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Mitosis: Cell Division ‘’Dance of Chromosomes’’ Chromosome The Key Roles of Cell Division • The ability to reproduce is one of the key features that separates life from non-life. • All cells have the ability to reproduce, by making exact copies of themselves. • In unicellular organisms, division of one cell reproduces the entire organism • In multicellular organisms, cell division is needed for: • Development of an embryo from a sperm/egg • Growth • Repair Asexual Reproduction • Asexual reproduction is reproduction that involves a single parent producing an offspring. • The offspring produced are, in most cases, genetically identical to the single cell that produced them. • Prokaryotic organisms (like bacteria) reproduce asexually, as do some eukaryotes (like sponges). Also termed Binary fission. Prokaryotic Chromosomes • Prokaryotic cells lack nuclei. Instead, their DNA molecules are found in the cytoplasm. • Most prokaryotes contain a single, circular DNA molecule, or chromosome, that contains most of the cell’s genetic information. Eukaryotic Chromosomes • In eukaryotic cells, chromosomes are located in the nucleus, and are made up of chromatin. • Chromatin is composed of DNA and histone proteins. • DNA coils around histone proteins to form nucleosomes. • The nucleosomes interact with one another to form coils and supercoils that make up chromosomes. Sexual Reproduction • In sexual reproduction, offspring are produced by the fusion of two sex cells – one from each of two parents. These fuse into a single cell before the offspring can grow. • The offspring produced inherit some genetic information from both parents. • Most animals and plants, and many single-celled organisms, reproduce sexually. Contrasting Reproduction types Cell Division • Cells duplicate their genetic material before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA. • A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into daughter cells. • A cell’s endowment of DNA (its genetic information) is called its genome. • DNA molecules in a cell are packaged into chromosomes. Terminologies • DNA: is the hereditary material in humans and almost all other organisms. Most DNA is located in the cell nucleus (where it is called nuclear DNA. • GENE: A gene is a locus (or region) of DNA that encodes a functional RNA or protein product, and is the molecular unit of heredity. • CHROMOSOME: In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure. •. Terminologies • CHROMATID: A chromatid is one copy of a newly replicated chromosome, which typically is joined to the other copy by a single centromere. Before replication, one chromosome is composed of one DNA (double-helix) molecule. • CHROMATIN: is a complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells. • NUCLEOSOME: The nucleosome is the fundamental subunit of chromatin. Each nucleosome is composed of a little less than two turns of DNA wrapped around a set of eight proteins called histones, which are known as a histone octamer. Terminologies • CENTROMERE: The centromere is the part of a chromosome that links sister chromatids. During mitosis, spindle fibers attach to the centromere via the kinetochore. • KINETOCHORE:A kinetochore is a protein structure that forms on a chromatid during cell division and allows it to attach to a spindle fiber on a chromosome. • HISTONES: are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes. They are the chief protein components of chromatin, acting as spools around which DNA winds, and playing a role in gene regulation. Chromosome Terminologies • TELOMERASE: is a ribonucleoprotein that adds the polynucleotide "TTAGGG" to the 3' end of telomeres, which are found at the ends of eukaryotic chromosomes. • TELOMERE: is a region of repetitive nucleotide sequences at each end of a chromatid, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. • SEPARASE: also known as separin, is a cysteine protease responsible for triggering anaphase by hydrolyzing cohesin, which is the protein responsible for binding sister chromatids during the early stage of anaphase. Terminologies • COHESIN: Protein complexes that glues sister chromatids at the centromere. • CONDENSIN: is an elongated complex of several proteins that binds and encircles DNA. In contrast to cohesin, which binds two sister chromatids together, condensin binds a single chromatid at multiple spots, twisting the chromatin into a variety of coils and loops. • SEPARASE: also known as separin, is a cysteine protease responsible for triggering anaphase by hydrolyzing cohesin, which is the protein responsible for binding sister chromatids during the early stage of anaphase. Chromosomes • The genetic information that is passed on from one generation of cells to the next is carried by chromosomes. • Every cell must copy its genetic information before cell division begins. • Each daughter cell gets its own copy of that genetic information. • Cells of every organism have a specific number of chromosomes. Chromosome Chromosomes During Cell Division • In preparation for cell division, DNA is replicated and the chromosomes condense • Each duplicated chromosome has two sister chromatids, which separate during cell division • The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached Phases of the Cell Cycle • The cell cycle consists of • Interphase (cell growth and copying of chromosomes in preparation for cell division) • Mitotic (M) phase (mitosis and cytokinesis) • Interphase (about 90% of the cell cycle) can be divided into sub phases: • G1 phase (“first gap”) • S phase (“synthesis”) • G2 phase (“second gap”) G1 Phase: Cell Growth • In the G1 phase, cells increase in size and synthesize new proteins and organelles. S Phase: DNA Replication • In the S (or synthesis) phase, new DNA is synthesized when the chromosomes are replicated. G2 Phase: Preparing for Cell Division • In the G2 phase, many of the organelles and molecules required for cell division are produced. M Phase: Cell Division • In eukaryotes, cell division occurs in two stages: mitosis and cytokinesis. • Mitosis is the division of the cell nucleus. • Cytokinesis is the division of the cytoplasm. Prophase/Prometaphase • During prophase, the first phase of mitosis, the duplicated chromosome condenses and becomes visible. • The centrioles move to opposite sides of nucleus and help organize the spindle. • The spindle forms and DNA strands attach at a point called their centromere. • The spindle forms and DNA strands attach at a point called their centromere. • The nucleolus disappears and nuclear envelope breaks down. Prophase Prometaphase Metaphase • During metaphase, the second phase of mitosis, the centromeres of the duplicated chromosomes line up across the center of the cell. The spindle fibers connect the centromere of each chromosome to the two poles of the spindle. • The spindle fibers connect the centromere of each chromosome to the two poles of the spindle. Anaphase • During anaphase, the third phase of mitosis, the centromeres are pulled apart and the chromatids separate to become individual chromosomes. • The chromosomes separate into two groups near the poles of the spindle. Telophase • During telophase, the fourth and final phase of mitosis, the chromosomes spread out into a tangle of chromatin. • A nuclear envelope re-forms around each cluster of chromosomes. • The spindle breaks apart, and a nucleolus becomes visible in each daughter nucleus. Cytokinesis • Cytokinesis is the division of the cytoplasm. • The process of cytokinesis is different in animal and plant cells. • The cell membrane is drawn in until the cytoplasm is pinched into two equal parts. • Each part contains its own nucleus and organelles. The Cell Cycle Control System • The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock • The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received. For many cells, the G1 checkpoint seems to be the most important one • If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide • If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a non dividing state called the G0 phase Loss of Cell Cycle Controls in Cancer Cells • Cancer cells do not respond normally to the body’s control mechanisms • Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue • If abnormal cells remain at the original site, the lump is called a benign tumor • Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors Phase Interphase Prophase Chromosome Appearance & Location DNA copies itself; chromatin Chromosomes coil up Important Events DNA replication, cell grows and replicates organelles Nuclear envelope disappears, spindle fibers form Chromosomes line up in the middle Spindle fibers connect to chromosomes Anaphase Chromosome copies divide and move apart Spindle fibers pull chromosome copies apart to opposite poles Telophase Chromosomes uncoil back into chromatin Nuclear envelopes reform, 2 new nuclei are formed, spindle fibers disappear Metaphase Cytokinesis Chromatin Division of the rest of the cell: cytoplasm and organelles I.P.M.A.T INTERPHASE PROPHASE METAPHASE ANAPHASE TELOPHASE Apoptosis: Programmed Cell Death Apoptosis or programmed cell death, is carefully coordinated collapse of cell, protein degradation , DNA fragmentation followed by rapid engulfment of corpses by neighboring cells. EXAMPLES The formation of the fingers and toes of thefetus The sloughing off of the inner lining of the uterus The formation of the proper connections between neurons in the brain . Apoptosis is needed to destroy cells Examples: Cells infected with viruses Cells of the immune system Cells with DNA damage Cancer cells • THE END Meiosis SEXUAL REPRODUCTION Sets of Chromosomes in Human Cells • Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes. • A karyotype is an ordered display of the pairs of chromosomes from a cell. • The two chromosomes in each pair are called homologous chromosomes, or homologs. • Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters. APPLICATION TECHNIQUE Pair of homologous replicated chromosomes Centromere Sister chromatids Organisms that reproduce Sexually are made up of two different types of cells. 1. 2. Somatic Cells are “body” cells and contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc. Gametes are the “sex” cells and contain only ½ the normal number of chromosomes…. called the “Haploid” number (the symbol is n)….. Sperm cells and ova are gametes. 3. n = number of chromosomes in the set… so….2n means 2 chromosomes in the set…. Polyploid cells have more than two chromosomes per set… example: 3n (3 chromosomes per set) Gametes • The Male Gamete is the Sperm and is produced in the male gonad the Testes. • The Female Gamete is the Ovum (ova = pl.) and is produced in the female gonad the Ovaries. • During Ovulation the ovum is released from the ovary and transported to an area where fertilization, the joining of the sperm and ovum, can occur…… fertilization, in Humans, occurs in the Fallopian tube. Fertilization results in the formation of the Zygote. (fertilized egg) Sperm + Ovum (egg) Zygote Chromosomes • If an organism has the Diploid number (2n) it has two matching homologues per set. One of the homologues comes from the mother (and has the mother’s DNA).… the other homologue comes from the father (and has the father’s DNA). • Most organisms are diploid. Humans have 23 sets of chromosomes… therefore humans have 46 total chromosomes….. The diploid number for humans is 46 (46 chromosomes per cell). Homologous Chromosomes Pair of chromosomes (maternal and paternal) that are similar in shape and size.  Homologous pairs (tetrads) carry genes controlling the same inherited traits.  Each locus (position of a gene) is in the same position on homologues.  Humans have 23 pairs of homologous chromosomes. 22 pairs of autosomes 1 pair of sex chromosomes Autosomes (The Autosomes code for most of the offspring’s traits) In Humans the “Autosomes” are sets 1 - 22 Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. ** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male. In Humans the “Sex Chromosomes” are the 23rd set XX chromosome - female XY chromosome - male Meiosis is the process by which ”gametes” (sex cells) , with half the number of chromosomes, are produced. During Meiosis diploid cells are reduced to haploid cells Diploid (2n)  Haploid (n) If Meiosis did not occur the chromosome number in each new generation would double…. The offspring would die. Meiosis in males is called spermatogenesis and produces sperm. Meiosis in females is called oogenesis and produces ova. Meiosis • Prior to meiosis I, there are 46 chromosomes, each with two chromatids (total of 92 chromatids). At the end of meiosis I (the reduction division), there will be 23 chromosomes (each having two chromatids, for a total of 46 chromatids) in each cell. In meiosis II, the 23 chromosomes split at the centromeres and the chromatids from each respective chromosome migrate to opposite poles. So, at the end of meiosis I, you have 23 chromosomes, each with two chromatids and at the end of meiosis II, you also have 23 chromosomes, however each has a single chromatid. Interphase I  Similar to mitosis interphase.  Chromosomes replicate (S phase).  Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.  Centriole pairs also replicate.  Nucleus and nucleolus visible. Meiosis I (four phases)  Cell division that reduces the chromosome number by one-half.  Four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I Prophase I • Longest and most complex phase. 90% of the meiotic process is spent in Prophase • Chromosomes condense. • Synapsis occurs: homologous chromosomes come together to form a tetrad. • Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids). During Prophase I “Crossing Over” occurs Crossing Over is one of the major occurrences of Meiosis  During Crossing over segments of nonsister chromatids break and reattach to the other chromatid. The Chiasmata (chiasma) are the sites of crossing over. Crossing Over: During Crossing over segments of nonsister chromatids break and reattach to the other chromatid. The Chiasmata (chiasma) are the sites of crossing over. nonsister chromatids creates variation (diversity) in the offspring’s traits. chiasmata: site of crossing over Tetrad variation Metaphase I  Shortest phase  Tetrads align on the metaphase plate.  INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2n Anaphase I  Homologous chromosomes separate and move towards the poles.  Sister chromatids remain attached at their centromeres. Telophase I  Each pole now has haploid set of chromosomes.  Cytokinesis occurs and two haploid daughter cells are formed. Meiosis II…….Reduction Division • No interphase II • (or very short - no more DNA replication) • Remember: Meiosis II is similar to mitosis • Prophase II • Metaphase II • Anaphase II • Telophase II : four haploid daughter cells produced. • gametes = sperm or egg Telophase II: Nuclei splits into two, followed by cytokinesis. Oogenesis Spermatogenesis Question:  A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? Answer: 10 chromosomes (haploid) Question:  A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? Answer: 10 chromosomes Non-disjunction Non-disjunction is one of the anomalies in meiosis • Non-disjunction is the failure of homologous chromosomes, or sister chromatids, to separate during meiosis. • Non-disjunction results with the production of zygotes with abnormal chromosome numbers…… • …. An abnormal chromosome number (abnormal amount of DNA) is damaging to the offspring. Non-disjunctions usually occur in one of two fashions. • The first is called Monosomy, the second is called Trisomy. • If an organism has Trisomy 18 it has three chromosomes in the 18th set • Trisomy 21…. Three chromosomes in the 21st set. • If an organism has Monosomy 23 it has only one chromosome in the 23rd set. Common Non-disjunction Disorders • • • • Down’s Syndrome – Trisomy 21 Turner’s Syndrome – Monosomy 23 (X) Kleinfelter’s Syndrome – Trisomy 23 (XXY) Edward’s Syndrome – Trisomy 18 Amniocentesis • An Amniocentesis is a procedure a pregnant woman can have in order to detect some genetics disorders…..such as non-disjunction. Amniocentesis Amniotic fluid withdrawn Karyotype (picture of an individual’s chromosomes) One of the ways to analyze the amniocentesis is to make a Karyotype What genetic disorder does this karyotype show? Trisomy 21….Down’s Syndrome • THE END Mendelian Patterns of Inheritance Fundamentals of Genetics • Genetics is the branch of science that studies how the characteristics of living organisms are inherited. • An allele is a specific version of a gene. • Examples: eye color, hair color, earlobe type • The two different alleles are on the same part of a chromosome. Fundamentals of Genetics • Gene: A hereditary unit consisting of a sequence of DNA that occupies a specific location on a chromosome and determines a particular characteristic in an organism. • An allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. • Locus: (plural loci) is the specific location of a gene or DNA sequence or position on a chromosome. Fundamentals of Genetics • The interaction of alleles determines the appearance of the organism. – The genotype of an organism is the combination of alleles that are present in an organism’s cells • Ex. BB, Bb, bb • Homozygous – two identical alleles • Heterozygous – two different alleles – The phenotype of an organism is how it appears outwardly and is a result of an organism’s genotype • Blue eyes, brown eyes Fundamentals of Genetics • A dominant allele masks the recessive allele in the phenotype of an organism – Dominant allele is usually shown by a capital letter – Recessive alleles are usually shown by a lower-case letter. • B – brown-eyes, b – blue-eyes • BB, Bb, bb. Fundame ...
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gt7428
School: UT Austin

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Meiosis

Meiosis is the process of sexual reproduction.
In humans, each cell has 23 pairs of chromosomes, making a total of 46. A karyotype is an image
display of these chromosomes. Each pair creates homologous chromosomes, which are two
chromosomes of the same length that contain information for the same traits. One set of
chromosomes (23) comes from the mother while the other comes from the father.
There are two different cell types in the body. Somatic cells are regular body cells that contain
two sets of chromosomes (2n). Gametes are the sex cells used for reproduction and they only
contain one set of chromosomes (n). N refers to the number of chromosomes (in humans, 23).
Humans have two forms of gametes, the egg and the sperm, that combine to create a zygote, or
fertilized egg.
Another way of saying that a cell has two sets of chromosomes (2n) is that it is diploid.
There are 22 pairs of autosomes and 1 pair of sex chromosomes (either XX for female or XY for
male, based on gender). Autosomes are shared for both genders and contain most of the genetic
information necessary for bodily function.
Meiosis is the process of forming gametes, or in other words the process of going from a 2n cell
to an n cell. In males it is called spermatogenesis and in females it is called oogenesis.
Meiosis follows the same steps as meiosis, with a different end result.
First the chromosomes replicate. Then the cell divides for the first time:

-

Prophase I – crossing over can occur here, meaning that a pair of chromosomes can
exchange genetic material, creating a new combination of genes

-

Metaphase I – tetrads align on the central metaphase plate, facing either direction in a
process called independent assortment

-

Anaphase I – the homologous chromosomes separate from each other

-

Telophase I – cytokinesis occurs to form two separate cells

At this point, there are two cells, each with 23 chromosomes. However, there are two copies of
each chromosome in each cell. In the next phase, these pairs separate.

-

Prophase II – genetic information prepares for separation

-

Metaphase II – pairs align on metaphase plate

-

Anaphase II – pairs separate

-

Telophase II – cytokinesis occurs

At this point, there are now four haploid (n) daughter cells, either sperm or egg, that only have
one of each of the 23 chromosomes. Meiosis is complete.
There are some potential problems that can occur during meiosis.
-

Non-disjunction – the homologous chromosomes or sister chromatids fail to disconnect
during meiosis. This creates zygotes (sperm or egg) with an irregular number of
chromosomes, which is quite damaging to the offspring. It can lead to a series of different
disorders. The first is monosomy where only one chromosome has been passed on to the
offspring and the other is trisomy, where there are three chromosomes.
o Trisomy 21 is also known as Down Syndrome
o Monosomy 23 is known as Turner’s Syndrome
o Trisomy 23 is known as Kleinfelter’s Syndrome
o Trisomy 18 is known as Edward’s Syndrome

An amniocentesis can be done during a woman’s pregnancy to test the baby’s chromosomes for
any abnormalities. A karyotype can be made to have a picture of the baby’s chromosomes to
show any genetic disorders or nondisjunction disorders.


Mendelian Patterns of Inheritance
Fundamentals of Genetics
Genetics studies how the...

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