1. Cell division functions in 3 things : reproduction, growth, and repair The division of a unicellular organism reproduces an entire organism, increasing the population. Here s one amoeba dividing into 2. Fig. 12.1
Cell division is also central to the development of a multicellular organism that begins as a fertilized egg or zygote. Multicellular organisms also use cell division to repair and renew cells that die from normal wear and tear or accidents. Fig. 12.1b Fig. 12.1c
Cell division requires the distribution of identical genetic material - DNA - to two daughter cells. What is remarkable is the accuracy with which DNA is passed along, without changing, from one generation to the next. A dividing parent cell duplicates its DNA, moves the two identical copies to opposite ends of the cell, and then splits into two daughter cells, each with the same DNA.
2. Genome A cell s entire set of genetic information, packaged as DNA, is called its genome. In prokaryotes, the genome is often a single, long, circular DNA molecule. In eukaryotes, the genome consists of several shorter, linear DNA molecules. How many in yours? A human cell must duplicate about 3 m of DNA and separate the two copies such that each daughter cell ends up with a complete genome.
DNA molecules are packaged into chromosomes. Every eukaryotic species has a characteristic number of chromosomes in the nucleus. Human somatic cells (body cells) have 46 chromosomes. Human gametes (sperm or eggs) have 23 chromosomes, half the number in a somatic cell. Fig. 12.2
Each duplicated chromosome consists of two sister chromatids which contain identical copies of the chromosome s DNA, plus proteins. As they condense, the region where the strands connect shrinks to a narrow area, the centromere. Later, the sister chromatids are pulled apart and repackaged into two new nuclei at opposite ends of the parent cell. Fig. 12.3
The process of the formation of the two daughter nuclei is what is specifically what is referred to as mitosis, and is usually followed by division of the cytoplasm, cytokinesis. These processes take one cell and produce two cells that are the genetic equivalent of the parent.
1. The mitotic phase alternates with interphase in the cell cycle: an overview The mitotic (M) phase of the cell cycle alternates with the much longer interphase. The M phase includes mitosis and cytokinesis. Interphase accounts for 90% of the cell cycle. Let s watch Fig. 12.4
Interphase has three subphases: G 1 phase ( first gap ) centered on growth (protein synthesis, respiration, etc.), the S phase ( synthesis ) when the chromosomes are copied, the G 2 phase ( second gap ) where the cell completes preparations for cell division (like microtubule formation), and divides (M).
By late interphase, the chromosomes have been duplicated but are loosely packed. The centrosomes have been duplicated and begin to organize microtubules into an aster ( star ). Fig. 12.5a
In prophase, the chromosomes are tightly coiled, with sister chromatids joined together. The nucleoli disappear. The mitotic spindle begins to form and appears to push the centrosomes away from each other toward opposite ends (poles) of the cell. Fig. 12.5b
During prometaphase (late prophase), the nuclear envelope fragments and microtubules from the spindle interact with the chromosomes. Microtubules from one pole attach to each chromosome at the centromere region. Fig. 12.5c
The spindle fibers push the sister chromatids until they are all arranged at the metaphase plate or equator, an imaginary plane equidistant between the poles. This lining up defines metaphase. Fig. 12.5d
At anaphase, the centromeres divide, separating the sister chromatids. Each is now pulled toward the pole to which it is attached by spindle fibers. By the end, the two poles have identical collections of chromosomes. Fig. 12.5e
At telophase, two nuclei begin to form, surrounded by the fragments of the parent s nuclear envelope. Chromatin becomes less tightly coiled. Cytokinesis, division of the cytoplasm, begins. Let s watch again Fig. 12.5f
Fig. 12.5 left
Fig. 12.5 right
3. Cytokinesis divides the cytoplasm: a closer look Cytokinesis follows mitosis. In animals, the first sign of cytokinesis (cleavage) is the appearance of a cleavage furrow in the cell surface near the old metaphase plate. See Animation 12.3.8 Fig. 12.8a
On the inside of the cleavage furrow, a ring of actin microfilaments and the motor protein myosin form. Contraction of the ring pinches the cell in two. This is often referred to as the purse-string method. Fig. 12.8a
Cytokinesis in plants, which have cell walls, involves a completely different mechanism. During telophase, vesicles from the Golgi connect at the metaphase plate, forming a cell plate. The plate enlarges until its membranes fuse with the plasma membrane at the perimeter, with the contents of the vesicles forming new wall material in between. Fig. 12.8b
Introduction The frequency of cell division varies Some human cells divide frequently throughout life (skin cells), others have the ability to divide, but keep it in reserve (liver cells), and mature nerve and muscle cells do not appear to divide at all after maturity, although there is current evidence that they may under certain conditions. Cancer (tumour) cells seem to have lost such controls over their division, and divide repeatedly. Any cells in an organ can become cancerous.
CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES Section A: An Introduction to Heredity 1. Offspring acquire genes from parents by inheriting chromosomes 2. Like begets like, more or less: a comparison of asexual and sexual reproduction
Introduction Living organisms are distinguished by their ability to reproduce their own kind. Offspring resemble their parents more than they do less closely related individuals of the same species. The transmission of traits from one generation to the next is called heredity or inheritance. However, offspring differ somewhat from parents and siblings, demonstrating variation. Genetics is the study of heredity and variation.
1. Offspring acquire genes from parents by inheriting chromosomes Parents endow their offspring with coded information in the form of genes, which are specific sections of a chromosome. Your genome is derived from the thousands of genes that you inherited from your mother and your father. Genes program specific traits that emerge as we develop from fertilized eggs into adults. Your genome may include a gene for freckles, which you inherited from your mother.
2. Like begets like, more or less: a comparison of asexual and sexual reproduction In asexual reproduction, a single individual passes along copies of all its genes to its offspring. Single-celled eukaryotes reproduce asexually by mitotic cell division to produce two identical daughter cells. Even some multicellular eukaryotes, like hydra, can reproduce by budding cells produced by mitosis. Fig. 13.1
Sexual reproduction results in greater variation among offspring than does asexual reproduction. Two parents give rise to offspring that have unique combinations of genes inherited from the parents. Offspring of sexual reproduction vary genetically from their siblings and from both parents. Fig. 13.2
1. Fertilization and meiosis alternate in sexual life cycles In humans, each somatic cell (all cells other than sperm or ovum) has 46 chromosomes. Each chromosome can be distinguished by its size and the position of the centromere. A karyotype display of the 46 chromosomes shows 23 pairs of chromosomes, each pair with the same length and centromere position. Pair #1 is the longest, #2 second, etc. These homologous chromosome pairs carry genes that control the same inherited characters. There are a pair of blood type genes, for instance, one from that person s mom and one from their dad.
Karyotypes, ordered displays of an individual s chromosomes, are often prepared with lymphocytes, a type of white blood cell, or those from a fetus. Fig. 13.3
The occurrence of homologous pairs of chromosomes is a consequence of sexual reproduction. We inherit one chromosome of each homologous pair from each parent. This is a key concept!!! The 46 chromosomes in a somatic cell can be viewed as two sets of 23, a maternal set and a paternal set. Sperm cells or ova (gametes) have only one set of chromosomes. A cell with a single chromosome set is haploid. For humans, the haploid number of chromosomes is 23 (n = 23).
IN sexual reproduction, a haploid sperm reaches and fuses with a haploid ovum. The fertilized egg (zygote) now has two haploid sets of chromosomes bearing genes from the maternal and paternal family lines. It will divide by mitosis to produce a new organism. The zygote and all cells with two sets of chromosomes are diploid cells (2n). For humans, the diploid number of chromosomes is 46 (2n = 46). In fruit flies, the diploid number is 8 (2n = 8).
As an organism develops from a zygote to a sexually mature adult, the zygote s genes are passes on to all somatic cells by mitosis. Gametes, which develop in the gonads, are not produced by mitosis. If gametes were made by mitosis they would be diploid, and the fusion of gametes would produce offspring with four sets of chromosomes after one generation, eight after another, and so on. Instead, gametes undergo the process of meiosis, in which the chromosome number is halved. For this reason, it is also called reduction division. Human sperm or ova have a haploid set of 23 different chromosomes, one from each homologous pair.
Fertilization restores the diploid condition by combining two haploid sets of chromosomes. Fertilization and meiosis alternate in sexual life cycles. Fig. 13.4
3. Meiosis reduces chromosome number from diploid to haploid: a closer look Many steps of meiosis resemble steps in mitosis. Both are preceded by the replication of chromosomes. However, in meiosis, there are two consecutive cell divisions, meiosis I and meiosis II, which results in four daughter cells. Replication of chromosomes only occurs before the first division, though. Each final daughter cell has only half as many chromosomes as the parent cell.
Meiosis reduces chromosome number by copying the chromosomes once, but dividing twice. The first division, meiosis I, separates homologous chromosomes. The second, meiosis II, separates sister chromatids. Watch Or here, here and here. Fig. 13.6
Division in meiosis I occurs in four phases: prophase, metaphase, anaphase, and telophase. During the preceding interphase the chromosomes are replicated to form sister chromatids. These are genetically identical and joined at the centromere. Also, the single centrosome is replicated. This is just like mitosis. Fig. 13.7
In prophase I, the chromosomes condense and homologous chromosomes pair up to form tetrads. This does NOT happen in mitosis. Synapsis is the process by which homologous chromosomes come together. A spindle forms from each centrosome and spindle fibers attach to each chromosome and begins to move the tetrads around. Fig. 13.7
At metaphase I, the tetrads are all arranged at the metaphase plate. How is this NOT like mitosis? Microtubules from one pole (not from both as in mitosis) are attached to one chromosome of each tetrad, while those from the other pole are attached to the other. In anaphase I, the homologous chromosomes separate and are pulled toward opposite poles. Fig. 13.7
In telophase I, movement of homologous chromosomes continues until there is a haploid set (one from each pair) at each pole. Each chromosome still consists of two sister chromatids (it is said to be double stranded). Cytokinesis by the same mechanisms as mitosis usually occurs simultaneously. In some species, nuclei may reform, but there is no further replication of chromosomes. Fig. 13.7
Meiosis II is very similar to mitosis. During prophase II a spindle apparatus forms, attaches to each sister chromatid, and moves the double stranded chromomsomes around. Spindle fibers from one pole attach to one sister chromatid and those of the other pole to the other sister chromatid. Fig. 13.7
At metaphase II, the double stranded chromosomes are arranged in single file at the metaphase plate. At anaphase II, the centomeres of sister chromatids separate and the now separate sisters travel toward opposite poles. Fig. 13.7
In telophase II, separated sister chromatids, now called single stranded chromosomes, arrive at opposite poles. Cytokinesis separates the cytoplasm. At the end of meiosis, there are four haploid daughter cells, each with one single stranded chromosome from each pair. Let s sing it! Fig. 13.7
Mitosis and meiosis have several key differences. The chromosome number is reduced by half in meiosis, but not in mitosis. Mitosis produces daughter cells that are genetically identical to the parent cell and to each other. Meiosis produces cells that differ genetically from the parent and each other. Mitosis produces two cells, meiosis produces 4 cells. What are some similarities between mitosis and meiosis?
Mitosis produces two identical daughter cells, but meiosis produces 4 very different cells. Watch! Fig. 13.8
Benchmark Clarification for SC.912.L.16.17 Students will: Differentiate the process of meiosis and meiosis Describe the role of mitosis in asexual reproduction, and/or the role of meiosis in sexual reproduction, including how these processes may contribute to or limit genetic variation Describe specific events occurring in each of the states of the cell cycle and/or phases of mitosis Explain how mitosis forms new cells and its role in maintaining chromosome number during asexual reproduction Content Limits: Items will focus on the relationship between mutations and uncontrolled cell growth, rather than a specific mutation that may result in uncontrolled growth
Of the phases shown, which is the first in meiosis?
In many plants, a new plant can grow from a piece of the parent plant. Strawberries reproduce this way, from runners, and African violets can be grown from a leaf. Piece of potato tuber can be used to grow new potato plants, as shown below. This method of producing offspring is dependent on which process? A. mutation during mitosis B. mitosis during asexual reproduction C. self-pollination during regeneration D. meiosis during sexual reproduction
Which of these is most likely to result from the processes of mutation and crossing over during reproduction? A. Offspring that are genetically identical to their parents B. Offspring that are genetically identical to each other C. Decreased genetic variation among offspring D. Increased genetic variation among offspring
Which statements (in box below) about cell division are correct? A. statements 1 and 3 B. statements 1 and 4 C. statements 2 and 3 D. statements 2 and 4
Which statement is correct? A. Meiosis is a way to reproduce, but mitosis is not. B. Meiosis is a way to create diversity, but mitosis is not. C. During mitosis, chromosomes are copied, but, during meiosis chromosomes double. D. During mitosis, chromosome numbers double, but during meiosis, chromosome numbers remain constant.
Study the sequence below. Which cellular process missing from the sequence produces cells having a chromosome number of 2n?
Which best explains how meiosis is a contributing factor to genetic variation within a species? A. Meiosis reduces the number of mutations within an organism B. Meiosis produces daughter cells that will contain identical chromosomes C. Meiosis results in offspring that contain alleles from only one parent gamete D. Meiosis allows for crossing over of chromosomes, resulting in new gene combinations
A cheetah is multicellular. A paramecium is unicellular. How do these two organisms differ in terms of how they produce offspring? A. The cheetah uses sexual reproduction, the paramecium uses meiosis B. The cheetah uses binary fission, the paramecium uses asexual reproduction C. The cheetah uses asexual reproduction, the paramecium uses binary fission D. The cheetah uses sexual reproduction, the paramecium uses asexual reproduction.
What would most likely result if mitosis was not accompanied by cytoplasmic division? A. Two cells, each with one nucleus B. Two cells, each without a nucleus C. One cell with two identical nuclei D. One cell without a nucleus
When does crossing-over occur during meiosis? A. when the DNA of the ciploid cell is copied B. when homologous chromosomes move to opposite ends of the dividing cell C. when spindle fibers move the chromosomes toward the midline of the dividing cell D. when homologous chromosomes pair and portions of chromatids break off and are exchanged
During meiosis, homologous chromosomes line up next to each other. If one arm of a chromatid crosses over the arm of another chromatid, what results? A. an additional sex cell is created B. independent assortment of genetic material C. a possible change in the offspring cell s function D. additional variation in the DNA combination of each sex cell formed