Introduction to Genetics and Cellular Reproduction PDF

Chapter 2 INTRODUCTION TO GENETICS AND CELLULAR REPRODUCTION At the end of this chapter, pre-service teachers are expected to: 1. Explain what chromosomes are; 2. Recall the cell cycle and the regulation mechanisms; 3. Illustrate and differentiate mitosis and meiosis, and discu...

Chapter 2 INTRODUCTION TO GENETICS AND CELLULAR REPRODUCTION At the end of this chapter, pre-service teachers are expected to: 1. Explain what chromosomes are; 2. Recall the cell cycle and the regulation mechanisms; 3. Illustrate and differentiate mitosis and meiosis, and discuss what happens in each phase of the two types of cell division; Introduction When you look at your parents, you can see features that you share with them. The sharing of features can be explained by heredity, where traits are passed on from parents to offspring. Yet when you look at your brothers and sisters, even if you share the same parents, each one of you can be considered unique based on the combination of traits each possesses. That is variation, which demonstrates differences among individuals. Genetics is the study of heredity and variation. It aims to understand how traits can be passed on to the next generation and how variation arises. Every living thing undergoes reproduction. The nutrients taken by an individual will provide for energy for metabolic processes, for growth and development as well as reproduction. The cellular level of reproduction, in the form of cell division, provides for the backdrop for the organismal level of reproduction. The Chromosome All living things contain what we call the genetic material at serves as the set of instructions that direct the activities and functions of the cells. These genetic materials, also known as the deoxyribonucleic acid or DNA, are passed on from one generation to the next to ensure the continuity of life. In eukaryotic cells (cells with organelles), the DNA are bound with proteins and are organized as beads on strings to form chromosomes.  The number of chromosomes in a cell is characteristic of the species to which it belongs. For example, humans have 46 chromosomes while rice have 24. The table shown at the right summarizes the chromosome numbers of some common organisms. The Cell Cycle The chromosomes of a cell change form as the cell transitions from one stage to another in a typical cell cycle. The cell cycle may be divided into two stages: the interphase where the chromosomes are long and extended and are also referred to as chromatin, and the cell division phase where the chromosomes become condensed or thickened. The Cell Cycle is an ordered set of events, culminating in cell growth and division into two daughter cells. Non-dividing cells not considered to be in the cell cycle. A eukaryotic cell cannot divide into two, the two into four, etc. unless two processes alternate: 1. doubling of its genome (or DNA) in S phase (synthesis phase) of the cell cycle; and 2. halving of that genome during mitosis (M phase). The four (4) stages of the cell cycle are: 1. G1 stage stands for "GAP 1". During this time, there is growth and preparation of the chromosomes for replication 2. S stage stands for "Synthesis". This is the stage when DNA replication occurs. 3. G2 stage stands for "GAP 2". During this time, there is preparation for mitosis. 4. M stage stands for "mitosis", and is when nuclear (chromosomes separate) and cytoplasmic (cytokinesis) division occur. Mitosis is further divided into 4 phases. Regulation of the Cell Cycle How cell division (and thus tissue growth) is controlled is very complex. The following terms are some of the features that are important in regulation, and places where errors can lead to cancer. Cancer is a disease where regulation of the cell cycle goes awry and normal cell growth and behavior is lost. a. Cdk (cyclin dependent kinase, adds phosphate to a protein), along with cyclins, are major control switches for the cell cycle, causing the cell to move from G1 to S or G2 to M. b. MPF (Maturation Promoting Factor) includes the CdK and cyclins that triggers progression through the cell cycle. c. p53 is a protein that functions to block the cell cycle if the DNA is damaged. If the damage is severe this protein can cause apoptosis (cell death). 1. p53 levels are increased in damaged cells. This allows time to repair DNA by blocking the cell cycle. 2. A p53 mutation is the most frequent mutation leading to cancer. An extreme case of this is Li Fraumeni syndrome, where a genetic defect in p53 leads to a high frequency of cancer in affected individuals. d. p27 is a protein that binds to cyclin and cdk, blocking entry into S phase. Recent research (Nature Medicine 3, 152 (1997)) suggests that breast cancer prognosis is determined by p27 levels. Reduced levels of p27 predict a poor outcome for breast cancer patients. Cell Division: Mitosis and Meiosis Mitosis is nuclear division plus cytokinesis, and produces two identical daughter cells during several stages. Interphase is often included in discussions of mitosis, but interphase is technically not part of mitosis, but rather encompasses stages G1, S, and G2 of the cell cycle. Mitosis takes place among somatic or body cells. In actively dividing animal cells, the whole process takes about an hour. 1. Prophase. Prophase occupies over half of mitosis. The nuclear membrane breaks down to form a number of small vesicles and the nucleolus disintegrates. A structure known as the centrosome duplicates itself to form two daughter centrosomes that migrate to opposite ends of the cell. The centrosomes organize the production of microtubules that form the spindle fibers that constitute the mitotic spindle. The chromosomes condense into compact structures. Each replicated chromosome can now be seen to consist of two identical chromatids (or sister chromatids) held together by a structure known as the centromere. The centromere may divide the chromosome into the shorter arms, also called the p arms (‘p’ stands for petite in French) and the longer q arms. If the chromosomes are stained using Giemsa, alternating dark and light regions will appear. These are the heterochromatin and euchromatin, respectively. The heterochromatin are more coiled and dense than the euchromatin. 2. Prometaphase. The chromosomes, led by their centromeres, migrate to the equatorial plane in the mid-line of the cell - at right-angles to the axis formed by the centrosomes. This region of the mitotic spindle is known as the metaphase plate. The spindle fibers bind to a structure associated with the centromere of each chromosome called a kinetochore. Individual spindle fibers bind to a kinetochore structure on each side of the centromere. The chromosomes continue to condense. 3. Metaphase. The chromosomes align themselves along the metaphase plate of the spindle apparatus. 4. Anaphase. The shortest stage of mitosis. The centromeres divide, and the sister chromatids of each chromosome are pulled apart - or 'disjoin' - and move to the opposite ends of the cell, pulled by spindle fibers attached to the kinetochore regions. The separated sister chromatids are now referred to as daughter chromosomes. (It is the alignment and separation in metaphase and anaphase that is important in ensuring that each daughter cell receives a copy of every chromosome.) 5. Telophase. The final stage of mitosis, and a reversal of many of the processes observed during prophase. The nuclear membrane reforms around the  chromosomes grouped at either pole of the cell, the chromosomes uncoil and become diffuse, and the spindle fibers disappear. Cytokinesis serves as the final cellular division to form two new cells. In plants a cell plate forms along the line of the metaphase plate; in animals there is a constriction of the cytoplasm. The cell then enters interphase - the interval between mitotic divisions. Meiosis 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). Through meiosis, gametogenesis or the production of sperm (spermatogenesis) and eggs (oogenesis) takes place. It should be taken into consideration that meiosis is not a cycle. Meiosis I Meiosis I has two main purposes: 1. It serves as the reduction division, where it reduces the number of chromosomes in half, making the daughter cells haploid (when the parent cell was diploid); and 2. It is during this stage that most of the genetic recombination occurs through the process of crossing over. 1. Prophase I – this phase has the following sub-stages: Leptotene. Each chromosome is made up of two long threads of sister chromatids as a result of replication during the S phase of the cell cycle. Zygotene. The chromosomes begin to pair off. Pairs of chromosomes are called homologous chromosomes, and this pairing process is exact. Pachytene. The chromosomes contract due to repeated coiling. Crossing over takes place during this stage where a segment of a sister chromatid of one chromosome is exchanged with the same segment of the sister chromatid of the homologous chromosome through the formation of a cross-linkage of the segments called a chiasma. After crossing over, the sister chromatids of each chromosome may no longer be identical with each other based on the genetic material they contain. Diplotene. The chromosomes begin to uncoil.  Diakinesis. The paired chromosomes disperse in the nucleus. 2. Metaphase I – tetrads line up at the equator and the spindle has completely formed. 3. Anaphase I – spindle fibers form and attach to the centromeres of the chromosomes. The homologous chromosomes separate from each other completely and start their movement toward the poles of the cells as they are pulled by the spindle fibers. As the centromere of each chromosome does not divide, the sister chromatids remain together. 4. Telophase I - Chromosomes with two chromatids decondense and a nuclear envelope reforms around them. Each nucleus is now haploid. Meiosis II Meiosis II takes place in order to reduce the amount of DNA back to normal, that is, to split the chromosomes so that each daughter cell has only one chromatid per chromosome. Phases of meiosis II: 1. Prophase II Chromosomes with two chromatids become visible as they condense (and the nuclear envelope and nucleus disappear, and the spindle in forming). 2. Metaphase II Chromosomes with two chromatids line up at the equator. The spindle is fully formed. 3. Anaphase II Chromosomes split, so that a chromosome with only one chromatid heads toward each pole. 4. Telophase II Chromosomes with only one chromatid decondense and get surrounded by new nuclear envelopes. The four daughter cells are now all haploid and have the right amount of DNA. They are ready to develop into sperm or eggs now. * * * END * * *