Genetics - Chapter 12
Meiosis I Products
Begins with one diploid cell containing one haploid set of chromosomes derived from the mother and one haploid set of chromosomes derived from the father Ends with two haploid nuclei each containing one haploid set of dyads After cytokinesis, each of the two progeny cells have a nucleus with a haploid set of dyads
Chromosome Structure
Centromere - constriction along the length of a chromosome that contains proteins essential to the behavior of chromosomes during cellular division Location on a Chromosome - p arm: the smaller arm of a chromosome - q arm: the larger arm of a chromosome - regions: divided into numbers, 1 being the closest to the centromere - subregions are indicated by decimals of the region ex - ChromosomeArmRegion.Subregion 7q31.2 Location of Centromere: 1) Metacentric Chromosome: centromere about the center with two equal arms 2) Submetacentric Chromosome: one arm slightly longer than the other 3) Acrocentric Chromosome: one arm is a short stalk, the other is a long bulb 4) Telocentric Chromosome: one long arm bc centromere is at the end of the chromosome
Genic Sex Determination
Eukaryotic organisms that do not have sex chromosomes rely on an allelic difference in one or several genes to distinguish the "sex" of the organism - this is called the Genic System of Sex Determination
3. Metaphase
Kinetochore microtubules orient the sister chromatids along the metaphase plate. At this point the chromosomes are the most dense.
End result of two meiotic divisions
One diploid cell (2N) becomes four haploid cells (N), each with one chromosome from the homologous pairs (tetrad) from meiosis I, and due to crossing over in prophase I, these chromosomes are not exact copies
1A. Early Prophase
Prior to Prophase, in Interphase, each chromosome has replicated and now consists of two sister chromatids and the centrioles have duplicated to produce two pairs Centriole pairs begin to move apart. Chromosomes shorten and thicken as they condense and start to become visible. The Nucleolus begins to disappear.
Meiosis II
Prophase II, Prometaphase II, Metaphase II, Anaphase II, Telophase II
Sex Chromosomes
Sex Chromosomes: chromosomes that differentiate between a male and female (found in all cells along with autosomes) X Chromosome: sex chromosome of which females have 2 and males have 1 Y Chromosome: sex chromosome of males NOTE: sex is determined by whether or not the sperm that fertilizes the egg, which always has two X chromosomes, is carrying an X or a Y chromosome GENERALLY SPEAKING: Heterogametic Sex (males): the male can produce two kinds of gametes (X or Y) Homogametic Sex (females): the female can only produce one kind of gamete (X) NOTE: random fusion of male and female gametes produces progeny that is 1/2 female and 1/2 male
Sex Determination in Drosophilia and Caenorhabditis
Sex determination in the fruit fly and the nematode is dependent on the ratio of the number of X chromosomes to the number of sets of autosomes - this is called the X Chromosome-Autosome Balance System of Sex Determination NOTE: for these animals, the Y chromosome plays no role in sex determination Dosage compensation in drosophila occurs not by the inactivation of one of the two female X chromosomes, but rather the increased transcriptional activity of the one X chromosome of males Nematodes are hermaphroditic - they have both sex organs
Meiosis in Animals
Sexual Reproduction: meiosis produces haploid gametes, during fertilization a haploid male and female gamete fuse to form a diploid zygote, that diploid zygote then goes through cycles to mitosis for form a haploid organism Gametes are produced in specialized cells: Spermatogenesis - production of sperm (male gametes called spermatozoa) within the testes Oogenesis - production of eggs (female gametes)
Eukaryotic Chromosomes
The eukaryotic genome is distributed among multiple, linear chromosomes, with the number of chromosomes being characteristic to each specie Diploid (2N): organisms with two copies of each type of chromosome in their nuclei, produced by the fusion of two haploid gametes (mature reproductive cells that are specialized for sexual fusion) Zygote: the diploid product of the fusion of a haploid male and female gamete Haploid (N): a gamete with only one set of chromosomes Genome: complete set of genetic info in a haploid chromosome set (a diploid organism will have two complete genomes) Ex - humans are diploid, we have 46 chromosomes, 23 pairs of chromosomes, and two complete genomes Homologous Chromosomes - members of a chromosome pair in a diploid organism that contain the same genes and that pair during meiosis (each member of the pair is called a homolog) Nonhomologous Chromosomes - Chromosomes that contain different genes and that do not pair during meiosis Sex Chromosomes - what makes the distinction between male and female cells - one sex will have matched chromosomes and one will have unmatched chromosomes - females have a matched pair of sex chromosomes (XX) and males an unmatched pair of sex chromosomes (XY) Autosomes - chromosomes other than sex chromosomes Karyotype - a complete set of all the metaphase chromosomes in a cell - chromosomes are ordered in terms of size smallest to largest NOTE: chromosomes are typically identified during metaphase bc this is the point at which they are most condensed
4. Anaphase
The joined centromeres of sister chromatids separate, giving rise to two daughter chromosomes. Once the paired kinetochores on each chromosome separate, the sister chromatids undergo disjunction (separation), and the daughter chromosomes move toward the opposite poles. As the kinetochore microtubules shorten, the daughter chromosomes are pulled two the poles. Once the they have officially reached the two poles, chromosome separation is complete. Cytokinesis (cell division) begins.
Metaphase I
The kinetochore microtubules align the tetrads on the metaphase plate in pairs of homologous chromosomes (sister chromatids pairs)
1B. Late Prophase
The mitotic spindle (spindle apparatus), which is a structure consisting of fibers made of microtubules make of proteins called tubulins, begins to assemble between the centrioles outside of the nucleus. Each duplicated chromosome becomes visible as two sister chromatids. The nuclear envelop breaks down.
2. Prometaphase
The nuclear envelop is now broken and the developing spindle now enters the former nuclear area. A specialized protein complex called a kinetochore binds to each centromere. These kinetochores are the site where the chromosomes are attached to the spindle microtubules known as kinetochore microtubules. At least one kinetochore microtubule extends from each pole and attaches to the kinetochore bound to the centromere of the sister chromatids NOTE: Nonkinetochore microtubules do not bind to the kinetochores but are attached to the spindle poles and terminate in the middle of the spindle
5. Telophase
The two sets of daughter chromosomes are assembled at opposite ends of the cell. The nuclear envelope begins to form around each group of chromosomes as they begin to uncoil and decondense. The microtubules of the spindle disappear and the nucleolus reforms. Nuclear division is complete and the cell has two individual nuclei.
Gene Segregation in Meiosis
Three significant results: 1) Meiosis generates haploid nuclei with half the number of chromosomes found in the diploid cell that entered meiosis (occurs because in meiosis for every one replication event there are two meiotic divisions). However, the fusion of male and female haploid cells during fertilization produces a diploid zygote and restore the 2N 2) In metaphase I, each maternally derived and paternally derived chromosome has an equal change of aligning on either side of the metaphase plate (the basis of Mendell's second law - random alignment leads to Independent Assortment). Because of random alignment, each haploid cell generated by meiosis is likely to have chromosomes from both the maternal and paternal chromosomes NOTE: a maternal chromosome is the chromosome that contains the maternal centromere and a paternal chromosome is the chromosomes that contains the paternal centromere Formula for number of possible chromosome arrangements at the metaphase plate in meiosis: 2^(n-1) *where n is the number of chromosome pairs in a diploid cell Formula for the number of possible chromosome combinations in the nuclei resulting from independent assortment of chromosomes in meiosis: 2^n *where n is the number of chromosome pairs in a diploid cell NOTE: this large number of possible combinations of maternal and paternal chromosomes results in nuclei produced by meiosis that are genetically unique 3) The crossing over events that occur between maternal and paternal chromosomes during meiosis I (prophase I) generates even more variation in final combinations
Meiosis in Plants
Two phases: Gametophyte - haploid stage in which the gametes are produced Sporophyte - diploid stage in which haploid spore are produced by meiosis Alteration of Generations
Nondisjunction of X Chromosomes
Typically during meiosis, the homologous chromosomes will separate in meiosis 1 and the sister chromatids will separate in meiosis 1 Nondisjunction: if the chromosomes or chromatids fail to separate and move to opposite poles in anaphase Primary Nondisjunction: nondisjunction of an autosomal chromosome X Chromosome Nondisjunction: two X chromosomes fail to separate, so eggs produced either have two X chromosomes or zero X chromosomes instead of one X for each Aneuploidy: abnormal condition in which one or more whole chromosomes of a normal set are missing (ex - XO) Secondary Nondisjunction: this a disjunction of the X chromosomes of progeny that have already experienced primary nondisjunction - in secondary nondisjunction, you will have a two X chromosomes moving to one pole and the Y chromosome moving to the other pole, resulting in eggs that are either XX or Y - these eggs are then fertilized by X or Y sperm cells, resulting in two surviving class, the XXY females and the XY males (the XXX and YY classes die early in development)
Sex Linkage
Wild type: a strain, organism, or allele that is most prevalent in the wild population of an organism with respect to phenotype and genotype Mutant alleles: a result of mutational changes to the wild-type allele NOTE: wild-type = w+ and mutant allele = w Hemizygous: used to refer to X Linked genes in males because the gene is only present once in the organism (bc males only have one X chromosome) so there is no homologous gene on the Y chromosome Crisscross Inheritance: pattern of gene transmission from father to daughter to male grandchild Sex-Linked Inheritance: gene locus is on a sex chromosome so inheritance patterns differ from Mendel's laws NOTE: a result for a reciprocal cross will show differences between males and females if the gene is sex-linked but will be the same if it is an autosomal gene
X-Linked Dominant Inheritance
X-Linked Dominant Trait: a trait resulting from a dominant mutant allele on an X chromosome (enamel hypoplasia) - more frequent in females than males - there is no Father-Son Interitance
X-Linked Recessive
X-Linked Recessive Trait: a trait resulting from a recessive mutant allele carried on the X chromosome (ex - Hemophilia) Characteristic of X-Linked Recessive 1) woman must be homozygous to express mutant trait 2) A mother can pass it on to a son, but when he becomes a father he can only pass it on to his daughters (No Father-Son Inheritance) 3) Many more males should exhibit the mutant trait than females 4) All son will express the mutant trait if the mother is homozygous for the trait 5) The sons of a heterozygous mother should should a 1:1 ratio of expressing and not expressing the mutant allele (a+/Y and a/Y) 6) A carrier (heterozygous) mother and a normal father will result in all normal daughters and a 1:1 ratio of normal to mutant in the sons 7) A male expressing the trait and a homozygous normal mother will produce all normal children
Y-Linked Inheritance
Y Linked Trait (Holandric): a trait resulting from a mutant gene that is carried on the Y chromosome (ex - Hairy ears) - every son of a mutant father will express the mutant trait - only males, no females will express the trait
Prophase I
begins with the chromosomes having already duplicated, with each consisting of two sister chromatids attached at the centromere NOTE: though Mitotic Prophase and Prophase I both begin with a homologous chromosome pairs, but crossing over only occurs in meiosis Leptonema - the replicated chromosomes begin to condense, once a cell enters leptonema, it has committed to meiosis Zygonema - homologous pairs find each other and align via synapsis, the formation of a zipperlike structure called a synaptonemal complex along the length of the chromatids, which perfectly align the sister chromatids, base pair for base pair NOTE: telomeres play a crucial role in synapsis because they move the chromosomes around so that the homologs align Pacchynema - starts when synapsis concludes, at this point, each synapsed set of homologous chromosomes consists of four chromatids and is known as a Bivalent or a Tetrad. These Tetrads preform crossing over, then the synaptonemal complex is dissembled Crossing over - the reciprocal physical exchange of chromosome segments at corresponding positions along pairs of homologous chromosomes Recombinant Chromosome: a chromosome that emerges from meiosis with a different combination of alleles as a result of crossing over Diplonema - after the synaptonemal complex has dissembled, the homologous chromosomes begin to move apart to form chismata, sites at which homologous chromosomes as tightly associated due to crossing over Diakineisis - chromosomes condense to the point at which the for chromatids of the tetrad are visible
Chromosome Theory of Inheritance
genes are located on chromosomes
Dosage Compensation
if gene expression of the X chromosome is not equalized between XX females and XY males, the condition is lethal early in development Dosage Compensation is a mechanism to avoid this issue In mammals, a somatic cell of a normal XX female contains a highly condensed mass of chromatin named a Barr Body (a female XX somatic cell has one Barr body, while a male XY somatic cell has no Barr body) Lyon Hypothesis 1) the Barr body is a highly condensed and mostly genetically inactive X chromosome after it has undergone lyonization - this leave a single X chromosome that is transcriptionally the equivalent of the single X chromosome of a male 2) the X chromosome inactivated is randomly chosen from the maternally and paternally derived X chromosomes in a process that is independent from cell to cell (Note that once a maternal or paternal X chromosome has been inactivated, it will stay inactive in all mitotic cell progeny) NOTE: is X inactivation is an example of an epigenetic phenomenon - a heritable change in gene expression that occurs without a change in DNA sequence (X inactivation is the epigenetic silencing of one X chromosome to compensate dosage) X inactivation of females that are heterozygous for X Linked traits act as genetic mosaics because cells show either one or the other phenotype (ex- orange and black patches of a calico cat)
Genotypic Sex Determination
sex chromosomes playing a decisive role in the inheritance and determination of sex Y Chromosome Mechanism of Sex Determination - in all mammals, including humans, Y chromosome mechanism of sex determination occurs because the Y chromosome determines the sex of an individual - this occurs because the Y chromosome carries an important gene that sets the switch toward male sexual determination, the product of this gene is called testis-determining factor, which causes the tissue that will become gonads to differentiate into testes instead of ovaries - in absence of the testis-determining factor supplied by a gene on the Y chromosome, the gonads will develop as ovaries Turner Syndrome (45, XO) - a person with turner syndrome is born with XO at their sex chromosome locus as a result of nondisjunction - an XO person is female, sterile, and still have two completely normal sets of autosomal chromosomes - leaving her with a total of 45 chromosomes ( 22 pairs and 1 X sex chromosome) - issues in early development of females with Turner Syndrome indicates that 2 X chromosomes are needed to develop normally Klinefelter Syndrome (47, XXY) - a person with Klinefelter Syndrome is born with XXY at their sex chromosome locus as a result of nondisjunction - a person with Klinefelter Syndrome (XXY) is male, and still has two completely normal sets of autosomal chromosomes - resulting in a total of 47 chromosomes (22 autosomal pairs and 3 sex chromosomes) - issues with early development of Klinefelter patients indicate a pairing of 1 X and 1 Y sex chromosome is necessary for normal male development
Anaphase II
the centromeres separate and the now-daughter chromosomes are pulled to the opposite poles of the the spindle. One sister chromatid from each pair goes to each pole - at this point each sister chromatid is now considered a chromosome
Telophase II
the chromosomes begin to decondense and the nuclear envelop forms around each haploid set of chromosomes as cytokinesis takes place, forming four complete haploid gametes
Prophase II
the chromosomes condense and the spindle fiber begins to form outside of the nuclear envelop
Anaphase I
the chromosomes in each tetrad separate into dyads, so that the chromosomes of each homologous pair disjoin and migrate to opposing poles NOTE: key difference is that for meiosis Anaphase I is that though the homologous chromosomes have separated from one another, the sister chromatids are still attached within each homolog
Cytokinesis (cell division)
the division of the cytoplasm that usually occurs after the nuclear division stage (mitosis) and is completed before the end of telophase. Cytokinesis separates the the two new nuclei into separate daughter cells For animals: a constriction forms in the middle of the cell and continues until two cells are produced For plants: a new cell wall and cell membrane are formed between the two new nuclei to form a cell plate. Cell wall material then coats each side of the plate, resulting in the two progeny cells.
Telophase I
the dyads or chromosomes (consisting of sister chromatids) complete their migration to the opposite opes of the cell and the spindle disassembles. Cytokinesis then occurs and two haploid cells are formed
Meiosis I
the first meiotic division in which the chromosome number is reduced from diploid to haploid Consists of: Prophase I, Prometaphase I, Metaphase I, Anaphase I, and Telophase I
Metaphase II
the movement of the kinetochore microtubules perfectly aligns the chromosomes on the metaphase plate
Prometaphase II
the nuclear envelop formed in telophase I breaks down and the spindle fiber enters the former nuclear area - kinetochore microtubules form on opposite poles and attach to the kinetochores on each chromosome
Prometaphase I
the nucleoli disappears, the nuclear envelope breaks down, and the meiotic spindle that has been forming between the centriole pairs enters the former nuclear area. Kinetochore microtubules attach to both the sister kinetochores of each the chromosome as well as to either one of the poles
Mitosis
the physical nuclear division of the cell cycle, which is often followed by cell division (cytokinesis) Cell Cycle - the cycle of growth, mitosis, and cell division - occurs in two phases within a somatic cell - Mitotic Phase (M phase) and Interphase Interphase - G1: Presynthesis Gap - cell prepares for DNA replication - S: synthesis of new DNA (doubles), DNA of each chromosome is doubled, giving two exact copies called sister chromatids which are held together by the replicated but unseparated centromeres - G2: Postsynthesis Gap - cell prepares for cell division Chromatid: one of the two distinct subunits of all replicated chromosomes that first become visible in prophase Sister Chromatids: exact copies, one the exact replicant of the other, held together by the replicated by unseparated centromere Daughter Chromosomes: what the sister chromatids are referred to after separation In a Diploid: There is originally a pair of homologous chromosomes - this pair of homologous chromosomes then replicates in interphase, resulting in two pairs of sister chromatids - these sister chromatids separate, resulting in two pairs of daughter chromosomes each NOTE: Mitosis is a continuous process in diploids and haploids Mitosis is divided into five stages: 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase
Meiosis
the two successive divisions of a diploid nucleus after one DNA replication (chromosome duplication the original diploid nucleus contains one haploid set from the father and one from the mother For animals: Meiosis results in the formation of haploid gametes (eggs for females and sperm for males by a process called gametogenesis) For plants: Meiosis results in the formation of haploid meiospores (by sporogenesis), which then undergo mitosis to produce gamete-bearing, multicellular complex called a gametophyte Before meiosis, the DNA of the homologous chromosomes replicate, and these chromosomes are then paired and divided in Meiosis I and Meiosis II NOTE: the meiosis of one diploid cell produces four haploid cells (gametes)