenine equaled the amount of thymine and the amount of guanine equaled the amount of cytosine. The DNA would be known as Chargaff’s rule
- Watson and Crick used the chemical model to solve the structure. Watson and crick attempted to combine all that had been learned so far about DNA structure into a single coherent model.
Franklin’s crystallography results convinced them that the DNA molecule must be helical-it must have a spiral shape like a spring. Density measurements and previous model building results suggested that there are two polynucleotide chains in the molecule. Modeling studies also showed that the strands run in opposite directions, that is, they are anti-parallel.
- Watson and crick suggested that:
o The nucleotide bases are on the interior of the two strands, with a sugar phosphate backbone on the outside
o To satisfy Chargaff’s rule, a purine on one strand is always paired with a pyrimidine on the opposite strand. These base pairs have the same width down the double helix, a uniformity shown by x ray diffraction
- Four features summarize the structure of DNA
o It is a double stranded helix of uniform diameter
o It is right handed. Hold your right hand with the thumb pointing up. Imagine the curve of the helix following the direction of your fingers as it winds upward, and you have the idea
o It is antiparallel. Each strand is built in the 5-3 direction, but the two strands run in opposite directions to one another. So at one end of the double stranded molecule there is a 3OH exposed on the deoxyribose sugar of one strand and a 5 phosphate on the other
o The outer edges of the nitrogenous bases are exposed in the major and minor grooves. These grooves exist because the helices formed by the backbones of the two DNA strands are not evenly spaced relative to one another. The exposed outer edges of the base pairs are accessible for additional hydrogen bonding. Notice that the arrangements of unpaired atoms and groups differ in the AT base pairs compared with the GC base pairs. Thus the surfaces of the AT and GC base pairs are chemically distant. Allow other molecules, such a proteins, to recognize specific base pair sequences and bind to them. The atoms and groups in the major groove are more accessible, and tend to bind other molecules more frequently, than those in the minor groove. This binding of proteins to specific base pair sequences is the key to protein DNA interactions, which are necessary for the replication and expression of the genetic information in DNA
- The genetic material performs four important functions, and the DNA structure proposed by Watson and Crick was elegantly suited to three of them
o Storage of genetic information. With its millions of nucleotides, the base sequence of a DNA molecule can encode and store an enormous amount of information. Variations in DNA sequences can account for differences among species and individuals
o Precise replication during the cell division cycle. Replication could be accomplished by complementary based pairing, A with T and G with C.
o Susceptibility to mutations – the structure of DNA suggested an obvious mechanism for mutations: they might be simple changes in the linear sequence of base pairs.
o Expression of the coded information as phenotypes – the way this function is accomplished is not obvious in the structure of DNA. However, the nucleotide sequence of DNA is copied into RNA. The linear sequence of nucleotides in RNA is translated into a linear sequence of amino acids- a protein. The folded forms of proteins determine many of the phenotypes of an organism
- Semiconservative replication
each strand of the parental DNA acts as a template for a new strand which is added by base pairing
Conservative mode of replication would show
the parental DNA intact with both strands labeled, and the new DNA with both strands unlabeled. This does not occur. Instead, the resulting DNA molecules are always hybrids providing experimental evidence to support the semiconservative model of replication.
- DNA replication involves a number of different enzymes and other proteins. It takes place in two general steps
o The DNA double helix is unwound to separate the two template strands and make them available for new base pairing
o As new nucleotides form complementary base pairs with template DNA, they are covalently linked together by phosphodiester bonds, forming a polymer whose base sequence is complementary to the bases in the template strand. The template DNA is read in the 3-5 direction
- During DNA synthesis, nucleotides are added to
the 3” end of the growing new strand-the end at which the DNA strand has a free hydroxyl group on the 3’ carbon of its terminal deoxyribose,
- A free nucleotide can have one of three
phosphate groups attached to its pentose sugar, The raw materials for DNA synthesis are the four nucleotides deoxyadenosine triphosphate (dATP), deoxythymidine triphosphate (dTTP), deoxycytidine triphosphate (dCTP), and deoxyguanosine triphosphate (dGTP) or deoxyribonucleotides.
- These nucleotides can carry
three phosphate groups.
- During DNA synthesis, the two outer phosphate groups
groups are released in an exothermic reaction. This provides energy for the formation of a phosphodiester bond between the third phosphate group of the incoming nucleotide and the 3’ position of the sugar at the end of the DNA chain
- DNA replication begins with
the binding of a large protein complex to a specific site on the DNA molecule.
- This dna protein complex contains several different proteins
among them the enzyme DNA polymerase, which catalyzes the addition of nucleotides as the new DNA chain grows.
- All chromosomes have at least one region called
the origin of replication, to which the pre-replication complex binds.
- Binding occurs when
when proteins in the complex recognize specific DNA sequences within the ori
- Once the pre-replication complex binds to it,
the DNA unwinds and replication proceeds in both directions around the circle, forming two replication forks.
Ecoli
- The single circular chromosome of the bacterium Escherichia coli has 4x10^6 base pair4s of DNA. The 245 bp ori sequence is at a fixed location on the chromosome.
- Once the pre-replication complex binds to it, the DNA unwinds and replication proceeds in both directions around the circle, forming two replication forks.
- The replication rate in E. coli is approximately 1— bp per second, so it takes about 40 minutes to fully replication the chromosome.
- Rapidly dividing e coli cells divide every 20 minutes. In these cells, new rounds of replication begin at the ori of each new chromosome before the first chromosome has fully replicated, In this way the cell can divide in less time than the time needed to finish replicating the original chromosome
- Eukaryotic chromosomes are much longer than those of prokaryotes-up to a billion bp- and are
linear, not circular. If replication occurred from a single ori, it would take weeks to fully replicate a chromosome. So eukaryotic chromosomes have multiple origins of replication, scattered at intervals of 10,000 – 40,000
- DNA polymerase elongates a polynucleotide to
a preexisting strand. However it cannot begin this process without a short starter strand, called a primer
- In most organisms this primer is a
a short single strand of rna, but in some organisms its dna
- The primer is complementary to
the DNA template and is synthesized one nucleotide at a time by an enzyme called a primase. The DNA polymerase then adds nucleotides to the 3’ end of the primer and continues until replication of that section of DNA has been completed.
- Then RNA primer is
degraded, DNA is added in its place and resulting DNA fragments are connected by the action of other enzymes. When dna replication is complete each new strand consists of DNA only
- DNA polymerase are much larger than their
substrate, the dNTPs, and the template DNA, which is very thin.
- Molecular models of the enzyme substrate template
template complex from bacteria show that the enzyme is shaped like an open hand with a palm, a thumb, and fingers.
- Within the palm is the
the active site of the enzyme, which brings together each dNTP substrate and the template.
- The finger regions have precise shapes that can
recognize the different shapes of the four nucleotide bases. They bind to the bases by hydrogen bonding and rotate inwards
- Most cells contain more than one kind of DNA polymerase, but
only one of them is responsible for chromosomal DNA replication.
- Most cells contain more than one kind of DNA polymerase, but only one of them is responsible for chromosomal DNA replication.
- The others are involved in primer removal and DNA repair.
- Most cells contain more than one kind of DNA polymerase, but only one of them is responsible for chromosomal DNA replication.
- The others are involved in primer removal and DNA repair.
- A single replication fork opens in
one direction.
- One newly synthesized strand – the leading strand-is oriented so that it can
grow continuously at its 3 end as the fork opens up.
- The other new strand-the lagging strand- must be synthesized differently
because it grows in the direction away from the replication fork
- Synthesis of the lagging strand requires the synthesis of relatively
relatively short, discontinuous stretches of sequence (100 – 200 nucleotides in eukaryotes; 1,000-2,000 nucleotides in prokaryotes)
- These discontinuous stretches are synthesized just as
the leading strand is, by the addition of new nucleotides one at a time to the 3 end, but the new strand grows away from the replication fork
- These stretches of new DNA are called
Okazaki fragments (Reiji)
- To summarize, while the leading strand grows continuously forward, the lagging strand grows in shorter backward stretches with gaps between them
- To summarize, while the leading strand grows continuously forward, the lagging strand grows in shorter backward stretches with gaps between them
- A single primer is needed to initiate synthesis of the leading strand, but each Okazki fragments requires
requires its own primer to be synthesizes by the primase. DNA polymerase then synthesizes an Okazki fragment by adding nucleotides to one primer until it reaches the primer of the previous fragment.
- At this point, a different DNA polymerase removes the old primer and replaces it with
DNA. Left behind is tiny nick-the final phosphodiester linkage between the adjacent Okazaki fragments is missing
- The enzyme DNA ligase catalyzes the
formation of the diester bond linking the fragments and making the lagging strand whole.
- DNA replication may appear complex, but it occurs with astonishing speed and accuracy. There’s less than
1 base in a million error.
- DNA replication would not proceed as rapidly as it does if DNA polymerase went through such a cycle for
each nucleotide. Instead polymerase is processive-that is, it catalyzes many sequential polymerization reactions each time it binds to a DNA molecule
o Substrate bind to enzyme -> many products are formed -> enzyme is released -> cycle repeats
- When the terminal RNA primer is removed from the replicating end of a linear eukaryotic chromosome,
no DNA can be synthesized to replace it because there is no 3” end to extend.
- So the new chromosome has a bit of single stranded DNA at the end. This situation activates a mechanism for
for cutting off the single stranded region, along with some of the intact double stranded DNA. Thus the chromosome becomes slightly shorter with each cell division
- Another problem with chromosome ends is that they must be
protected from being joined to other chromosomes by the DNA repair system. When DNA is damaged by external and internal agents, it is repaired by a combination of DNA polymerase and DNA ligase activities. This system might mistakenly recognize chromosome ends as breaks and join two chromosomes together. This would create havoc with genomic integrity.
- To prevent chromosomes from joining,
many eukaryotes have strings of repetitive sequences at the ends of their chromosomes called telomeres.
- In humans, the repeated sequence TTAGGG, is repeated about 25000 times
These repeats bind special proteins that protect the ends from being joined together by the DNA repair system.
- In addition, the repeats form loops that have a similar protective role. So telomeres act like a plastic tip of a shoelace.
- Each human chromosome can lose
lose 50 – 200 bp of telomeric DNA after each round of DNA replication and cell division.
fter 20 – 39 cell divisions, the chromosome ends become
short enough to lose their protective role, and the chromosomes lose their integrity.
- An enzyme called telomerase catalyzes the addition of any
lost telomeric sequences in these cells. Telomerase contains an RNA sequence that acts as template for the telomeric dna repeat sequence
- There is a relationship between telomeres length and aging:
the average telomere length is shorter in older individuals. Furthermore, when a gene expressing high levels of telomerase is added to human cells in culture, their telomeres do not shorten. Instead of living 20-30 cell generations and then dying, the cells become immortal. It remains to be seen how this finding relates to the aging of a whole organism
- Accurate transmission of genetic information is essential for the proper
functioning and even the life of a single cell or multicellular organism.
- DNA polymerases sometimes insert a
a base that is not complementary to the template.
- In eukaryotes, the error rate is about
1 incorrect base in 100,000. With a genome size of 4z10^9, this would produce 40,000 errors after every cell division.
- This is an intolerable mutation rate for survival in the long term. However, if eukaryotic DNA sequences are
studied before and after a cell cycle, the actual frequency of DNA errors is 10^-10 per cell cycle. This means that most of the errors are repaired. There are two major repair mechanisms
o Proofreading occurs right after DNA polymerase inserts a nucleotide. When a DNA polymerase recognizes a mispairing of bases, it removes the improperly introduced nucleotide and tries again
o Mismatch repair occurs after DNA has been replicated. A second set of proteins surveys the newly replicated molecule and looks for mismatched base pairs that were missed in proofreading. A portion of the DNA including the incorrect nucleotide is removed and replaced by DNA polymerase
- The polymerase chain reaction allows researchers to make
multiple copies of short DNA sequences in a test tube- a process referred to as DNA amplification. T
The PCR technique uses
o A sample of double stranded DNA, often from a biological sample, to act as the template
o Two short, artificially synthesized primers that are complementary to the ends of the sequence to be amplified
o The four dNTPs
o A dna polymerase that can tolerate high temperatures without becoming denatured
o Salts and a buffer to maintain a near neutral pH
- PCR is a
cyclic process in which a sequence of steps is repeated over and over again. Since DNA replication is fast even in a test tube, it takes only a short time to go from 1 to 2 to 4 million short segments of DNA. The PCR technique requires the base sequences at each end of the amplified fragment be known ahead of time, so that complementary primers, usually 15 – 30 bases long, can be made in the laboratory.
- Because of the uniqueness of DNA sequences
a pair of primers this length will usually bind to only a single region of DNA in an organism’s genome.
- This specificity is a key to the power of
PCR to amplify just a small part of a larger DNA molecule
- Existing alleles are subject to change by
by mutation and can give rise to new alleles-in fact, a single gene can have many alleles.
- Alleles do not always show
simple dominant recessive relationships. A single allele may have multiple phenotypic effects, and a single character may be controlled by multiple genes
- The expression of a gene is generally affected by
interactions with other genes and with the environment
- Gene are subject to
mutations, which are rare, stable, and inherited changes in the genetic material.
- An allele can
mutate to become a different allele
- For example, we can envision that at one time all pea plants
plants made spherical seeds and had the seed shape allele S. At some point a mutation in S resulted in a new allele, s(wrinkled). If this mutation occurred in a cell that underwent meiosis, some of the resulting gametes would carry the s allele, and some offspring of this pea plant would carry the s allele in all their cells
- By producing phenotypic variety
mutations are the raw material for evolution.
- A new allele may become more or less
prevalent in a population, depending on its effect on the fitness of individuals carrying it
- Wild type
the allele that is present in most individuals in nature
- Other alleles of that gene are usually called
mutant alleles, and they may produce different phenotypes.
- The wild type and mutant alleles are inherited according to
Mendelian laws. A gene with a wild type allele that is present less than 99 percent of the time is said to be polymorphic
- Many genes have alleles that are neither
dominant nor recessive to one another. Instead, the heterozygotes have an intermediate phenotype in a situation called incomplete dominance.
- For example, if a true breeding red snapdragon is crossed with a white one, all the F1 flowers are an intermediate pin. Such cases appear to support the old blending theory of inheritance.
- However, further crosses indicate that this apparent blending can still be explained in terms of Mendelian genetics.
- The red and white snapdragon alleles have not disappear, as those colors reappear in the F2 generation.
- Sometimes two alleles of a gene both produce their phenotypes when
present in a heterozygote-a phenomenon called codominance.
- For example, in humans the gene I encodes en enzyme involved in attachment of sugars to a glycoprotein on the surface of red blood cells. There are three alleles of the gene: Ia, Ib, and Io. The Ia and Ib alleles both encode active enzymes, but the enzymes attach different sugars to the glycoprotein; the Io allele does not encode an active enzyme, so no sugar is attached at that position on the glycoprotein. When two different alleles are present, both alleles are expression (both enzymes are made, so both types of glycoproteins are made). The A and B glycoproteins are antigenic: if a red blood cell with the A glycoprotein on its surface gets into the bloodstream of a person who lacks the Ia allele, the recipient mounts an immune response and produces antibodies against the “nonself” cells. While the A and B glycoproteins are anti-genic in people who do not have the Ia or Ib alleles the O glycoprotein does not provoke an immune response. This makes people who are IoIo good blood donors in transfusions.
- Epistasis occurs when
the phenotypic expression of one gene is affected by another gene. For example. Two genes determine coat color in Labrador retrievers
o Allele B (black pig) is dominant to b (brown)
o Allele E (pigment deposition in hair) is dominant to e (no deposition, so hair is yellow)
- An EE or Ee dog wit BB or Bb is black; one with bb is brown. A dog with ee is yellow regardless of the Bb alleles present. Clearly gene E determines the phenotypic expression of gene B, and is therefore epistatic to B
- Perhaps the most dramatic example of interacting genes is
is hybrid vigor. Multiple gene count for the same phenotype
- Hybrid vigor – most complex phenotypes are determined by multiple genes
- Quantitative traits
traits conferred by multiple genes because they need to be measured rather than assessed qualitatively. For example, grain yield must be measured, whereas a simple trait such as pea seed color can be assessed by eye
he phenotype of an individual does not result from
its genotype alone.
- Genotype and environment interact to determine
the phenotype of an organism. This is especially important to remember in the era of genome sequencing.
- Environmental variables such as
light, temperature, and nutrition can affect the phenotypic expression of a genotype.
- Two parameters describe the effects of genes of environment on phenotype
o Penetrance – the proportion of individuals in a group with a given genotype that actually show the expected phenotype. For example, many people who inherit a mutant allele of the gene BRCA1 develop breast cancer in their lifetimes. But for some reasons that are not yet clear and must involve other genes and the environment, some people with the mutation do not develop breath cancer. So the BRCA1 mutation is said to be incompletely penetrant
o Expressivity – the degree to which a genotype is expressed in an individual. For example, a woman with the BRCA1 allele may develop both breast and ovarian cancer as part of the phenotype, but another woman with the same mutation may only get breast cancer. So the mutation is said to have variable expressivity
- The lifespan of an organism, from birth to death, is intimately linked to
cell reproduction
- Cell division plays important rolls in
the growth and repair of tissues in multicellular organisms, as well as in the reproduction of all organisms.
- Although the details vary widely, organisms have two basic strategies for reproducing themselves:
sexual reproduction and sexual reproduction. These two strategies make use of different types of cell division.
- Asexual reproduction is
reproduction is a rapid and effective means of making new individuals, and it is common in nature.
- The offspring resulting from asexual reproduction are clones of the parent organism-they are genetically identical to each other and the parent.
- Any genetic variation among the parent and offspring are due to changes called mutations
- Mutations
– alterations in dna sequence caused by environmental factors or errors in dna replication.
- In most cases, single celled prokaryotes reproduce by binary fission
- A cell of the bacterium e coli is the whole organism, so when it divides to form two new cells, it is reproducing.
- Similarly single celled eukaryotes can reproduce asexually through
MITOSIS
- Mitosis
a type of cell division that also produces two genetically identical cells
- Sexual reproduction
involves the fusion of two specialized cells called gametes, and can result in offspring with considerable genetic variation
- Gametes are formed by
meiosis
- Meiosis
a process of cell division that results in daughter cells with only half the genetic material of the original cell
- During meiosis,
, the genetic material is randomly separated and reorganized so that the daughter cells differ genetically from one another. Because of this genetic variation, some offspring of sexual reproduction may be better adapted than others to survive and reproduce in a particular environment
- Meiosis thus generates
genetic diversity that is the raw material for natural selection and evolution
- DNA in eukaryotic cells is organized into
multiple structures called chromosomes
- Each chromosome consist of
a single molecule of DNA and associated proteins.
- somatic cells
- In multicellular organisms, the body cells that are not specialized for reproduction
- Homologous pairs
– somatic cells each contain two sets of chromosomes, and the chromosomes occur in pairs
- One chromosome of each pair comes from the organism’s female parent, and the other comes from its male parent.
- For example, in human with 46 chromosomes, 23 come from the mother and 23 come from the father with a chromosome 1 from each parent and so on
- The two chromosomes in a homologous pair bear corresponding,
though not identical genetic information
- For example, a homologous pair of chromosomes in a plant may carry different versions of a gene that controls seed shape.
- Gametes contain only a single set of
chromosomes-one homolog from each pair. The number of chromosomes in a gamete is denoted by n, and the cell is said to be a haploid. During sexual reproduction, two haploid gametes fuse to form a zygote in a process called fertilization. The zygote thus has two sets of chromosomes, just as the somatic cells do. The chromosome number in the zygote is denoted by 2n, and the cells are said to be a diploid
- In many familiar organisms, the zygote divides by
mitosis, producing a new, mature organism with diploid somatic cells.
- All sexual life cycles involve meiosis to produce
produce haploid cells.
- In some cases, gametes develop immediately after meiosis.
- In others, each haploid cell divides and develops into a haploid organism – the haploid stage of the life- which eventually produces gametes by mitosis.
- The fusion of gametes results in a zygote and begins the diploid stage of the life cycle.
o In haplontic organisms, including most protists, fungi, and some green algae, the tiny zygote is the only diploid cell in the life cycle. After it is formed it immediately undergoes meiosis to produce more haploid cells. These are usually spores, which are the dispersal units for the organism. A spore germinates to form a new haploid organism, which may be single celled or multicellular. Cells of the mature haploid organism produce gametes that fuse to form the diploid zygote
o Most plants and some fungi display alternation of generations. As in many haplontic organisms, meiosis gives rise to haploid spores, which divide by m9itosis to form a haploid life stage called the gametophyte. In flowering plants the gametophytes are very small: the male gametophyte is the pollen, and the female gametophyte is the embryo sac-a part of the flower. In mosses and liverworts the gametophytes are larger and multicellular. The gametophyte forms gametes by mitosis, and they fuse to produce the diploid zygote. The zygote divides by mitosis to become the diploid sporophyte in which in turn produces haploid spores by meiosis
o In diploidic organisms, which include animals, brown algae and some fungi, the gametes are the only haploid cells in the life cycle and the mature organism is diploid
- The essence of sexual reproduction is the
random selection of half of the diploid chromosome set to make a haploid gamete, followed by fusion of two haploid gametes from separate parents to produce a diploid cell.
- Cell reproduction by either binary fission or mitosis produces
two genetically identical cells.
- For multicellular organisms, mitosis is a way to
build tissues and organs during development and to repair damaged tissues once development is complete
- In order for any cell to divide, the following events must occur
o There must be a reproductive signal. This signal initiates cell division and may originate from either inside or outside the cell
o Replication of DNA must occur so that each of the two new cells will have complement of genes to complete cell functions
o The cells must distribute the replicated DNA to each of the two new cells. This process is called segregation
o Enzymes needed for cell division must be synthesized, new organelles must be formed, and new material must be added to the plasma membrane. The division of the cytoplasm to form two daughter cells is called cytokinesis
- In prokaryotes, cell division results in the reproduction of the entire single celled organism.
The cell grows in size, replicates its DNA, and then separates the cytoplasm and DNA into two new cells by a process called binary fission
- In most prokaryotic cells, almost all of the genetic information is carried on one single chromosome.
- Two regions of the prokaryotic chromosome play functional roles in cell reproduction
o Ori – the site where replication of the circular chromosome starts
o Ter – the site where the replication ends
- Chromosome replication takes place as the DNA is threaded through a replication complex of proteins near the center of the cell.
- Replication begins at the ori site and moves toward the ter site.
- When replication is complete, the two daughter dna molecules separate and segregate from one another at opposite ends of the cell
- Replication begins near the center of the cell, and as it proceeds, the ori region move toward opposite ends of the cell.
- DNA sequences adjacent to the ori region bind proteins that are essential for this segregation. This is an active process, since the binding proteins hydrolyze ATP
- Components of the prokaryotic cytoskeleton are involved in the segregation process.
- A bacterial protein that is structurally related to actin but functionally related to tubulin provides a filament along which the ori regions and their associated proteins move
- The actual division of a cell and its contents into two cells begins immediately after chromosome segregation
- Initially there is a pinching in of the plasma membrane caused by the contraction of a ring of fibers on the inside surface of the membrane
- Replication of DNA
– unlike prokaryotes, eukaryotes have more than one chromosome. But the replication of each eukaryotic DNA molecule is similar to replication in prokaryotes, in that it is achieved by threading the long strands through replication complex. DNA replication occurs only during a specific stage of the cell cycle
- Segregation of DNA
when a cell divides, one copy of each chromosome must end up in each of the two new cells- for example, each new somatic cell in a human will have all 46 chromosomes (23 pairs). In eukaryotes, the newly replicated chromosomes are closely associated with each other. They become highly condensed, and a mechanism called mitosis segregates them into two new nuclei. The cytoskeleton is involved in this process
- Cytokinesis –
cytokinesis follows mitosis. The process in plant cells is different than in animal cells
- Cell cycle
period from one division to the next
- In eukaryotes, the cell cycle can be divided into
mitosis and cytokinesis-referred to as the M phase- and a much longer interphase.
- During interphase
- Interphase has three sub phases called, G1, S, and G2.
- G1 – is quite variable, and a cell may spend a long time in this phase carrying out its specialized functions.
- S phase – the cell’s DNA is replicated
- G2 – the cell makes preparations for mitosis-for example, by synthesizing components of the microtubules that will move the segregating chromosomes to opposite ends of the dividing cell
- In mitosis, a single nucleus gives rise to
two daughter nuclei that each contain the same number of chromosomes as the parents nucleus
- While mitosis is a continuous process in which each event flows smoothly into the next, it is convenient to subdivide it into a series of stage:
prophase, prometaphase, metaphase, anaphase, and telophase
- During interphase only the nuclear envelope and the nucleolus are
- The appearance of the nucleus changes as the cell enters
- Before the S phase of interphase,
each chromosome contains one very long double stranded DNA molecule. During prophase the chromosomes become much more tightly coiled and condensed
- After DNA replication,
each chromosome has two DNA molecules, known as sister chromatids. The chromatids are held together at a region called the centromere until separation during mitosis.
- During prophase the chromosomes become so compact that
they can be seen clearly with a light microscope after staining with special dyes.
they can be seen clearly with a light microscope after staining with special dyes.
- The centrosome consists of a pair of centrioles
each one a hollow tube formed by nine triplets of microtubules. During S phase the centrosome becomes duplicated, and at the g2-to-m transition, the two centrosomes separate from one another, moving to opposite sides of the nucleus. Eventually these identify poles toward which chromosomes move during segregation
- The positions of the centrosomes determine
the plane at which the cell divides; therefore they determine the spatial relationship between the two new cells. This relationship may be of little consequence to single free living cells such a yeasts, but it is important for development in a multicellular organism.
- For example, during the development of an embryo, the daughter cells from some divisions must be positioned correctly to receive signals to form new tissues.
- Each of the two centrosomes, when positioned on opposite sides of the nucleus, serves as
a pole, toward which the chromosomes move. Tubulin dimers from a round the centrosomes aggregate into microtubules that extend from the poles into the middle region of the cell. Together these microtubules make up a spindle.
- The spindle forms during
prophase and prometaphase, when the nuclear envelope breaks down
- There are two type of microtubules in the spindle:
o Polar microtubules form the framework of the spindle and run from one pole to the other
o Kinetochore microtubules – which form later, attach to the kinetochores on the chromosomes. The two sister chromatids in each chromosome become attached to kinetochore microtubules from opposite sides of the cell. This ensures that the two chromatids will be pulled toward the opposite poles
- Separation fo the chromatids and movement of the daughter chromosomes is the central feature of
mitosis. It accomplishes the segregation that is needed for cell division and completion of the cell cycle
- In prometaphase
the nuclear envelope breaks down and the compacted chromosomes, each consisting of two chromatids, attach to the kinetochore microtubules.
- In metaphase
the chromosomes line up at the midline of the cell
- In anaphase
the chromatids separate, and the daughter chromosomes move away from each other toward the poles
- Two mechanisms operate to move the chromosomes along.
First, the kinetochores contain a protein called cytoplasmic dynein that act as a molecular motor. It hydrolyzes ATP to ADP and phosphate, thus releasing energy that move the chromosomes along the microtubules toward the poles. This accounts for about 75% of the force of motion.
- Second, the kinetochore microtubules shorten from the poles, drawing the chromosomes toward them, accounting for about 25 percent of the motion
- Telophase occurs after
the chromosomes have separated and is the last phase of mitosis.
- During this period, a nuclear envelope forms around each set of new chromosomes, nucleoli appear, and the chromosomes become less compact. The spindle also disappears
- The eukaryotic cell cycle can be divided into four stages:
G1, S, G2, and M.
- Progression through these phases is tightly regulated.
For example, at the end of G1, the G1-S transition marks a key decision point for the cell: passing the R point usually means the cell will proceed with the rest of the cell cycle and divide
- What event cause a cell to enter the S or M phase? A first indication that there were substances that control these transitions came from experiments involving cell fusion.
- Experiments show that a cell in S phase produces and activated DNA replication. Similar experiments pointed to a molecular activator for entry into M phase
- The molecular activators revealed by the cell fusion experiments turned out to be
protein kinases. These enzymes, common in cell signal transduction, catalyze the phosphorylation of target proteins that regulate the cell cycle
o Cell cycle regulator + ATP -> cell cycle regulator-P + ADP
- The class of protein kinases involved in cell regulation is
called cyclin dependent kinases. CDKs are activated by binding to the protein cyclin. This binding changes the shape of a Cdk such that its active site is exposed and is an example of allosteric regulation
- There are several Cdk’s that regulate the cell cycle at specific stages called
cycle checkpoints. Each Cdk has its own cyclin to activate it an the cyclin is made only at the right time.
- After the Cdk acts, the cyclin is broken by a protease. So a key event controlling transition from one cycle phase to the next is the synthesis and subsequent breakdown of a cyclin.
- Cyclins are synthesized in response to various molecular signals, including growth factors
o Growth factor -> cyclin synthesis -> Cdk activation -> cell cycle events
- G1-S cyclin-Cdk catalyzes the phosphorylation of a protein called
RB. In many cells RB acts as an inhibitor of the cell cycle at the R point. To begin S phase, a cell must overcome the RB block. Here is where G1-S-cyclin-CDK comes in: it catalyzes the addition of a phosphate to RB. This causes a change in the three dimensional structure of RB, thereby inactivating it. With RB out of the way, the cell cycle proceeds
.o RB -> RB-P + ADP
- Although the nucleus divides twice during meiosis, the DNA is
- Although the nucleus divides twice during meiosis, the DNA is
- Keep in mind the overall functions that meiosis has evolved to serve
o To reduce the chromosome number from diploid to haploid
o To ensure that each of the haploid products has a complete set of chromosomes
o To generate genetic diversity from the products
- Aneuploidy
– condition of having an extra chromosome or missing a chromosome
- The first type of cell death, necrosis,
occurs when cells are damaged by mechanical means or toxins, or are starved of oxygen or nutrients. These cells often swell up and burst, releasing their contents into the extracellular environment. This process often results in inflammation
- More typically cell death is due to apoptosis. Apoptosis is genetically programmed series of events that result in cell death.
o The cell is no longer needed by organism – for example, before birth, a human fetus has weblike hands, with connective tissue between the fingers. As development proceeds, this unneeded disappears as its cells undergo apoptosis in response to specific signals
o The longer cells live, the more prone they are to genetic damage that could lead to cancer. This is especially true of epithelial cells on the surface of an organism, which may be exposed to radiation or toxic substances such cells normally die after only days or weeks and are replaced by new cells
o In apoptosis – the cell becomes detached from its neighbors, hydrolyzes its DNA into small fragments, and forms membranous lobes that break into cell fragments
- Apoptosis is also used by plant cells in an important defense mechanism called the hypersensitive response. Plans can protect themselves from disease by undergoing apoptosis at the site of infection by a fungus or bacterium
