Ch 3: Reproduction and Chromosome Transmission

Back to the Basics

General features (44-45). The chromosomes are structures within living within living cells that contain the genotypic material (44). Genes are physically located on the chromosomes. The chomoromes contain a very long segment of DNA and proteins, with the proteins bound to the DNA to provide it with an organized structure (44). Bacteria and archaea are prokaryotes because their chromosomes are not contained within a separate nucleus of the cell (44) These organisms have a single type of circular chromosome in the cytoplasm called a nucleoid. Eukaryotes have internal membranes that enclose highly specialized compartments . These compartments form membrane bounded organelles with specific functions (44).  The nucleus is a particularly conspicious organelle. It is bound by the nuclear envelope, and contains most of the genetic material found within chromosomes.  The mitochondria and chloroplasts (in plants) have their own extranuclear/extrachromosomal DNA.     Karyotypes (45-47) Cytogenics is the field of genetics that involves the microscopic examination of chromosomes. The most basic observation that a cytogeneticist can make is to examine the chromosomal composition of a particular cell (45)  In dividing cells, the chromosomes become more tightly coiled, which shortens them and increases their diameter (46). Distinctive shapes and numbers of chromosomes become visible with a light microscope. Human cells contain 23 pairs of chromosomes (total of 46) (46) Cells that that are not gametes or precurser of gametes are somatic cells. The gametes are also called germ cells (47). A photographic representation of the chromosomes within a cell is called a karyotype (47) Eukaryotic Chromosomes Inherited in Sets (47).  Most eukaryotes have their chromosomes in pairs. Their somatic cells have 2 sets of chromosomes. This makes them diploid (47). The members of a pair of chromosomes are called homologues. Each type of chromosome is found in a homologous pair (47). Homologues are nearly identical in size, have the same banding pattern, and contain a similar composition of genetic material. The homologues can, however, carry different alleles for the same genes. The sequence of bases in one homologue would usually differ less than 1% from another homologue. This doesn’t apply to the gametes. The physical location of a gene is its locus (47).

Cell Cycle Overview

Bacteria reproduce asexually. This is when a preexisting cell divides to produce 2 new cells (48). Precise transmission of chromosomes during every cell division is critical in order for all the cells to receive the correct amount of genetic material (48). Binary fission In bacteria, the circular chromosome is in direct contact with the cytoplasm (48). Prior to division, bacterial cells copy their chromosomal DNA, producing 2 identical copies of the genetic material (48). Once material is divided, the bacteria undergo binary fission, where 2 daughter cells with a copy of the chromosomal genetic material are made (48). Recent research shows that a protein called FtsZ moves to the division site and recruits other proteins to produce a new cell wall between the daughter cells. Eukaryotic Cell Cycle Cells that are destined to divide progress through the cell cycle (49). This cycle consists of the G (gap) phase, S (synthesis) phase, and M (mitosis) phase, along with the G1 and G2 phases (49). G1, S, and G2 are collectively called interphase. Some cells stay in a G0 phase, where they don’t divide (49). G1 phase: The cell may prepare to divide. It may accumulate molecular changes that allows it to progress through the rest of the cell (49). This is called a restriction point. If the cell passes here, it’s cleared to finish the cycle. S phase: The chromosomes are replicated. The cells has twice as many chromatids as compared to the G1 phase (49) The 2 copies of the chromosomes are called the chromatid. They are joined at the center by a centromere to form a pair of sister chromatids. The kinetochore is a group of proteins bound to the centromere. These help hold the sister chromatids together and play a role in sorting. G2 phase: Cell accumulates the materials that are necessary for nuclear and cell division. It then progresses into M phase (50). At this point, the cell would have 46 PAIRS of sister chromatids, giving it 92 chromatids total (49). Chromosome can refer either to a pair of sister chromatids during G2 and M phases or to structures at the end of M phase and during G1 that contain equivalent of 1 chromatid (50). M phase: Mitosis occurs! The replicated chromosomes are distributed ,and 1 nucleus is divided into 2 (50). 

Mitotic Spindle & Mitosis Steps

Mitotic spindle apparatus (50-51) The mitotic spindle apparatus (mitotic spindle) is formed from the centrosomes. (50) Each centrosome is located at a spindle pole.  In animal cells, a pair of centrioles at right angles to each other is found within each centrosome. This feature is not found in other species, and is not required for spindle formation.  Each centrosome organizes from microtubules. These microtubules are produced from the rapid polymerization of tubulin proteins (50) 3 types of microtubules in the mitotic spindle: (50-51) Aster microtubules: Emanate outward from centrosome. Important for the positioning of the spindle apparatus within cell, and later during cell division. Polar microtubules: Project toward region where the chromosomes will be found during mitosis (between 2 spindle poles). Ones that overlap play a role in separation of 2 poles. Kinetochore microtubules: Attach to kinetochore, form 3 layers. Inner plate makes direct contact with centromeric DNA, outer plate connects kinetochore microtubules, middle plate connects the inner and outer plates.  Spindle allows cells to organize and separate chromosomes so that daughter cells recieves same compliment of chromosomes (51) Transmission of Chromosomes During Division Requires Mitosis Interphase: The chromosomes are decondensed right before mitosis begins. Steps of mitosis (in order): Prophase: At this point, the chromosomes have already replicated to create chromatids. The nuclear membrane begins to dissociate into small vesicles, the chromatids condense, mitotic spindle begins to form, nucleolus disappears (53).  Metaphase: Spindle fibers interact with sister chromatids. The mitotic spindle forms. Sister chromatids align themselves along the metaphase plate. At this point, each pair of chromatids is attached to both poles by kinetochore microtubules, and is aligned into a single row. When this is finished, the chromatids can be equally distributed into 2 daughters.  Anaphase: Connection holding the pairs of chromatids together is broken. Each chromatid is an individual chromosome, and they migrate toward the pole to which they are attached due to shortening of kinetochore microtubules. The polar microtubules also elongates and 2 poles move further apart. By the end of this, each individual chromosome is linked to 1 of the 2 poles. Telophase: The chromosomes have reached their respective poles and are now decondensing. Nuclear membrane reforms to produce 2 separate nuclei. Cytokinesis': 2 nuclei are segregated into separate daughter cells. Cell organelles also segregate into the daughter cells.  In animal cells, a contractile ring composed of myosin motor proteins and actin filaments assembles adjacent to the plasma membrane. Myosin hydrolyzes ATP, the ring shortens, plasma membrane constricts to form a cleavage furrow. This continues until the cells are separated  In plants, the daughter cells are separated by a cell plate.  Mitosis and cytokinesis ultimately produce 2 daughter cells having the same number of chromosomes as mother cell. The daughters are genetically identical to each other. The critical consequence of this process is to ensure genetic consistenceny from one somatic cell to the next (54).

Meiosis I

During sexual reproduction, gametes are made that contain 1/2 the amount of genetic material (54).  The gametes fuse in fertilization. The process by which they are form is called gametogenesis (54). Gametes are typically haploid, meaning that they contain 1/2 the number of chromosomes that diploid cells contain . Compared to a diploid, the haploid contains a single set of chromosomes, giving them 23 chromosomes exactly (54). During meiosis, haploid cells are produced from a cell that was originally diploid (54). Meiosis Producing Haploid Cells The process begins  after a cell has progressed through the G1, S, and G2 phases of the cell cycle. There are then 2 successive divisions (54).  Prior to meiosis, the chromosomes are replicated in S phase  to produce pairs of sister chromatids. Then meiosis I and II occurs in a sequence just like mitosis.  The prophase of meiosis I has its own 5 phases: lepotene (replication), zygotene (homologues recognize each other and align) pachytene (syanpsis is complete), diplotena (synapses dissociate), and diakinesis (End of prophase) Leptotene: Replicated chromosomes begin to condense and become visible with a light microscope. Each structure is actually a pair of chromatids (54). Zygotene: Recognition process called synapsis, in which the homologous chromosomes recognize each other and then align themselves along their entire lengths (54). In most eukaryotes, a synaptonemal complex is formed between homologues. This complex is composed of parallel lateral elements bound to the chromosomal DNA and a central element that promotes binding of the lateral elements to each other via transverse filaments, This is not a required complex. It's unknown what its use is, but it may help maintain homologus pairing in situations where the normal process has failed. Either that or it may play a role in meiotic chromosome structure, or it may serve to regulate the process of crossing over (55-56) Diplotene: Synaptonemal complex has largely disappeared. Bivalent pulls apart slightly (56). A bivalent is also called a tetrad because it's composed of 4 chromatids. Diakinesis: Synaptonemal complex completely disappears (56). Before pachytene occurs crossing over happens. Crossing over is critical for proper segregation of chromosmes (56). The connection that results from crossing over is called a chiasma.  After prophase, the rest of meiosis I mostly continues as.... Normal. Here are some differences : (56). In metaphase, the bivalents are aligned along the metaphase plate in a double row, and the arrangement of the sister chromatids are random.  In metaphase, kinetochores are attached to the sister chromatids; one pair is of sister chromatids is linked to one of the poles and the homologous pair is linked to the opposite pole.  In anaphase, the 2 pairs of sister chromatids within a bivalent separate from each other. The connectionn holding these chromatids, however, is not broken. Instead, each joined pair of chromatids migrates to 1 pole, and the homologous pair of chromatids move to the opposite.  Meiosis can yield many different results acctoss different cells. This is consistent with Mendel's Law of Independent Assorrtment (56).  The sister chromatids can be randomly aligned along the metaphase plate in many possible ways (56). Because the homologues are very similar, but not idential, the random alignment allows for a vast amount of genetic diversity.  The end result of meiosis I is 2 cells, each with 3 pairs of sister chromatids. This is different from the diploid cell that made it; the original had its chromosomes in homologous pairs. (56).  

Meiosis II

We begin with two cells that have 1/2 the normal amount of chromosomes, joined as sister chromatids (56) In the example, it's 6 chromatids joined as 3 pairs of sister chromatids. In humans, it would probably be 23 chromosomes, joined as... 11 pairs of sister chromatids? Because one is a sex cell? I dunno, don't listen to me.  Big differences between end result of mitosis and end result of meiosis: (56) In meiosis, we end up with 4 daughter cells that contain 1/2 the normal amount of chromosomes. In mitosis, we end up with 2 daughter cells that contain the diploid amount of chromosomes (23 versus 46 in humans). In meiosis, the haplod cells produced are not genetically identical because they contain only 1 homologous chromosome from each parent. In mitosis, the daughter cells are identical.

Chromosome Theory of Inheritance

Chromosome Theory of Inheritance: ( 61) States that the inheritance patterns of traits can be explained by the transmission patterns of chromosomes during gametogenesis and fertilization.  Fundamental principals: Chromosomes contain the genetic material that is transmitted from parent to offspring. Chromosomes are replicated and passed along. This includes from parent to child and from cell to cell during development. Each type of chromosome retains its individuality. The nuclei of most eukaryotic cells contain chromosomes that are found in homologous pairs. One member of each pair is inherited from the mother, the other is inherited from the father. At meiosis, one of the members of each pair segregates into the daughter nucleus, and the homologues segregate into the other daughter nucleus.  During formation of haploids, different types of nonhomologous chromosomes segregate independently of each other. Each parent contributes 1 set of chromosomes, and the 2 sets are functionally equivalent. Mendel's law can be explained by the homologous pairing and segregation of chromosomes during meiosis (61-62) In essence, what is explained is why we get 1 copy of each allele, from each parent, and why traits are dominant or recessive, why you can carry a recessive trait but not show it, etc, etc (62). 

X-Linked Traits

The results of a study that Thomas Hunt Morgan did on fruit flies told us that for some traits, the pattern of transmission from parent to offspring depends on the sex of the offspring and the alleles that they carry (65).  The trait studied was eye color. Genes that are physically located on the X chromosome are called X-linked genes/alleles (65).  A testcross is when an individual with a dominant phenotype and unknown genotype is crossed with an individual with a recessive phenotype (65).