Biology Chapter 7

Difference between chromatids and chromosomes

Chromatids share centromere Chromosomes have their own centromere

Anaphase

Fourth phase of mitosis • The chromatids becomes disconnected as the protein bonds between them degrade. • The chromatids separate, and the daughter chromosomes move away from each other toward the poles. • The nonkinetochore spindle fibers begin to lengthen, and the poles begin to move father apart. • During late anaphase, the chromosomes reach the poles.

Prometaphase

Second phase of mitosis The nuclear envelope breaks down and the compacted chromosomes, each consisting of two chromatids, attach to the kinetochore microtubules

Metaphase`

Third phase of mitosis The chromosomes line up at the midline of the cell (the equatorial position called the metaphase plate) This is an important checkpoint in the process of cell division, if the chromosomes do not line up properly at the metaphase plate, mitosis will not proceed.

Cytokinesis

While mitosis only refers to the division of the nucleus, cytokinesis is the division of the cells cytoplasm and is the final stage of cell reproduction. It is different in plant and animals • At the end of cytokinesis, there are two daughter cells, each with identical genetic material. •Cell division may be uneven in the sense that one cell might receive more cytoplasm and organelles than the other cell.

Mitosis

• A small stage in the cell cycle of eukaryotic cells. Goes through multiple stages itself: 1. Prophase: Condensation of chromosomes; spindle assembly 2. Prometaphase: Nuclear envelope breakdown; chromosome attachment to spindle 3. Metaphase: Alignment of chromosomes at equatorial plate 4. Anaphase: Separation of chromatids; migration to the poles 5. Telophase: Chromosome decondense; nuclear envelope recloses 6. Cytokinesis: Cell separation; cell membrane and/or cell wall formation

Binary fission

• A type of asexual reproduction that prokaryotes typically divide by. • Steps of binary fission are: 1. Signal to initiate

Mitosis cell cycle

• Although Mitosis is continuous, it is convenient to subdivide it into phases • Mitosis is the set of processes in which the chromosomes become condensed and then segregate into two new nuclei. • Mitosis and cytokinesis are referred to as M phase. • M phase is followed by a much longer period called interphase, when the cell nucleus is visible and typical cell functions occur, including DNA replication in cells preparing to divide.

Independent assortment

• At anaphase I, it is a matter of chance which member of a homologous pair goes to which daughter cell. • The greater the number of chromosomes, the greater the potential for genetic diversity. • Humans have over 8 million different combinations of maternal and paternal chromosomes that can be produced

Cytokinesis in animal cells

• Begins with the furrowing of the cell membrane, cinching the cytoplasm between the two nuclei. • This is known as the formation of a contractile ring of microfilaments that pinch off as they contract. • This occurs along the metaphase plate.

Cell division

• Cell reproduction • Cell division is a process by which a parent cell duplicates its genetic material and then divides into two smaller similar cells. • Important in the growth of multicellular organisms and repair of tissues, as well as reproduction of all organisms • Either asexual reproduction or sexual reproduction

The G2 checkpoint

• Checkpoint during the G2 phase of Mitosis. • This checkpoint is triggered by DNA damage • The cell will not progress from the G2 phase to mitosis unless the DNA is undamaged and intact.

The M-phase checkpoint

• Checkpoint during the M phase of Mitosis. • This checkpoint is triggered by a chromosome that fails to attach to the spindle. • The cell will not go into anaphase unless the chromosomes are properly aligned along the metaphase place and unless the kinetochores are attached to the spindle fibers. • This checkpoint ensures that the cell is ready for division before it proceeds any further.

Meiosis

• Consists of two nuclear divisions that reduce the number of chromosomes to the haploid number. • The haploid cells produced by meiosis are genetically different from one another and from the parent cell • The function of meiosis is to: Reduce the chromosome number from diploid to haploid, & ensure that each haploid cell has a complete set of chromosomes • Meiosis consists of two nuclear divisions, Meiosis 1 and Meiosis 2

Prophase I

• First stage of Meiosis I • May last a long time • In Human Males: Prophase lasts about 1 week, and 1 month for the entire meiotic cycle • Human Females: Prophase I begins before birth, meiosis continues up to decades later during the monthly ovarian cycle and is completed only after fertilization.

Process of gametes through sexual reproduction

• Gametes contain only a single set of chromosomes, thus one homolog from each pair. • The number of chromosomes in a gamete is denoted by n, and the cell is said to be a haploid. • During sexual reproduction, two haploid gametes fuse to form a zygote in a process called fertilization. • The zygote thus has two sets of chromosomes, just as the somatic cells do. The chromosome number in the zygote is denoted by 2n and the cells are thus called a diploid.

Interphase

• Interphase is the larger part of the cell cycle in non reproducing eukaryotic cells. • Has 3 subphases called G1, S, and G2 (G stand for gap) • G1 (Gap 1) is variable and a cell may last a long time in this phase as it carries out its special function • S phase (Synthesis): The cells DNA is replicated • G2 (Gap 2) the cell makes preparations for mitosis; synthesizes microtubules for segregative chromosomes.

How is Meiosis II similar to mitosis

• It involves the separation of chromatids into daughter nuclei.

What two mechanisms move the chromosomes to opposite poles?

• Kinetochores have molecular motor proteins (kinesin and dynein), which move the chromosomes along the microtubules. • The kinetochores microtubules shorten from the poles, drawing the chromosomes toward the poles.

All sexual life cycles involve...

• Meiosis. • Gametes may develop immediately after meiosis • Or each haploid cell may develop into a haploid organism (haploid stage of the life cycle) that eventually produces gametes by mitosis. • Fertilization results in a zygote and begins the diploid stage of the life cycle.

Division in asexual eukaryotic cells

• Mitosis is cell division in somatic cells (nonreproductive) which result in two sets of chromosomes. • Mitosis also can occur in single celled eukaryotes • In mitosis, one nucleus produces two daughter nuclei, each containing the same number of chromosomes as the parent cell. • The basic steps to mitosis are: 1. Reproductive signal: Unlike prokaryotes, the signals for cell division are usually not related to the environment of a single cell, but to the function of the entire organism. • Most cells in a multicellular organism are specialized and do not divide. • Signals for cell division are related to the function of the entire organism rather than that one specific cell. 2. DNA replication: Unlike prokaryotes, eukaryotes have more than one chromosome, but, similarly, they are replicated by threading the long strands through replication complexes. • DNA replication only occurs during a specific stage of the cell cycle. 3. DNA segregation: Much more complex than in a prokaryote as eukaryotes have a nuclear envelope and there are multiple chromosomes. • When a cell divides, one copy of each chromosome (in humans we have 46) much end up in each of the two daughter cells. • In eukaryotes, the replicated chromosomes are initially attached to each other, become highly condensed and are pulled apart, segregating into the new nuclei with the help of the cytoskeleton. 4. Cytokinesis: The process is different in plant cells (as they have cell walls) than in animal cells.

Meiosis II

• Not preceded by DNA replication • Sister chromatids separate • End result: four haploid cells that are genetically identical.

Asexual reproduction

• Offspring are clones - genetically identical to the parent • Any genetic variations are due to mutations (changes in DNA sequences due to environmental factors or copying errors) • Single cell prokaryotes reproduce Asexually in a process called binary fission, while single cell eukaryotes can reproduce a sexually through mitosis (sometimes followed by cytokinesis). • Many multicellular eukaryotes can also reproduce by asexual means (Aspen trees) • The daughter cells have the same number of chromosomes as the parent cells.

Sexual reproduction

• Only occurs in eukaryotes • Two individuals contribute half of their genetic material to the offspring. • Two specialized cells, called gametes, are formed through meiosis, a process of cell division resulting in daughter cells with half the original cell's genetic material. • In sexual reproduction, gametes fuse and the genetic material is "scrambled" through genetic recombination, producing an offspring that is genetically different from either parent. • Thus, random selection of half the diploid chromosome set forms a haploid gamete, followed by fusion of the haploid gametes from the separate parents to make a diploid cell. • Contributes to genetic diversity • Forms the raw material for natural selection and evolution •The daughter cells have the number of chromosomes as the parent cells, thus they are haploids.

Meiosis I

• Overall, the homologous chromosomes separate, and the cells divide. The result is two haploid cells with two sister chromatids of each homologous chromosome. • Meiosis I is preceded by an S phase during which DNA is replicated. • Each chromosomes then consists of two chromatids. • At the end of meiosis I, two nuclei form, each with half the original chromosomes (one member of each homologous pair) • The centromeres did not separate, so each chromosome is still two sister chromatids.

G1 Interphase

• Part of mitosis cell division, before mitosis occurs. • During this phase, each chromosomes exists as a single molecule of unreplicated, double stranded DNA. •One chromosome come from the father and one comes from the mother. • The combination of each of these homologs form a homologous pair. • The chromosomes are located in the cells membrane bound nucleus. There is a single centrosome outside of the nuclear envelope.

Division in Prokaryotes

• Prokaryotes divide by binary fission. • The basic steps of binary fission are: 1. Signal to initiate: An internal or external signal initiates the asexual reproduction 2. DNA replication: Most prokaryotes have one circular chromosomes with two important regions: • Ori: the site were the replication starts • Ter: The site where the replication ends. Replication occurs as the DNA is threaded through a "replication complex" of proteins at the center of the cell. When the replication finishes, the two daughter DNA molecules separate and segregate from one another at opposite ends of the cell. 3. Segregation of DNA: As replication proceeds, the ori complexes move to the opposite end of the cell. • DNA sequences adjacent to the ori bind proteins that are essential for this segregation, using ATP. • An actin-like protein provides a filament in which ori and other proteins move. 4. Cytokinesis: After chromosome segregation, the cell membrane pinches in by contraction of a ring of protein fibers under the surface. • As the membrane pinches in, new cell wall materials are deposited, resulting in the separation of the two cells.

Growth factors

• Prokaryotic cells divide in response to environmental conditions • In eukaryotes, cell division is related to the needs of the entire organism. • Mammals thus produce growth factors that stimulate cell division and differentiation. • Progression of growth in eukaryotic cells is thus tightly regulated

Regulation of the cell cycle is controlled by...

• Proteins called cyclins and cyclin-dependent kinases (CSKs). • When a specific protein cyclin binds to a CDK, the CDK becomes activated and can phosphorylate regulatory proteins. • Each CDK has its own cyclin to activate it, and the cyclin is only made at the right time.. • Once the cell has passed through the appropriate checkpoint (G1, S, G2, M) , the cyclins dissociate from the cyclin CDK complex and the cyclins are degraded by proteases. • Note that these CDK-cyclin complexes can be both inhibitory and excitatory, meaning that they can either slow down or speed up the cell cycle. • If these complexes are damaged, the organisms could have serious problems, • If the cell cycle goes too fast, cell division will be rapid and uncontrollable, leading to cancer. • If the cell cycle goes too slowly, cells will continue to grow without dividing.

S Phase: DNA Synthesis

• Second part of mitosis cell division before mitosis occurs • During this phase, each homolog replicates itself, forming two sister chromatids of the same double stranded DNA. • These chromatids are attached by proteins across their length and also at the centromeres. • The centrosome also duplicates at this phase

Somatic cells

• Somatic cells are body cells not specialized for reproduction. • Each somatic cell contains two sets of chromosomes that occur in homologous pairs. • One homolog came from the female parent and one from the male parent and have corresponding genetic information.

Karyotype

• The condensed chromosomes for a given organism can be distinguished by their sizes and centromere positions. • Karyotype analysis was used to identify and classify organisms, but DNA sequencing is more commonly used today • Karyotype analysis is still used to identify chromosomes abnormalities.

Cytokinesis in plant cells

• The cytoplasm divides differently because plants have cell walls. •Cytokinesis occurs as vesicles from the Golgi apparatus move along the microtubules to the middle of the cell. • These vesicles contain enzymes that are necessary for the synthesis of cell wall components. •They will eventually form a cell plate membrane, which fuses with the cell's outer membrane to produce two daughter cells.

Prophase: Reoriented centrosomes

• The first step of mitosis. • In prophase, 3 structure appear: condensed chromosomes, reoriented centrosomes, and a spindle • Before the spindle forms, its orientation is determined, in many cells, by the centrosome, an organelle in the cytoplasm near the nucleus. • The centrosome consists of a pair of centrioles. • The position of the centrosomes determine the plane at which the cell divides; therefore determining the spatial relationship between the two new cells. • Centrosomes complete their migration to the poles of the cell and emanate spindle fibers that begin to enter the nuclear area. • The nuclear envelope begins to disappear, and the chromosomes become fully condensed. • Kinetochores begin to form at the centromeres. • Plant cells lack centrosomes but distinct microtubules at each end of the cell play the same role

Prophase: Condensing chromosomes

• The first step of mitosis. • In prophase, 3 structure appear: condensed chromosomes, reoriented centrosomes, and a spindle • During prophase, the chromosomes become much more tightly coiled and condensed • After DNA replication, each chromosome has two DNA molecules, known as sister chromatids, they become super compact. • Until they are separated during anaphase, these compact chromatids are held together at a region called the centromere, giving it an X shape. • Specialized protein structured called kinetochores assemble one each centromere, one on each chromatic. These structures are important for chromosome movement. • For a given organism, the number and sizes of the condensed chromosome constitute the karyotype. • Each chromosome has a particular length and the centromere is located at a particular position along its length.

Prophase: Spindle formation

• The first step of mitosis. • In prophase, 3 structure appear: condensed chromosomes, reoriented centrosomes, and a spindle • The mitotic spindle controls movement of chromosomes during mitosis. • During prophase, the assembly of the spindle microtubules begin at the centrosome, which serves as the microtubule organizing center. • Sister chromatids will soon connect to the kinectochore microtubules (prometaphase) from opposite sides so that the two chromatids will move to opposite poles • sister chromatids become daughter chromatids after separation. • The nonkinetochore microtubules are not attached to chromosomes; rather they elongate the cell by pushing the centrosomes towards opposite poles.

Prophase

• The first step of mitosis. • In prophase, 3 structure appear: condensed chromosomes, reoriented centrosomes, and a spindle.

Errors that can occur in meiosis

• The gametes formed from meiotic errors carry abnormal chromosomes, and when abnormal chromosomes and can lead to consequences for the offspring • Examples are: Nondisjunction, Polyploidy & Translocation

Crossing over of Meiosis

• The shuffling of genetic material • In prophase I homologous chromosomes (synapsis) and four chromatids form a tetrad, which consists of two chromosomes and four chromatids. This process is called synapsis. • The homologues seem to repel each other at the centromeres but then attach at the chiasmata, the chiasmata form as the chromosomes cross over each other. • Genetic material is thus shared between nonsister chromatids • Any four chromatids in the tetrad can participate, and a single chromatid can exchange material at more than one point. • This process, called intrachromosomal recombination, is important because it "scrambles" the genetic material. • Crossing over results in recombinant chromatids and increases genetic variation in products.

In order for any cell to divide, the following events must occur:

• There must be one or more reproductive signals, initiating cell division and originates inside or outside the cell. (These may be environmental factos such as nutrients) • DNA replication must occur so that each of the new cells have a complete set of genes • The cell must distribute the replicated DNA to each of the new two cells in DNA segregation • Finally, Cytokinesis occurs which is the division of the cytoplasm and separation of the two new cells.

G2 Interphase

• Third part of mitosis cell division, before mitosis occurs. • During this phase, the cell synthesizes components needed for mitosis and eventually receives the signal to enter mitosis.

The S-phase checkpoint

• This is the checkpoint during the S phase during Mitosis. • Triggered by incomplete replication or DNA damage. • Here, DNA synthesis will be delayed if the DNA is damaged. • It will not continue again until the DNA has been repaired. • This is a checkpoint that reduces the deleterious effects of DNA damage.

G1-S checkpoint

• This transition is called R, the restriction point • This checkpoint is triggered by DNA damage. • A cell will not replicate its genome unless its large enough and has enough energy and materials to eventually divide. • Thus it is called the restriction point because it is the point at which the cell becomes "committed" to the cell cycle and no extracellular growth factors are needed.

Nondisjunction

• When a homologous chromosome pair fails to separate at anaphase I, or sister chromatids fail to separate at anaphase II. • Both result in aneuploidy, an abnormal number of chromosomes • Most human embryos from an aneuploid zygote do not survive and lead to miscarriages. • The most common human aneuplody is trisomy 16 (three copies of chromosomes 16) • Trisomy 21 (3 copies of 21) is Down syndrome and is one of the few aneuploidies that allow survival.

Telophase

•Fifth phase of mitosis • Occurs after the chromosomes have separated and is the last phase of mitosis. • During this period, a nuclear envelope forms around each set of new chromosomes, nucleoli appear, and the chromosomes become less compact. • The chromosomes decondense back into chromatin. • The spindle also disappears at this stage, and there are then two new nuclei in a single cell.