Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
ft v¡tal: essential Over time, cells grow and the size of their cltoplasm increases. Eventually, cells reach the stage where they divide. This division of cells is how they reproduce. reproductive cell: specialised cell that is able to unite with a similar cell of the other sex and produce a new individual Cell division is the way that an organism produces new cells in order to grow and repair its body's tissues. mitotic spindle: structure made of fibres that chromosomes attach to during cell division condensation: thickening and shortening of chromosomes during mitosis (entromere: point that joins the two halves of a chromosome together cett division 6.1. Mitosis During cell division, two daughter cells are produced from the parent cell. The daughter cells are identical to each other. It is vital that an exact copy of the genetic information is transmitted to the daughter cells. To do this, the nucleus has to divide in a process called mitosis. The parent cell needs to have two copies of its genetic material to give to each of the daughter cells. So before mitosis begins, the cell's genetic information is duplicated. As a consequence, mitosis can provide a copy of the parent cell's genetic information to each daughter cell. All cells, with the exception of reproductive cells, divide this way. There are four main stages in mitosis: prophase, metaphase, anaphase and telophase. Prophase s,oind le Ín the first stage of mitosis, there are four maín processes. I forming Genetic material thickens and shortens to form chromosomes. I r The nucleolus disappears. Between thetwo ends of the cells (poles), protein fibres appear to make a mitotic ,"nrr.r"r----147 spindle. I At the end ofthe prophase, the nuclear membrane disappears. fragments of nuclear membráne Metaphase These are the main events during the second stage of mitosis. r The condensed chromosomes move within the mitotic spindle that connects the two centrosomes. The chromosomes attach themselves to the spindle fibres with their centromeres. The chromosomes line up along the middle of the cell. spindle pole ftl ururr + / Anaphase ch romatids There are two main stages of the anaphase. I The spindle ñbres split into two equal parts. This causes the chromosomes' centromeres to break. As a result, the two chromatids that make up each \ chromosome separate into two unconnected strands. I :( The spindle fibres contract and pull the chromatids to the opposite poles of the cell. From this point onwards, the chromatids are considered to be independent chromosomes. \ li {/----..-L- \ +-/ centrioles mid section Telophase ln the final stage of mitosis, four main events occur. I Once the chromatids have moved to the poles of the cell, the mitotic spindle disappears. r A nuclear membrane surrounds each of daqghter chromosomes. This j:roqn results in the formation of two daughter nuclei. I I The chromatids change to chromatin. The nucleolus reappears. nucleolus ch romatin If the mitotic process has been normal, the resulting daughter nuclei will contain the same genetic information. This information is an identical copy of the genetic information found in the parent nucleus. Mitosis is usually followed by the dividing of the cytoplasm into two parts. A plasma membrane forms and separates the two daughter nuclei' This process of cytoplasmic division is called cytokinesis. There are two different typ es of cytokinesis. I split lvb.); separate, break open chromatid: strand formed when a chromosome divides In animal cells, the cytoplasm stretches, thins out and eventually separates. I In plant cells, a wall called a cell plate forms which divides the cytoplasm in half. ! l' At the end of cytokinesis, two separate and complete daughter cells are produced. e¡J 0 ¿ 0 ffi EEI Why do the nuclear membrane and nucleolus disappear and then reappear during mitosis? Explain your answer in your exercise book. C¡okinesis in animal cells (A) and plant cells (B) cellsfl 6.2. Meiosis diploid: having a complete set of chromosomes haploid: having a single set of unpaired chromosomes somatic celh any cell of an organism that isn't a reproductive cell offspring: new individual or individuals that result from sexual or asexual reproduction 1 cell (2n) Sometimes cell division results in cells that aren't identical to the parent cell. This is the case during a special type of cell division called meiosis. During this process, cells with half the amount of genetic information are produced. The cells produced from meiosis are known as reproductive cells or gametes. The male and female gametes join together to form a zygote, which contains a full set of genetic information . The zygote then divides repeatedly to create a new and complete individual. informationlnJ I information: n 1st mitosis ü 2 cells (n) I Sperm (male gamete) fertilising an egg (female gamete) i .\K i informat¡o1f -\ d .a\ 2nd mitosis parents gametes * fertilisatíon 4 cells (n) d Diagram of meiosis new individual information: n information: 2n l Meiosis is the process of cell division when a diploid cell with 2n chromosomes divides into 4 daughter haploid cells with n chromosomes If gametes contained the same number of chromosomes as somatic cells, the specific genetic characteristics of individuals could not be passed on to their offspring. This would mean that the specific genetic proiile of each species could not be maintained. The process of meiosis ensures that the correct number of chromosomes is passed onto the next generation. As a consequence, there are two consecutive cell divisions in meiosis. The first division is a special type of cell division that produces cells with half the number of chromosomes. The second division is the normal mitosis process. Meiosis happens at some point during the life cycle of most sexually reproductive organisms. ffi El-l 1¡u. or false? Discuss these statements with a partner. a) Only sexually reproductive animals have meiosis occur during their life cycle. b) Meiosis consists of two consecutive cell divisions because gametes have twice the number of chromosomes. l§l urvrr < Reduction division - This first cell division of meiosis produces cells with half the number ch¡omosomes. It has the same stages as normal mitosis, but with some special characteristics. of I In the prophase, chromosomes appear, but unlike in mitosis, they are in pairs. These pairs of homologous chromosomes exchange genetic material. This is called crossing over. r In the metaphase, pairs of homologous chromosomes (not individual chromosomes) become attached to the spindle fibres. I In the anaphase, complete chromosomes (not chromatids) are pulled to each spindle pole. I The telophase in meiosis is the same as for mitosis. At the end of the first division, there are two daughter haploid nuclei that have ¡z chromosomes. homologous: having the same structure or position H »tr Crossing over spindle microtubules \/ fragments of the nuclear / separation of homologous chromosomes that have crossed over membrane \/ homologous chromosomes prophase I anaphase I First division in meiosis cells l§ r Second division The second division process of meiosis is normal mitosis. rn this division, each chromosome sprits into chromatids. Each daughter ce:receives one set of chromatids. At the end of second divisiori, the cytoplasm divides and creates two separate ce[s. This is the rast e\-e:-: the process of meiosis. metaphase ll prophase il anaphasetl % -t4il, _( /á\\ ?t I r{Vtf Y,/ 'r l, -( cytokinesis telophase ll separation of sister chromatids i-rl La 4 haploid cells t I il c ,6Second division in meiosis and cytokinesis After the two divisions of meiosis, four daughter cels are produced; each one has half the number of chromosomes-of the originai parent cell. In other words, one diproid cen resurts in four haploid ,"i.oau"iire cens. EZ @l urum < What is trossing over,and why is it an important part of meiosis? -