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Chromosomes are the nuclear components of the special organization, individuality, and function that are capable of self-reproduction.
By the end of this lesson, students will demonstrate how DNA is condensed (supercoiled) into structures called chromosomes by building a quick and simple chromosome model. This activity is great for teaching students about the levels of organization w/in genetic material. Using the model they built...
If the DNA from all 46 chromosomes in a human cell nucleus was laid out end to end, it would measure approximately two meters; however, its diameter would be only 2 nm. Considering that the size of a typical human cell is about 10 µm (100,000 cells lined up to equal one meter), DNA must be tightly packaged to fit in the cell’s nucleus. At the same time, it must also be readily accessible for the genes to be expressed. During some stages of the cell cycle, the long strands of DNA are condensed into compact chromosomes. There are a number of ways that chromosomes are compacted. In the first level of compaction, short stretches of the DNA double helix wrap around a core of eight histone proteins at regular intervals along the entire length of the chromosome (Figure 1). The DNA-histone complex is called chromatin. The beadlike, histone DNA complex is called a nucleosome, and DNA connecting the nucleosomes is called linker DNA. A DNA molecule in this form is about seven times shorter than the double helix without the histones, and the beads are about 10 nm in diameter, in contrast with the 2-nm diameter of a DNA double helix. The next level of compaction occurs as the nucleosomes and the linker DNA between them are coiled into a 30-nm chromatin fiber. This coiling further shortens the chromosome so that it is now about 50 times shorter than the extended form. In the third level of packing, a variety of fibrous proteins is used to pack the chromatin. These fibrous proteins also ensure that each chromosome in a non-dividing cell occupies a particular area of the nucleus that does not overlap with that of any other chromosome (see the top image in GENOMIC DNA). Figure 1. Double-stranded DNA wraps around histone proteins to form nucleosomes that have the appearance of “beads on a string.” The nucleosomes are coiled into a 30-nm chromatin fiber. When a cell undergoes mitosis, the chromosomes condense even further. DNA replicates in the S phase of interphase. After replication, the chromosomes are composed of two linked sister chromatids. When fully compact, the pairs of identically packed chromosomes are bound to each other by cohesin proteins. The connection between the sister chromatids is closest in a region called the centromere. The conjoined sister chromatids, with a diameter of about 1 µm, are visible under a light microscope. The centromeric region is highly condensed and thus will appear as a constricted area.
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DNA is found in the nucleus. It carries the genetic information in all eukaryotes. How is DNA organized? -its basic structure is the double helix -this is then wound around proteins (called histones) to form chromatin. Under an electron microscope, it looks like beads on a chain. This is the form that DNA is stored in between cell divisions -during cell division the DNA winds up more tightly and the chromatin coils on itself, looping and coiling to form thick rods called chromosomes, which are visible under the light microscope Image from: http://themedicalbiochemistrypage.org/dna.html What happens? DNA is copied when it is uncondensed, then it condenses into chromosomes that have 2 halves (each a copy of the other). Each half is called a chromatid. Sister chromatids are identical. The point at which the DNA narrows and the chromatids are connected is called the centromere. Each chromosome has many genes, each gene defines a single characteristic. The number and shape of chromosomes are species-specific. eg. Humans = 46 chromosomes, dogs = 78, pea = 14, fruit fly = 8 All sexually reproducing organisms have 2 sets of chromosomes, one from each parent (this is the diploid state). In humans a diploid cell has 46 chromosomes, half from the mother and half from the father (23). The chromosomes which carry the same kind of information are called homologous chromosomes. Cell division There are 2 types: - mitosis (számtartó sejtosztodás): purpose is growth and repair, 2 identical daughter cells are produced - meiosis (számfelező sejtosztodás): purpose is to produce gametes (sex cells) for reproduction, 4 genetically different cells are produced The cell cycle describes the typical cycle of a somatic (body) cell that will go through mitosis: Image from: http://www.cdli.ca/courses/biol3201/unit02/unit02_org01_ilo02/b_activity.html During the first growth phase, the cell simply grows and carries out its normal functions. At a certain point, the cell enters the synthesis phase, where the DNA is replicated. DNA replication refers to the creation of another DNA double helix using the first helix as a template. In order for this to occur: 1. The DNA double helix begins to unwind or unzip at one end to form a replication fork. Unwinding requires the help of an enzyme called a helicase. 2. Enzymes called DNA polymerases bind to the single strands of DNA. They then proceed to "read" the template strand (in the 5' to 3' direction) and add complementary nucleotides. Since the polymerase only travels in one direction, it will move more quickly along the leading strand, but on the lagging strand it will attach at the fork and move toward the end, until it meets up with a previously formed DNA strand fragment, then it will detach and reattach at the continuously unwinding replication fork. The fragments that are created in this way are called Okazaki fragments. They are "glued" together with the help of enzymes called ligases. The end result is two semi-conservative daughter double helixes- meaning that each double helix contains one strand from the original and one strand that is new. If you want to see a video: http://www.youtube.com/watch?v=teV62zrm2P0 Once DNA replication has occured, the nucleus then has 2 copies of all of its DNA and will continue to grow and carry out some normal functions, but it will also prepare for cell division, which is either mitosis or meiosis, depending on whether or not it is a cell that will simply copy itself, or a cell that is designed to produce gametes (eggs or sperm). Mitosis is divided into 4 phases: Prophase: -chromatin condenses to chromosome -nuclear envelope disintegrates and disappears -spindle (magorsó) forms Metaphase: -chromosomes line up at the equator Anaphase: -chromatids are pulled to opposite poles of the cell Telophase: -cell plasma divides -nuclear envelope reappears (don't worry about the extra stages in the image below!!) Image from: https://www.msu.edu/~robiemat/science.htm Image from : http://imcurious.wikispaces.com/Midterm+Exam+2010+Review+P1 Meiosis occurs to produce haploid cells that will be gametes (sperm and eggs). It is a division that reduces the chromosome number by half. It is divided into meiosis I and meiosis II Meiosis I Prophase I -chromatin condenses to chromosomes -chromosomes "find" their homologous pairs and crossing over occurs Metaphase I --nuclear membrane disappears -homologous chromosomes line up at the equator and attach to spindle fibres Anaphase I - chromosomes pairs are split as they are pulled to opposite poles Telophase I - cell plasma divides - nuclear membrane reforms Short interphase, with no DNA replication Meiosis II Prophase II -chromosomes condense - nuclear membrane disappears -spindle forms Metaphase II -chromosomes line up at the equator Anaphase II -chromatids are pulled to opposite poles of the cell Telophase II -cell plasma divides -nuclear membrane forms Image from: http://commons.wikimedia.org/wiki/File:Meiosis_diagram.jpg So mitosis and meiosis share some characteristics, but are also unique in many ways. The following diagram presents a comparison of the two. Be sure to consider how they are similar and how they are different. Image from: http://bioactive.mrkirkscience.com/09/ch9summary.html
Chromosomes, chromatids, and chromosome arms are some of the most challenging concepts for students when they study cell division. Use this coloring page to help students see the difference. This resource is unique because it comes with a reading, questions, and a coloring page that students label and color. Concepts/terms taught in this resource: Chromatids Chromosomes Chromosome Arms Nucleosomes Chromatin Histones If you like this resource, you may like this cell division unit bundle of diagrams, cut-and-paste activities, and readings/questions! Do you like this resource and want to check out other coloring diagrams and activities I have created? Check these out! Photosynthesis and Cell Respiration Coloring and Cut-and-Paste Activities Cell Transport Coloring Diagrams Cell and Organelle Coloring Diagrams and Questions Protein Synthesis: Replication, Transcription, and Translation Activities Contact Us! If you have any questions or concerns, please reach out to us on the question and answer section of my store and we will get back to you quickly! Terms of Use: Purchasing my teaching resources allows you to: * make copies for your own classes only. * place this file on your own password-protected class page or server (Blackboard, Google Drive, etc) AS LONG AS no other teacher has access to that class webpage. This resource is for you, the purchaser, alone. You are not allowed to distribute this digital resource to other teachers or post this resource on any webpage or server that is available for public view. If you and a team of teachers would like to use this resource together, please purchase additional licenses on the resource purchase page. Failure to comply with these terms of use is a copyright infringement and a violation of the Digital Millennium Copyright Act (DMCA). Clipart and elements found in this PDF are copyrighted and cannot be extracted and used outside of this file without permission or license. All files are non-editable PDFs. They are non-editable to protect the images that are copyrighted and purchased through licenses. Thanks for understanding! © Bethany Lau All Rights Reserved.
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The human nature called compassion is innate or implanted in the living substance, living matter or protoplasm.
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