1.
Prokaryotic Cell Division:
- RememberŠ the definition of a prokaryote is a single-celled life form with no true nucleus. Instead, the cell has a nucleoid region with a single chromosome
- Prokaryotes divide by asexual reproduction, also known as binary fission: division (fission) produces two (binary) daughter cells that are identical to the parent cell
- Before division, the chromosome replicates inside the cell and attaches to the plasma membrane. The cell elongates, and pulls the chromosomes apart and into different cells as the plasma membrane and cell wall reform between the two new cells
- In addition to lacking a true nucleus, prokaryotic cells have no true organellesŠ which makes cell division much more rapid
- Since there is only one chromosome, there are no spindle fibers to organize and separate chromosomes in prokaryotic cells
2.
Eukaryotic Cell Division:
- Cell division in eukaryotes involves nuclear division and cytokinesis (division of cytoplasm)
- DNA is organized into chromatin consisting of chromosomes and protein inside a true nucleus
- Normally, most eukaryotic cells have two copies of each chromosome (2n, or diploid state). The reproductive cells (or gametes) have only one copy of each chromosome (n or haploid state). Some eukaryotes (plants and fungi) have an adult haploid state, which can divide and produce more haploid cells. In animals, the haploid state is restricted to the reproductive cells, which do not divide following meiosis.
- Each type of eukaryote has a characteristic number of chromosomes
Cell Cycle:
Consists of Interphase and Mitosis
The time required for cell division is relatively constant for a given cell type of a given organism (usually between 14 and 24 hours)
Interphase: consists of G1, S, and G2 stages.
- S phase is the synthesis stage of the cell cycle, when the DNA is replicated. Duplicated chromosomes are held together at the centromeres of each, and are referred to as sister chromatids
- G1 stage is a growth (formerly gap) stage during which the organelles increase in number to produce enough for two new cells
- G2 stage is also a growth stage in which metabolism provides new metabolites and energy for the mitotic division
Mitosis: M stage
Prophase: chromatin condenses and the nuclear membrane completes disintegration. Spindle fibers begin to assemble from microtubules in centrosomes. Animal cells have centrioles and form short asters prior to formation of spindle fibers, plant cells do not.
Prometaphase: Chromosomes attach to the spindle fibers, and are moved to the center of the cell (metaphase plate). Spindle fibers attach to the kinetochores (attachment point of centromeres) of duplicated chromosomes
Metaphase: Chromosomes align at metaphase plate attached to kinetochore spindle fibers
Anaphase: Chromosomes move toward opposite poles of the cell due to disassembly of spindle fibers
Telophase: Chromosomes are at opposite poles of the cell; nuclear envelope reforms around each set of chromosomes, and spindle disappears. Cytokinesis beginsŠ
Cytokinesis:
- Animal cells divide by means of a cleavage furrow
- Plant cells divide using a cell plate to allow formation of a new plasma membrane and cell wall between the two new cells. Cell wall is too rigid for cleavage furrow
Cell cycle control: S and M cyclins (cycle in quantity during cell cycle) combine with S and M cyclin-dependent kinases, respectively, to phosphorylate and activate critical molecules involved in the cell cycle
- S and M cyclins are degraded after function (cyclins:
- S and M cdk¹s are generally recycled (no cycling)
Cancer Development:
- Cancer development is promoted by:
1. Inactivation of tumor suppressor proteins (p53, brca1)
- Loss of the p53 protein results in a lack of apoptosis (programmed
cell death) in response to DNA damageŠ cells with DNA damage are then allowed to divide & pass the mutations to new cells
2. Activation of cellular oncoproteins (c-myc, h-ras)
Cancer Cells:
- nonspecialized cells that have abnormal chromosomes (mutations) & divide uncontrollably
- no contact inhibition; cells grow in layers, clusters (tumors)
- metastasize: migrate in blood to new sites in the body & form new tumors
Note: Recognize and be familiar with similarities and differences between:
a. Prokaryotic and eukaryotic cell division
b. Plant and animal cell cell division
Chapter 10: Meiosis and Sexual Reproduction
Meiosis is the type of nuclear division that reduces the chromosome number from the diploid (2n) number to the haploid (n) number
In animals, the adult is always diploid, and only the gametes (reproductive cells) are haploid (diplontic cycle)
In plants, the haploid stage (which produces gametes) alternates with the diploid stage (alternation of generations)
In protists and many fungi, the adult is always haploid; only the zygote (2n product of fertilizationŠn+n=2n) is diploid (haplontic cycle)
Chromosomes in eukaryotes have homologues (the corresponding chromosome from the other parent cell).
- Homologous chromosomes (1 maternal in origin, the other paternal) code for the same genetic traits, but have their own alleles (their own copy of a specific genetic locus or gene)
- Homologous chromosomes usually differ somewhat in DNA sequence, but are largely the same in content (genetic information)
Meiosis has 2 divisions:
Meiosis I: reduction division; the chromosome number is halved from the 2n number to the n number
- In
metaphase of meiosis I, homologous
pairs of duplicated chromosomes line up at the metaphase plate, and during
anaphase, duplicated homologs separate
- Also in metaphase of meiosis I, paired homologous chromosomes form bivalents (close pairing of homologs), and may exchange genetic information by forming chiasmata (crossing over of arms of homologous chromosomes)
- This results in tremendous possibility for genetic variation
- Also, the order of pairing for homologous chromosomes (which one of pair (maternal or paternal) goes to each daughter cell) is random
Meiosis II: mitotic-like division (since duplicated chromosomes are separated, instead of pairs of duplicated chromosomes in meiosis I), except each daughter cell is haploid in chromosome number
- In metaphase of meiosis I, duplicated chromosomes (only 1/2 the original #, since homologues were separated in meiosis I) line up at the metaphase plate, and during anaphase, duplicated chromosomes separate
The remaining events of the stages of meiosis I and meiosis II (chromatin condensation, nuclear envelope breakdown and reformation, spindle fiber assembly and disassembly, cytokinesis) are similar to mitosis
- Four daughter cells are produced by meiosis (compared to 2 in mitosis)
- The chromosome number goes from 2n (diploid) to n (haploid); following mitosis, the chromosome number of the daughter cells is the same as that of the parent cell
- The daughter cells of meiosis are not genetically identical to the parent cell
In animal cells, gametes are produced by spermatogenesis in males, and oogenesis in females
- Spermatogenesis results in four haploid spermatids
- Oogenesis results in 1 haploid egg and two or three polar bodies
Fertilization results in the formation of diploid zygote
- the egg arrests (stops dividing) in metaphase of meiosis II until fertilization occurs. If fertilization does not occur, the egg never completes meiosis and is lost.
Blending Concept of Inheritance: Before Mendel, it was widely believed that each parent contributes equally to the appearance of the offspring, and that parents of contrasting appearance always produce offspring of intermediate appearance.
- Mendel proposed a particulate theory for inheritance, whereby each parent passes on a component of a given inheritable trait (appearance was broken down into specific traits, or observable features of an organism)
Mendel¹s Law of Segregation: Each organism has 2 factors for each trait, and the factors segregate during formation of gametes, such that each gamete has only one factor for each trait.
- The factors described above eventually became known as alleles (alternate forms of a gene)
Mendel studied the inheritance of simple, distinguishable traits in garden peas such as stem length and seed color.
- Plants of contrasting appearance (Parents, or P generation) for a given trait were cross-pollinated, and the offspring were observed.
- Reciprocal crosses were performed: the pollen of a short plant was used to cross-pollinate a tall plant, and the pollen of a tall plant was used to cross-pollinate a short plant). In either caseŠ
- When 2 true-breeding plants (offspring always have the same appearance as the parent(s) for a given trait) of contrasting appearance for a specific trait were cross-pollinated, all of the resulting offspring had the appearance of only one of the parents for that trait.
- When the offspring from this first test cross (F1 generation) were cross-pollinated, the appearance (characteristic) of the other parent reappeared in the offspring (F2 generation) at a low frequency (1/4). The other 3/4 of the offspring again looked like the F1 generation.
This type of cross, involving parents of contrasting appearance for only one trait, is called a monohybrid cross (the offspring are a hybrid of the 2 parents for the trait)
- Mendel always found that, regardless of the trait studied (seed shape, pod color, etcŠ), the F2 generation of a monohybrid cross always had a 3:1 ratio regarding their appearance for that trait.
From this work, the concept of dominance for a given trait arose.
- One allele is dominant, and masks the expression of the other allele
- The other allele is recessive, and requires both alleles be recessive for expression of the characteristic
- The dominant allele (for which the trait is named) is identified by an uppercase letter, while the recessive allele is identified by the same letter in lowercase
- Alleles occur on homologous chromosomes (the same chromosome in the cells of a given individual, one of maternal origin, the other of paternal origin) at the same location, which is called the genetic locus for that gene.
- An individual with 2 of the same allele (either dominant or recessive) for a given trait is homozygous for that trait (e.g.: for stem length, TT (tall) or tt (short))
- An individual with different alleles (one dominant, the other recessive) for a given trait is heterozygous (for stem length, Tt (appears tall because only the dominant allele is expressed if present))
The explanation for Mendel¹s results:
- The F1 parents had 2 separate copies of each genetic factor (gene), one dominant and the other recessive
- The factors segregated in the gametes, each gamete receiving only one copy of each factor
- Random fusion of all possible gametes occurred upon fertilization
The genotype of an individual for a trait refers to which alleles are present for that trait (TT, tt, or Tt genotypes)
The phenotype of an individual for a trait refers to the appearance (expressed characteristic) for that trait (tall or short phenotype)
Laws of probability
- Multiplicative Law
- Additive Law
Punnett Squares : predict the probable results of a genetic cross
(Be able to construct & interpret)
Monohybrid testcross: used to determine if an individual with a dominant phenotype is homozygous or heterozygous for a trait
- Cross individual with a homozygous recessive individual
Dihybrid Cross: determines the ratio of offspring phenotypes for 2 traits
(e.g.: for peas, homozygous tall, yellow (TTYY) vs. homozygous short, green (ttyy))
- In the F2 generation (the result of the TtYy
vs. TtYy cross), the offspring
always have a 9:3:3:1 phenotypic ratio
(dominant for both traits: dominant for1, recessive for the other
trait: recessive for 1 trait, dominant for the other trait: recessive for both
traits)
Chapter 12: Chromosomes
and Genes
Traits do not always follow simple dominance
- Incomplete dominance: neither allele is fully dominant
- the offspring of a cross of true-breeding parents of contrasting appearance for a trait exhibiting incomplete dominance are of intermediate appearance between the 2 parents ( like the blending concept, but of course is the exception, not the rule)
- Example: Red-flowered four o¹clock vs.
white-flowered four o¹clock (both true-breeding) yields offspring all with pink
flowers. If this generation is
crossed, the white-flowered offspring reappear in the next generation
- Codominance: both alleles are fully expressed
- the offspring of a
similar monohybrid cross in this case express both characteristics
- Example: blood types in humans; both A and B types are dominant (if present, will be expressed). The homozygous recessive individual has blood type O
Pleiotropy: situation where a gene (pleiotropic gene) affects more than one phenotypic characteristic of the individual
- Example: Marfan Syndrome: the gene for fibrillin (strengthens elastic fibers in connective tissue, affecting many characteristics of an individual) is not expressed
Epistasis: multiple alleles (from multiple genes) affect the expression of a given trait. If any one of these characteristics or gene products is not expressed (homozygous recessive for a dominant characteristic, or the gene is lost to mutation), the trait is not expressed.
Multiple alleles: some genes have more than 2 alleles controlling the expression of a given trait
- Blood type in humans is controlled by three alleles : A, B, and O (neither A or B)
Environmental effects on PhenotypeŠ (see text)
Polygenetic Inheritance: one trait is controlled by several genes at different genetic loci on the same pair of homologous chromosomes. Each gene has a contributing (dominant) and noncontributing (recessive) allele. The affects of the different genes are additive, resulting in many different possible phenotypes (environment also plays a role)
Chromosomal Theory of Inheritance:
- chromosomes contain genes, and thus they behave similarly during meiosis and fertilization (individual chromosomes are not changed during these processes)
Sex Chromosomes:
- autosomes (nonsex chromosomes) are normally the same in number and type in males and females
- the sex of the individual is determined by the type of sex chromosomes (X and Y) present
- sex chromosomes in females are XX and in males are XY
- sex chromosomes contain sex-specific genes (sex-determining region of Y (SRY))
The X chromosome also has genes for traits that are not
sex-specific
- the gene for eye color in fruit flies is on the X chromosome
- the gene for color blindness in humans is on the X chromosome
Genes that inherited together at a high frequency are said to be linked: they are in close proximity to each other on the same chromosome
- linked genes belong to the same linkage group
- linkage can be determined by the frequency of crossing over during meiosis for a set of genes. Linked genes will have a high frequency of crossing over
Mutations:
Polysomy: more than the normal number of copies of a given gene/chromosome
Polyploidy: more than the normal number of chromosomes for a given species
Inversion: one arm of a chromosome is inverted with respect to its normal orientation
Translocation: a piece of one chromosome joins with another different chromosome
Point mutation (substitution): one nucleotide (base) is substituted for another
Insertion: one or several nucleotides are inserted at a specific site in the DNA sequence
Deletion: one or several nucleotides are deleted (lost) at a specific site in the DNA sequence
Frame-shift mutation: one of the above mutations that changes the reading frame of the DNA/RNA sequence from the point of the mutation (protein sequence changes)
Chapter 13: Human Genetics
Humans normally have 23
pairs of chromosomes: 22 pairs of
autosomes, and 1 pair of sex chromosomes.
- under normal circumstances, the sex chromosomes in females are XX, and in males are XY
- a karyotype displays all chromosomes in the cells of an individual, lined up according to homologous pairs
Nondisjunction causes chromosomal abnormalities
- Nondisjunction is the failure of homologous chromosomes to separate during meiosis I, or the failure of daughter chromosomes (duplicated chromatids) to separate during meiosis II
- Nondisjunction leads to gametes that have too few or too many chromosomes
- when these gametes fuse with a
normal gamete during fertilization, a monosomy (2n-1) or a trisomy (2n+1) can result in the zygote (and the
offspring)
- Most trisomies, and nearly all monosomies, of autosomes are fatal (Individuals with trisomy 13 (Patau syndrome), and trisomy 18 (Edward syndrome) have a life span of under 1 year)
- The most common trisomy in humans is trisomy 21, which leads to Down¹s Syndrome
- Many trisomies and monosomies of sex chromosomes exist, and are often not fatal syndromes
- Males with Jacob syndrome (XYY) have symptoms associated with an increase in male hormones, and males with Klinefelter syndrome (XXY) have symptoms associated with an increase in female hormones
- Females with Turner syndrome (X0) are missing an X chromosome, and tend to have underdeveloped feminine characteristics
- Females with Triplo-X syndrome (XXX) tend to have fairly normal sex-specific characteristics
Trinucleotide repeats cause human disorders
- Fragile X syndrome is caused by a trinucleotide repeat on the X chromosome, causing the end of the chromosome to nearly break away from the rest of the chromosome
- The syndrome does not appear for several generations, apparently due to a minimum number of repeats necessary for development of symptoms ( the number of repeats increases with successive generations)
- Symptoms of Fragile X syndrome, as in X-linked disorders, tend to be most severe in males
Human Autosomal Dominant disorders (may be homozygous dominant or heterozygous for the abnormality for the syndrome to appear)
- A dominant autosomal human disorder, Huntington Disease, is also caused by a trinucleotide repeat
- Huntington Disease leads to a progressive degeneration of brain cells, the symptoms of which do not appear until middle age
- Neurofibromatosis is caused by mutation of a tumor-suppressor gene on chromosome 17
Human Autosomal Recessive disorders (must be homozygous recessive for the abnormality for the syndrome to appear)
- Tay-Sachs disease is caused by the loss of the enzyme Hexosaminidase A (Hex A)
- Cystic Fibrosis is caused by a gene located on chromosome 7
- Phenylketonuria is caused by a loss of an enzyme necessary for the normal metabolism of the amino acid phenylalanine, encoded by a gene on chromosome 12
Pedigrees follow the pattern of inheritance of a disorder in families
- the pattern of inheritance (autosomal dominant or recessive, or sex-linked) can often be determined from a pedigree chart
Incomplete dominance in human disorders
- Sickle-Cell disease is a disorder that exhibits incomplete dominance in humans
- The disease is caused by a mutation in the gene for the oxygen-carrying blood cell protein hemoglobin
- Individuals with sickle-cell disease have 2 alleles for the abnormal hemoglobin
- Individuals with sickle cell trait have 1 normal and one sickle-cell allele for hemoglobin
Chapter 14: DNAŠThe Genetic Material
Searching for the genetic material
Features of the genetic material:
- able to store information for development & metabolism
- stable (high fidelity replication and transmission across generations)
- able to undergo rare mutations (source of evolution)
Griffith¹s Transformation Experiment:
- Transformed heat-killed R strain bacteria with S strain DNA to restore virulence
Avery et alŠ The transforming material is digested by DNase enzyme
- DNA is the transforming material
Hershey & Chase
- Bacteriophage DNA, and not its protein, is the genetic material that enters the host to reproduce the virus
Chargaff¹s rules:
- The amount of A,T,G, and C in DNA varies from species to species
- In each species, the amount (%) of A=T and G=C (A pairs with T and G pairs with C)
Watson
& Crick DNA model:
- DNA is a double-helical molecule, with a sugar-phosphate backbone on the outside and paired bases (A-T and G-C, complementary base pairing) on the inside
DNA replication is semiconservative:
- each newly replicated DNA molecule consists of 1 old strand from the original double- stranded DNA molecule, and 1 newly synthesized strand
- The parent DNA molecule unwinds and unzips (breaks apart strands (enzyme helicase)), and each old strand serves as template for the new strands
Replication is carried out by the enzyme DNA Polymerase, as well as some additional protein factors
- Replication is unidirectional (5¹ to 3¹). One strand (the leading strand) is synthesized continuously, while the other strand (the lagging strand which is in the 3¹ to 5¹ direction) is synthesized discontinuously (in short fragments called Okazaki fragments) in the 5¹ to 3¹ direction
- DNA Polymerase has a proofreading activity to correct replication errors (adding the wrong base). The corrected error rate (after proofreading) is 1 in 1 billion bases
Prokaryotic replication is much more rapid than eukaryotic replication
only one chromosome is replicated, usually bidirectionally
Eukaryotic replication occurs on chromosomes at multiple origins of replication
- replication
begins by the formation of replication forks, where DNA unwinding occurs
- actively
replicating DNA is called a replication bubble
Chapter 15: Gene Activity
- Genes specify enzymes: the bread mold fungus was used to show that loss of a gene that encoded an enzyme in a metabolic pathway could be determined by growing the mold in the presence or absence of individual metabolites in the pathway, and finding out where in the pathway the missing enzyme functions
- Genes specify polypeptides (proteins are composed of 1 or more polypeptides)
- some genes only specify RNA molecules (tRNA and rRNA)
Gene
Expression:
DNA is transcribed to RNA in the nucleus
primary mRNA is modified in the nucleus:
- 5¹ cap and poly-A tail are added
- splicing of exons together with removal of introns
- Transcription is carried out by a 5¹ to 3¹ RNA Polymerase, as well as additional protein factors
- The result of these modifications is mature mRNA
Mature mRNA is translated to protein in the cytoplasm
- Translation occurs at the ribosomes
- Many ribosomes may synthesize protein from the same mRNA molecule at the same time (polyribosomes)
- tRNA molecules carry amino acids to the ribosome during translation (a tRNA for each amino acid)
- rRNA along with proteins comprise the structure of the 2 subunits of the ribosome
- Ribosome subunits associate immediately prior to translation, and dissociate following translation
- Ribosomes bind mRNA at the 5¹ cap, assisted by rRNA components, and begin translation, usually, at the first AUG (start) codon
- The initiator (AUG/methionine) binds to the P site to begin translationŠ all following tRNAs bind to the A site, and transfer their amino acids to the growing polypeptide at the P site
- Following translation, a release factor cleaves the complete polypeptide from the last tRNA and the ribosome, and the polypeptide leaves the ribosome
Mutations:
Point mutations: (Substitutions) change the identity of a single nucleotide in DNA (can lead to disorders, as is the case with sickle-cell disease)
Insertions & Deletions: addition of 1 or more nucleotides (insertion), or removal of 1 or more nucleotides (deletion) to DNA
- can lead to frame-shift mutation: at the point of insertion or deletion, every codon that follows is changed
- can introduce stop codon
- multiples of 3 bases only change 1 codon
Mutagens (ultraviolet light, chemical carcinogens (pesticides, cigarette smoke) cause mutations
- DNA repair enzymes can correct some mutations