Chromosomes.docx

CHROMOSOMES Terminology Chromosomes/chromatids As help file for the understanding of the webpages on the cell cycle a number of terms and facts related to chromosomes and chromatids are highlighted here:

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Chromosomes and chromatids occur as rod or threadshaped structures in the nucleus of eukaryotic cells. Prokaryotes usually have only a single circular chromosome (in the webseries we will not focus on this group). The namechromosome (from the Greek chroma = color and soma = body) originates from the fact that chromosomes can be observed as stainable bodies in a light microscope during cell division.

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Chromosomes and chromatids contain chromatin, which mainly consists of extremely long stands of DNAmaterial (Deoxyribonucleic acid) that functions as carrier of genes and regulatory elements. Besides, chromatin contains Histones (chromosome proteins) and other proteins involved in the packaging of the DNA strands during condensation at cell division (see figure E here below) and small

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quantities of RNA. Sister chromatids (with -tid at the end), are two identical (= exactly the same) parts (Chromatids) arising from replication of a chromosome. (In the figure here next the sister chromatids A1 and A2 are an exact copy of each other, as well as the set B1 and B2, whereas homologs A's and B's show small differences). These two parts retain the denomination chromatids as long as they are bound together through the centromere, which is for example the case during the entire S phase following duplication of DNA (replication). This connection is vible as a constriction during mitosis or meiosis. During anaphase in mitosis and anaphase II in meiosis the two sister chroamtids are pulled apart at the centromer. According to current definitions, each single chromatid is regarded as a own chromosome after separation of the chromatids during cell division. In most organisms chromosomes occur in pairs, the so-called homolog chromosomes (homolog = similar/corresponding). In contrast to the sister chromatides the homolog chromosomes consist of two (slightly) different copies of the same chromosome; homolog chromosomes carry yet the same genes, but the two copies of each allel can be either identicalor different of each other. A single chromosome contains only on single long unbranched double-stranded DNA molecule that displays the typical double-helix structure. This double-strands DNA is formed by one phosphate group alternating with one desoxyribose group coupled to each other by nucleic acids (adenine, guanine, thymine en cytosine). These nucleic acids form consistent pairs (AT and CG). Of each complementary strands of the DNA molecule the antisense can be read (used as a template) for the synthesis of proteins, but not thesense. The ploidy refers to the number of different copies of each chromosome present in a cell. Most plants and animals are diploid, indicated by 2n, which means that there are twocopies of each chromosome per cell. Their gametes, however, are haploid, indicated by n (one single copy). Bacteria and some plants and fungi are haploid organisms. The number of chromatids or chromosomes coding for the same (corresponding) genes within a cell, is sometimes indicated by the small letter c. For example, a cell that was 2c before replication, will become 4 c after replication, thus when the DNA has been doubled in preparation of mitosis, because four samples of DNA stands coding for the same genes are present, but the ploidy will remain unchanged: if the cell was 2n, it is still 2n after replication and it was 4n it remains 4n. Besides hosting the genes that function as archive for genetic information, chromosomes also bear pieces of DNA between the genes which have a structural function. This is the case for the telomere and centromerethat are involved in replication and cell division. The centromere (centron = middle, meros = part) is the region of the chromosome where the chromatids that arise from replication are held together. The centromere hosts the kinetochore, a protein complex where the spindle filaments attach during mitosis or meiosis. Because the centromere remains relatively little spiralized during prophase and metaphase it can be distinguished as a primiry "pinch".

Nones, Monica Christianne M.

Stages of Mitosis

What is mitosis? Broadly speaking, mitosis simply refers to a type of cell division in which one cell (the mother) divides to produce two new cells (the daughters) that are genetically identical to itself. However, in the context of the cell cycle, mitosis also has a narrower definition: it refers to just one part of the overall division process, the part in which the DNA of the nucleus is split into two equal sets of chromosomes. The great majority of the cell divisions that happen in your body, or in the bodies of plants, animals, fungi, and other eukaryotic organisms, involve mitosis. During development and growth, mitosis populates an organism’s body with cells, and throughout an organism’s life, it replaces old, worn-out cells with new ones. For single-celled eukaryotes like yeast, mitotic divisions are actually a form of reproduction, adding new individuals to the population. In all of these cases, the “goal” of mitosis is to pass on genetic information as accurately as possible, so that each daughter cell gets a perfect, full set of chromosomes. Cells with too few or too many chromosomes usually don’t function well, and may not even be able to survive. So, when cells undergo mitosis, they don’t just divide their DNA at random and toss it into piles for the two daughter cells. Instead, they split up their duplicated chromosomes via a coordinated, elegant process that’s often compared to a dance. Here, we’ll look at the different phases of mitosis, which can be thought of as the steps of this dance.

Phases of mitosis Mitosis consists of four basic phases: prophase, metaphase, anaphase, and telophase. Some textbooks list five, breaking prophase into an early phase (simply called prophase) and a late phase (called prometaphase). These phases occur in strict sequential order, and cytokinesis - the process of dividing the cell contents to make two new cells - starts in anaphase or telophase.

Stages of mitosis: prophase, metaphase, anaphase, telophase. Cytokinesis typically overlaps with anaphase and/or telophase. You can remember the order of the phases with the famous mnemonic: [Please] Pee on the MAT. But don’t get too hung up on names – what’s most important to understand is what’s happening at each stage, and why it’s important for the division of the chromosomes. -Late G2 phase. The cell has two centrosomes, each with two centrioles, and the DNA has been copied. At this stage, the DNA is surrounded by an intact nuclear membrane, and the nucleolus is present in the nucleus. Let’s start by looking at a cell right before it begins mitosis. This cell is in interphase (late G_22start subscript, 2, end subscript phase) and has already copied its DNA, so the chromosomes in the nucleus each consist of two connected copies, called sister chromatids. You can’t see the chromosomes very clearly at this point, because they are still in their long, stringy, decondensed form. This animal cell has also made a copy of

its centrosome, an organelle that will play a key role in orchestrating mitosis, so there are two centrosomes. (Plant cells generally don’t have centrosomes with centrioles, but have a different type of microtubule organizing center that plays a similar role.)

-Early prophase. The mitotic spindle starts to form, the chromosomes start to condense, and the nucleolus disappears. In early prophase, the cell starts to break down some structures and build others up, setting the stage for division of the chromosomes.  

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The chromosomes start to condense (making them easier to pull apart later on). The mitotic spindle begins to form. The spindle is a structure made of microtubules, strong fibers that are part of the cell’s “skeleton.” Its job is to organize the chromosomes and move them around during mitosis. In prophase, the spindle grows between the centrosomes as they move apart. The nucleolus (or nucleoli, plural), a part of the nucleus where ribosomes are made, disappears. This is a sign that the nucleus is getting ready to break down.

-Late prophase (prometaphase). The nuclear envelope breaks down and the chromosomes are fully condensed. In late prophase (sometimes also called prometaphase), the mitotic spindle begins to capture and organize the chromosomes.   

The chromosomes finish condensing, so they are very compact. The nuclear envelope breaks down, releasing the chromosomes. The mitotic spindle grows more, and some of the microtubules start to “capture” chromosomes.

Anatomy of the mitotic spindle. Diagram indicating kinetochore microtubules (bound to kinetochores), non-kinetochore microtubules (not bound to kinetochores), and the aster. The aster is an array of microtubules that radiates out from the centrosome towards the cell edge. Diagram also indicates the centromere region of a chromosome, the narrow "waist" where the two sister chromatids are most tightly connected, and the kinetochore, a pad of proteins found at the centromere. Microtubules can bind to chromosomes at the kinetochore, a patch of protein found on the centromere of each sister chromatid. (Centromeres are the regions of DNA where the sister chromatids are most tightly connected, and they form the narrow "waist" of a duplicated chromosome). Microtubules that bind a chromosome are calledkinetochore microtubules. Microtubules that don’t bind to kinetochores (inventively called non-kinetochore microtubules) can overlap with and grab on to microtubules from the opposite pole, stabilizing the spindle. More microtubules radiate from each centrosome towards the edge of the cell, forming a structure called the aster(Greek for “star”).

Metaphase. Chromosomes line up at the metaphase plate, under tension from the mitotic spindle. The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles. In metaphase, the spindle has captured all the chromosomes and lined them up at the middle of the cell, ready to divide. 

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All the chromosomes align at the metaphase plate (not a physical structure, just a term for the plane where the chromosomes line up). At this stage, the two kinetochores of each chromosome should be attached to microtubules from opposite spindle poles. Before proceeding to anaphase, the cell will check to make sure that all the chromosomes are at the metaphase plate with their kinetochores correctly attached to microtubules. This is called the spindle checkpoint and helps ensure that the sister chromatids will split evenly between the two daughter cells when they separate in the next step. If a chromosome is not properly aligned or attached, the cell will halt division until the problem is fixed.

Anaphase. The sister chromatids separate from one another and are pulled towards opposite poles of the cell. The nonkinetochore microtubules push the two poles of the spindle apart, while the kinetochore microtubules pull the chromosomes towards the poles. In anaphase, the sister chromatids separate from each other and are pulled towards opposite ends of the cell. 

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Cohesin, the protein “glue” that holds the sister chromatids together, is broken down, allowing them to separate. Each is now considered its own chromosome. The microtubules attached to the chromosomes (kinetochore microtubules) get shorter, pulling chromosomes towards the poles. Microtubules not attached to chromosomes (non-kinetochore microtubules) elongate and push apart, separating the poles and making the cell longer. All of these processes are driven by motor proteins, molecular machines that can “walk” along microtubule tracks and carry a cargo. In mitosis, motor proteins carry chromosomes or other microtubules as they walk.

Telophase. The spindle disappears, a nuclear membrane reforms around each set of chromosomes, and a nucleolus reappears in each new nucleus. The chromosomes also start to decondense. In telophase, the cell is nearly finished dividing, and it starts to re-establish normal internal structures as cytokinesis takes place.  

The mitotic spindle is broken down into its building blocks. Two new nuclei form, one for each set of chromosomes. Nuclear membranes and nucleoli reappear.

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The chromosomes begin to decondense and return to their “stringy” form.

Cytokinesis in animal and plant cells. Cytokinesis in an animal cell: an actin ring around the middle of the cell pinches inward, creating an indentation called the cleavage furrow. Cytokinesis in a plant cell: the cell plate forms down the middle of the cell, creating a new wall that partitions it in two. Cytokinesis, the division of the cytoplasm to form two new cells, overlaps with the final stages of mitosis. It may start in either anaphase or telophase, depending on the cell, and finishes shortly after telophase. In animal cells, cytokinesis is contractile, pinching the cell in two like a coin purse with a drawstring. The “drawstring” is actually a band of filaments made of a protein called actin, and the pinch crease is known as thecleavage furrow. Plant cells, on the other hand, can’t be divided like this because they have a cell wall and are too stiff. Instead, a structure called the cell plate forms down the middle of the cell, splitting it into two daughter cells separated by a new wall.

When division is complete, it produces two daughter cells. Each daughter cell has a complete set of chromosomes, identical to that of its sister (and that of the mother cell). The daughter cells enter the cell cycle in G1. When cytokinesis finishes, we end up with two new cells, each with a complete set of chromosomes identical to those of the mother cell. The daughter cells can now begin their own cellular “lives,” and – depending on what they decide to be when they grow up – may undergo mitosis themselves, repeating the cycle.

Introduction to the three stages of interphase: The stages in a cell that is progressing towards cell division is called the cell cycle. The cell cycle is broadly divided into two phases: the interphase and the mitotic phase. Interphase is the phase during which the growth of the cells take place along with the metabolic activity but the nuclear division in the cell does not occur. The three stages included within the interphase are the G1, S and G2.

G1 Phase (the first Gap Phase):the first Stage of Interphase During the G1 phase or the Gap 1 phase, the protein synthesis and the RNA synthesis within the cell resumes that was interrupted during the process of mitosis. Growth and young cell maturation occurs, which accomplish the physiological function. G1 phase is the phase during which the cell cycle starts with the synthesis of RNA and proteins required by the young cells for their growth and maturity. The time period of the G1 phase of the interphase is varied highly among the different species’ eukaryotic cells. For example, faster renovation tissues like the mucosa and the endometrial epithelium require very short G1 periods as compared to the muscles or connective tissues that do not require frequent repair or renovation. G1 phase is usually termed as the prior to DNA synthesis phase. S Phase: the Second Stage of Interphase S phase:The DNA synthesis place as the name suggests, S which stands for synthesis. Soon after the G1 phase, DNA checking and subsequent repair occurs during the variable pause phase before the transition of the cell cycle to the S phase. The S phase of the interphase deals with the semi-conservative synthesis of DNA occurs. Replication of cellular DNA begins with the S phase, which when gets duplicated with the cell containing nearly double the amount of chromosomes, the cells from the S phase move into the G2 phase. G2 Phase (the Second Gap Phase): the third Stage of Interphase During the G2 phase, there is an increase in the synthesis of the RNA and the protein, which is followed by another round of proof reading and subsequent repair among the newly synthesized DNA sequences before the cell cycle transits to the mitotic cycle. The mitotic spindle formed from the cytokinetic fibers start forming and the cell ensures the number of chromosomes and the organelles present, which further leads the cell cycle from the interphase to the mitotic phase. Three Stages of Interphase of Interphase Conclusion To summarize, Interphase also known as the resting phase is cell cycle phase during which the cells are preparing themselves for the mitotic phase by the cell growth and maturing spanning over the three phases, G1, S and the G2

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). As in mitosis, meiosis is preceded by a process of DNA replication that converts each chromosome into two sister chromatids.

Meiosis I Meiosis I separates the pairs of homologous chromosomes.

In Meiosis I a special cell division reduces the cell from diploid to haploid.

Prophase I The homologous chromosomes pair and exchange DNA to form recombinant chromosomes. Prophase I is divided into five phases:

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Leptotene: chromosomes start to condense.

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Zygotene: homologous chromosomes become closely associated (synapsis) to form pairs of chromosomes (bivalents) consisting of four chromatids (tetrads).

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Pachytene: crossing over between pairs of homologous chromosomes to form chiasmata (sing. chiasma).

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Diplotene: homologous chromosomes start to separate but remain attached by chiasmata.

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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 Meiosis II separates each chromosome into two chromatids.

The events of Meiosis II are analogous to those of a mitotic division, although the number of chromosomes involved has been halved. Meiosis generates genetic diversity through:

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the exchange of genetic material between homologous chromosomes during Meiosis I

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the random alignment of maternal and paternal chromosomes in Meiosis I

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the random alignment of the sister chromatids at Meiosis II

Meiosis in females

Difference Between Mitosis and MeiosisNo: 1 Mitosis: Take place in the somatic cells of the body Meiosis: Take place in the germ cells. No:2 Mitosis: Occurs in both sexually as well as asexually reproducing organisms. Meiosis:Occurs only in sexually reproducing organisms. No:3 Mitosis: The cell divide only once.

Meiosis: There are two cell divisions, the first and the second meiotic divisions. No: 4 Mitosis: Interphase occurs prior to each division. Meiosis: Interphase precedes only in meiosis I. It does not occur prior to meiosis II. No: 5 Mitosis: DNA replication takes place during interphase I. Meiosis: DNA replication takes place during interphase I but not interphase II No: 6 Mitosis: The DNA replicates once for one cell division. Meiosis:The DNA replicates once for two cell divisions No: 7 Mitosis: The duration of prophase is short, usually of a few hours. Meiosis: Prophase is comparatively longer and may take days. No: 8 Mitosis: Prophase is comparatively simple. Meiosis: Prophase is complicated and divided into leptotene, zygotene, pachytene, diplotene, and diakinesis. No: 9 Mitosis: The cell divides only once and the chromosomes also divide only once. Meiosis: There are two cell divisions but the chromosomes divide only once. No: 10 Mitosis: There is no synapsis. Meiosis: Synapsis of homologous chromosomes takesplace during prophase. No: 11 Mitosis: The two chromatids of a chromosome do not exchange segments during prophase. Meiosis: Chromatids of two homologous chromosomes exchange segments during crossing over. No: 12 Mitosis: Each chromosomes consists of two chromatids united by a centromere. Meiosis:The two homologous chromosomes from bivalents or tetrads. Each bivalents has four chromatids and two centromers. No: 13 Mitosis: The arms of the prophase chromatids are close to one another. Meiosis: The arms of the chromatids are separated widely in prophase II. No: 14 Mitosis: Chromosomes are already duplicated at the beginning of prophase Meiosis: When prophase I commences the chromosomes appear single, (although DNA replication has taken place in interphase I).

No: 15 Mitosis: No bouquet stage is recorded. Meiosis: Chromosomes of animals and some plants show covergence towards one side during early prophase I. It is known as bouquet stage. No: 16 Mitosis: Pairing of chromosomes does not occur in mitosis. Meiosis:Pairing or synapsis of homologous chromosomes takesplace during zygotene of prophase I and continues upto metaphase I No: 17 Mitosis: A synaptionemal complex is absent. Meiosis: Synapsed homologous chromosome develop a synaptonemal complex. No: 18 Mitosis: Crossing over is absent. Meiosis:Crossing over or exchange of similar segments between nonsister chromatids of homologous chromosomes usually take place during pachytene stage. No: 19 Mitosis: Chiasmata are absent. Meiosis: Chiasmata or visible connections between homologous chromosomes of bivalents are observed during diplotene, diakinesis (prophase I) and metaphase I No: 20 Mitosis: In the metaphase plate all the centromeres line up in same plate. Meiosis: In metaphase I the centromeres are lined up in two planes which are parallel to one other. No: 21 Mitosis: The metaphase plate is made up of chromosome pairs. Meiosis: The metaphase plate is made up of paired chromosome pairs.

No: 22 Mitosis: Two chromatids of a chromosome (Progeny cells) are genetically similar. The genetic constitution of the daughter cells is identical to that of the parent cells. Meiosis: Two chromatids of a chromosome (Progeny cells) are often genetically different due to crossing over. The genetic constitution of the daughter cells differs from that of the parent cell. The chromosomes of daughter cells usually contain a mixture of maternal and paternal genes. No: 23 Mitosis: Division of the centromeres take place during anaphase. Meiosis: There is no centromeric division during anaphase I. Centromeres divide only during anaphase II. No: 24 Mitosis:The chromosomes separates simultaneously during anaphase. Meiosis: Short chromosomes separate early, separation of long chromosome is delayed. No: 25 Mitosis:Anaphase chromosomes are single stranded. Meiosis: Chromosomes are double stranded in anaphase I, but single stranded in anaphase II. No: 26 Mitosis: Similar chromosomes move towards the opposite poles in anaphase. Meiosis: Dissimilar chromosomes move towards the opposite poles both in anaphase I and II. No: 27 Mitosis: Spindle fibers disappear completely in telophase. Meiosis: Spindle fibers do not disappear completely in telophase I. No: 28 Mitosis: Nucleoli reappear at telophase. Meiosis: Nucleoli do not reappear at telophase I. No: 29 Mitosis: Cytokinesis follows every mitosis.It produces two new cells. Meiosis :Cytokinesis often does not occur after the first or reduction division. It is often simultaneous after second division to result in four new cells. No: 30 Mitosis: The chromosome number remains constant at the end of mitosis. Meiosis: The chromosome number is reduced from the diploid to the haploid. No: 31 Mitosis: It helps in multiplication of cells. Meiosis: Multiplication of cells is not involved. No: 32 Mitosis: Take part in healing and repair. Meiosis: Take part in the formation of meispores or gametes and maintenance of chromosome number of the race.