Chapter Two Lecture Objectives
Stages of the Cell Cycle.
1. Interphase 2. Prophase 3. Prometaphase 4. Metaphase 5. Anaphase 6. Telophase 1. Interphase: This has DNA synthesis. G1, S, G2 occurs. You cannot see chromosomes with a light microscope. It has G1, where the cell grows physically large, copies organelles, and constructs molecular building. It has S phase, where the cell synthesized a complete copy of the DNA within the nucleus and duplicates the centrosomes. G2 phase, where the cell grows large, makes proteins and organelles, and begins to prepare for mitosis. It ends when mitosis begins. 2. Prophase: The chromosome condense. DNA was already replicated. The mitosis spindle forms from centromeres for animals only. 3. Prometaphase: The nuclear envelope disappears. The microtubules contact chromatids. 4. Metaphase: The chromosomes arrange within single place = Metaphase plate. 5. Anaphase: The sister chromatids move toward opposites poles. After separation chromatids, chromosomes. 6. Telophase: Chromosomes arrive at spindle pores. The nuclear envelope reforms, which equal 2 nuclei. The chromosomes disappear from view. What regulates the cell cycle? The Cyclin Dependent Kinase (CDK). MPF = Cyclin B + CDK. It occurs within 16 hours!
Types of chromosomes in Eukaryotes (Centromere Position).
1. Metacentric Chromosomes: Metacentric chromosomes have the centromere in the center, such that both sections are of equal length. Human chromosome 1 and 3 are metacentric. 2. Submetacentric Chromosomes: Submetacentric chromosomes have the centromere slightly offset from the center leading to a slight asymmetry in the length of the two sections. Human chromosomes 4 through 12 are submetacentric. 3. Acrocentric Chromosomes: Acrocentric chromosomes have a centromere which is severely offset from the center leading to one very long and one very short section. Human chromosomes 13,15, 21, and 22 are acrocentric. 4. Telocentric Chromosomes: Telocentric chromosomes have the centromere at the very end of the chromosome. Humans do not possess telocentric chromosomes but they are found in other species such as mice.
Bacterial Binary Fission - Basics about how chromosome separates.
Bacterial Binary Fission: The process that bacteria uses to carry out cell division. Binary fission is similar in concept to the mitosis that happens in eukaryotic organisms (such as plants and animals), but its purpose is different. When cells divide by mitosis in the body of a multicellular organism, they cause the organism to grow larger or replace old, worn-out cells with new ones. In the case of a bacterium, however, cell division isn't just a means of making more cells for the body. Instead, it's actually how bacteria reproduce, or add more bacteria to the population. Binary fission has features in common with mitosis, but also differs from mitosis in some important ways. Like a human cell, a dividing bacterium needs to copy its DNA. Unlike human cells, which have multiple linear (rod-like) chromosomes enclosed in a membrane-bound nucleus, bacterial cells usually have a single, circular chromosome and always lack a nucleus. However, the bacterial chromosome is found in a specialized region of the cell called the nucleoid. Copying of DNA by replication enzymes begins at a spot on the chromosome called the origin of replication. The origin is the first part of the DNA to be copied. As replication continues, the two origins move towards opposite ends of the cell, pulling the rest of the chromosome along with them. The cell also gets longer, adding to the separation of the newly forming chromosomes. Replication continues until the entire chromosome is copied and the replication enzymes meet at the far side. Once the new chromosomes have moved to opposite cell ends and cleared the center of the cell, division of the cytoplasm can take place. In this process, the membrane pinches inward and a septum, or new dividing wall, forms down the middle of the cell. (Bacteria have a cell wall, so they must regenerate this wall when they undergo cell division.) Finally, the septum itself splits down the middle, and the two cells are released to continue their lives as individual bacteria.
What are the advantages of diploid state?
Chromosomes exist within homologous pairs within diploid organisms. 1) Less stochastic noise in genes expression. 2) Back-up allele of a gene in case one becomes non-functional (mutation, methylation, disruption of frame by a retrovirus). 3) Additional load of genetic information that can be transmitted from one generation to another. This is advantageous in case external conditions are modified every couple of generation in a way that a normally less fit allele becomes more fit (think a mutation that allows to better survive drought but that makes an organism grow slower in times of abundance).
Crossing over and segregation of homologous chromosomes - When during meiosis they occur and the impact these processes have on heredity?
Crossing over: The meiotic event referred to as crossing over results with genetic exchange between member of each homologous pair of chromosomes. This process creates intact chromosomes that are mosaics of the maternal and paternal homologs from which they arise, further enhancing the potential genetic variation within gametes and the production of offspring. During meiosis, crossing over occurs during prophase I. It is the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes, which contribute to genetic diversity. Following recombination, chromosome segregation occurs as indicated by the stages metaphase I and anaphase I in the meiosis diagram. Different pairs of chromosomes segregate independently of each other, a process termed "independent assortment of non-homologous chromosomes."
Major differences between prokaryotes and eukaryotes.
Eukaryotes: The presence of a nucleus and other membranous organelles will be the defining characteristic of eukaryotic organisms. Prokaryotes: The absence of a nucleus, nuclear envelope, or membranous organelles will be the defining characteristics of prokaryotic organisms. SUMMARY OF DIFFERENCES: Eukaryotes: Cells present Prokaryotes: Cells absense Eukaryotes: 5 - 100 microns Prokaryotes: 1 - 10 microns Eukaryotes: Multiple linear DNA Prokaryotes: One circular DNA Eukaryotes: Large DNA Prokaryotes: Small DNA Eukaryotes: Present membrane bound organelles Prokaryotes: Absent membrane bound organelles
Male and female gametogenesis.
Gametogenesis: Another name for the biological process of meiosis. It produces sperm cells in males and ovum, or eggs, cells in females. Gametogenesis: Spermatogenesis in male mammals, and oogenesis in female mammals.
What cells undergo meiosis and what is the purpose of this process and its consequences in the genetic composition?
Germ cells undergo meiosis. Germ cells are found in the male and female gonads and form sperm and eggs. Meiosis produces haploid gametes, which contain only one set of chromosomes, instead of diploid somatic cells, which contain two.
Meiosis in Eukaryotic cells - Stages and events in each stage.
Meiosis is the form of eukaryotic cell division that produces haploid sex cells or gametes (which contain a single copy of each chromosome) from diploid cells (which contain two copies of each chromosome). The process takes the form of one DNA replication followed by two successive nuclear and cellular divisions (Meiosis I and Meiosis II). As in mitosis, meiosis is preceded by a process of DNA replication that converts each chromosome into two sister chromatids. 1. Meiosis I separates the pairs of homologous chromosomes. In Meiosis I a special cell division reduces the cell from diploid to haploid. 2. Prophase I The homologous chromosomes pair and exchange DNA to form recombinant chromosomes. Prophase I is divided into five phases: A. Leptotene: chromosomes start to condense. B. Zygotene: homologous chromosomes become closely associated (synapsis) to form pairs of chromosomes (bivalents) consisting of four chromatids (tetrads). C. Pachytene: crossing over between pairs of homologous chromosomes to form chiasmata (sing. chiasma). D. Diplotene: homologous chromosomes start to separate but remain attached by chiasmata. E. Diakinesis: homologous chromosomes continue to separate, and chiasmata move to the ends of the chromosomes. Prometaphase I Spindle apparatus formed, and chromosomes attached to spindle fibres by kinetochores. Metaphase I: Homologous pairs of chromosomes (bivalents) arranged as a double row along the metaphase plate. The arrangement of the paired chromosomes with respect to the poles of the spindle apparatus is random along the metaphase plate. (This is a source of genetic variation through random assortment, as the paternal and maternal chromosomes in a homologous pair are similar but not identical. The number of possible arrangements is 2n, where n is the number of chromosomes in a haploid set. Human beings have 23 different chromosomes, so the number of possible combinations is 223, which is over 8 million.) Anaphase I: The homologous chromosomes in each bivalent are separated and move to the opposite poles of the cell Telophase I: The chromosomes become diffuse and the nuclear membrane reforms. Cytokinesis: The final cellular division to form two new cells, followed by Meiosis II. Meiosis I is a reduction division: the original diploid cell had two copies of each chromosome; the newly formed haploid cells have one copy of each chromosome. Meiosis II separates each chromosome into two chromatids. Meiosis generates genetic diversity through: The exchange of genetic material between homologous chromosomes during Meiosis I the random alignment of maternal and paternal chromosomes in Meiosis I the random alignment of the sister chromatids at Meiosis II
Comparison of mitosis and meiosis.
Mitosis: 1. Single Nuclear Division 2. Results with the same number of chromosomes. 3. Yields genetically identical cells. Meiosis: 1. Two divisions 2. Newly formed cells has 1/2 number of starting chromosomes. 3. Genetically variable cells. Mitosis: Mitosis provides a mechanism by which chromosomes, having been duplicated, are distributed into progeny cells during cell reproduction. Mitosis converts a diploid cell into two diploid daughter cells. Meiosis: Meiosis distributes one member of each homologous pair of chromosomes into each gamete or sport, which reduces the diploid chromosome number to the haploid chromosome number. Meiosis generates genetic variability by distributing various combinations of maternal and paternal members of each homologous pair of chromosomes into gametes or spores. During the stage of mitosis and meiosis: The genetic material will be condense into discrete structures called chromosomes. Comparison: Mitosis leads to the production of two cells, each with the same number of chromosomes as the parent cell. Meiosis leads to the reduction of genetic content and the number of chromosomes by precisely half. It's an essential reduction if sexual reproduction is to occur without doubling the amount of genetic material in each new generation. Meiosis produces sex cells (gametes or spores).
Concept of Ploidy.
PLOIDY: The number of sets of chromosomes in the nucleus of the cell. Polyploid: It's a term used to describe cells with three or more sets of chromosomes, which would be triploid or higher ploidy. Haploid: 1 Set Diploid: 2 Sets Triploid: 3 Sets Tetraploid: 4 Sets Pentaploid: 5 Sets And so on!
Mitosis in Eukaryotic cells - Stages and events in each stage!
The division cycle of most eukaryotic cells is divided into four discrete phases: M, G1, S, and G2. M phase (mitosis) is usually followed by cytokinesis. S phase is the period during which DNA replication occurs. The cell grows throughout interphase, which includes G1, S, and G2. The relative lengths of the cell cycle phases shown here are typical of rapidly replicating mammalian cells.
Homologous Chromosomes.
With the exception of sex chromosome, they exist within pairs with regard to these properties, and the members of each pair are called homologous chromosomes. For each chromosome exhibiting a specific length and centromere placement, another exists with identical features. In other words: The cell has two sets of each chromosome; one of the pair is derived from the mother and the other from the father. The maternal and paternal chromosomes in a homologous pair have the same genes at the same loci, but possibly different alleles. 46! There's an exception: In many species, one pair, consisting of the sex-determining chromosomes, is often not homologous within size, centromere placement, arm ratio, or genetic content. For example, women carry two homologous X chromosomes, males carry one Y chromosomes and one X chromosome.