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Transcript
Chromatid Gene – Sequence of DNA that codes for a particular protein or trait. Ex. Gene for pea plant flower color Allele – Different form of a particular gene Ex. P = purple, p = white Locus (loci) – the physical location of a gene on a chromosome Ex. Closer/farther from the centromere (colored bands on the chromosomes above) Different species have different numbers of chromosomes in their nucleus. Human body cells (somatic cells) contain 46 chromosomes. 44 autosomal chromosomes, 2 sex chromsomes Chromosomes easily visible during metaphase of Mitosis o For each replicated autosomal chromosome, there is another that is identical in length, genes and centromere location. How many pairs of chromosomes are in each cell? 22 Homologous Chromosomes (Homologous Pairs) Homologous Chromosomes – Pairs of chromosomes, one from a man and one from a woman, that carry the same genes at the same locations. Allele F Allele F Humans have 23 pairs of chromosomes 22 homologous pairs of chromosomes are called autosomal chromosomes (autosomes) o Non-sex chromosomes Women have one homologous pair of sex chromosomes (XX) Men have one pair of sex chromosomes (XY) o Homologous only at some locations We inherit one chromosome of each set (123) from our biological mothers and the other (123) from our biological fathers. Cells and Chromosome Number Di- or Dipl- two/double Diploid Cells are cell with two sets of chromosomes, one from each biological parent(123 + 123). Most cells contain homologous pairs o (1 and 1, 2 and 2, 3 and 3 etc) Ex. Body cells The Diploid number is the total number of chromosomes in the cell o Abbreviated “2n” What is the human diploid number? 2n = 46 Hapl- simple (think half) Haploid Cells are cells with one set of chromosomes These cells do not contain homologous pairs o (1, 2, 3 etc) Ex. Sex cells / Gametes/ Eggs and Sperm The Haploid number is half the Diploid number o Abbreviated “n” What is the human haploid number? n = 23 Haploid Egg (n) + Sperm (n) Zygote (2n) Diploid Fertilization What type of cell division forms gametes? Meiosis Why do cells undergo this type of division to form gametes? Keeps the chromosome number from doubling in each generation Zygote undergoes mitosis to form the multicellular adult Common Name Genus and Species Diploid Chromosome Number Buffalo Bison bison 60 Cat Felis catus 38 Cattle Bos taurus, B. indicus 60 Dog Canis familiaris 78 Donkey E. asinus 62 Goat Capra hircus 60 Horse Equus caballus 64 Human Homo sapiens 46 Pig Sus scrofa 38 Sheep Ovis aries 54 Chimpanzees, Gorillas and Orangutans have 48 chromosomes Meiosis Meiosis I XX Meiosis II X X I I I I MEIOSIS Meiosis – a type of cell division that produces 4 haploid cells from 1 diploid cell. Cells undergo Interphase prior to Meiosis Two nuclear divisions (and two cellular divisions) o Meiosis I o Meiosis II Purpose/Use – to produce sex cells/ gametes/ eggs and sperm for sexual reproduction with half the genetic material as the parent cell. (Haploid = 1 set of chromosomes) Location – Ovaries and testes Meiosis I Interphase Same events: G1, S, G2 Meiosis I Prophase I – Most complex and time consuming (90%) o Replicated chromatin supercoils into replicated chromosomes; Centromere visible o Homologous chromosomes pair – process called synapsis. Pair called a tetrad Crossing over can take place o Nucleoli disappear o Centrosomes visible and move towards the poles o Spindle fibers begin to form o Nuclear envelope breaks down o Some spindle microtubules attach to a protein structure called the kinetochore located at the centromere of each replicated chromosome o Other spindle microtubules connect to microtubules from the other centrosome Metaphase I – Middle – Homologous Pairs o Homologous Chromosome line-up along the metaphase plate Imaginary plane between the poles Kinetochores face the centrosome poles o Random Assortment/Independent Assortment Anaphase I – Separate – Homologous Pairs o Homologous Chromosomes separate Pulled by shortening microtubules to opposite poles Spindle/Spindle microtubule interactions elongate the cell Telophase I– reverse prophase o o o o Nuclear envelopes reform around genetic material Chromosomes uncoil to chromatin Nucleoli reform Meiotic spindles disappear Cytokinesis - Division of cytoplasm to both new cells Usually begins during telophase I Some cells enter an interphase after cytokinesis, other continue right into Meiosis II Questions: Are the daughter cells at the end of Meiosis I diploid or haploid? Haploid How are homologous pairs represented in the picture above? Size Are there homologous pairs in the daughter cells at the end of Meiosis I? No Are the daughter cells at the end of Meiosis I identical? No Meiosis I Meiosis II (“Mitosis Lite” – low calorie mitosis) Meiosis II Prophase II o Replicated chromatin supercoils into replicated chromosomes; Centromere visible o Nucleoli disappear o Centrosomes visible and move towards the poles o Spindle fibers begin to form o Nuclear envelope breaks down o Some spindle microtubules attach to a protein structure called the kinetochore located at the centromere of each replicated chromosome o Other spindle microtubules connect to microtubules from the other centrosome Metaphase II - Middle o Chromosomes line-up along the metaphase plate Imaginary plane between the poles Kinetochores face the centrosome poles Anaphase II – Separate – Sister chromatids separated o Centromere separates o Sister chromatids pulled by microtubules to opposite poles o Now each an unreplicated chromosome o Spindle/Spindle microtubule interactions elongate the cell Telophase II – reverse prophase o o o o Nuclear envelopes reform around genetic material Chromosomes uncoil to chromatin Nucleoli reform Meiotic spindles disappear Cytokinesis - Division of cytoplasm to both new cells Usually begins during telophase Result: 4 haploid daughter cells Not identical to each other Not identical to parent cell Used for sexual reproduction (eggs and sperm) Are cells diploid during Anaphase II? No Were there Homologous Chromosomes in Mitosis? Yes Male Meiosis Female Meiosis Meiosis I XX Meiosis II X X I I I Meiosis I Meiosis II I What is “wrong” with this diagram? Homologous Chromosomes – Pairs of chromosomes, one from a man and one from a woman, that carry the same genes at the same locations. Should be more pink Sources of Variation Alleles and Crossing Over Alleles – Different versions of the same gene. Gene for flower color o Purple allele and White allele Crossing Over Takes places during Prophase I of Meiosis Homologous chromosomes in tetrad o Chromosomes exchange corresponding segments of their chromatids (sections that carry the same genes) – 1-3 locations per chromosome pair Chiasmata (chiasma-singular) are sites where homologous chromatids are attached Tetrad Independent/Random Assortment Offspring produced from sexual reproduction are highly varied Not identical to each other (except identical twins) Not identical to parents What explains this variation? Question: How many different number combinations can you make from the following numbers: 1, 1, 2, 2? No combinations with the same number (ex. 11) Must use one of each number 21 and 12 are the same thing 4: 12, 12, 12, 12 Question: How many different number combinations can you make from the following numbers: 1, 1, 2, 2, 3, 3? Same rules 8: 123, 123, 123, 123, 123, 123, 123, 123 Question: How many different number combinations can you make from the following numbers: 1, 1, 2, 2, 3, 3, 4, 4? Same rules 16: 1234, 1234, 1234, 1234, 1234, 1234, 1234, 1234, 1234, 1234, 1234, 1234, 1234, 1234, 1234, 1234 What is the pattern? 2n 2 = number of different colors (number of sets) n = amount of different numbers of each color (Number of chromosomes/set) o 22 = 4 o 23 = 8 o 24 = 16 How does this relate to cells? Homologous pairs line up along the metaphase plate during Meiosis I The way one pair lines up is random or independent of the way other pairs line up. o XX or XX o XX xx or XX xx How many possible combinations for humans? Number chromosome sets = n = number of chromosomes in each set = 2 (23) 223 = about 8 million different combinations for each parent 8 million X 8 million = 64 trillion combinations in the zygote What is wrong with the image below? Cells should be “n” at the end of Meiosis I Karyotypes A picture/display of homologous chromosomes Arranged by size (largest to smallest*) o 23rd pair the sex chromosomes Use images of chromosomes from Metaphase of Mitosis Why? Shows: Sex of organism Women will have 23 homologous pairs on their karyotype Men will have 22 homologous pairs on their karyotype and one “X” and one “Y” Chromosomal abnormalities Number Size/Structure Does this karyotype belong to a man or a woman? How can you tell? Man: 23rd pair is one X and one Y chromosome Are these chromosomes replicated or unreplicated? Why? Replicated: picture was taken during metaphase of Mitosis. Genetic material replicated Process: White blood cells are separated from a blood sample Chemicals applied to arrest/freeze cell division at metaphase o Chromosomes replicated and supercoiled Chromosomes stained to see unique banding pattern o Allows homologous chromosomes to be paired in the picture/display Chromosomal Abnormalities: Number and Structure Most embryos with an abnormal number of chromosomes are not viable (not able to survive) Some number abnormalities have consequences less severe than death Trisomy – Three copies of a specific chromosome Trisomy 21 – 3 copies of the 21st chromosome leads to Down Syndrome Causes of Trisomy Nondisjunction during Meiosis Homologous chromosomes or sister chromatids fail to separate during Anaphase I or II of meiosis, respectively. o Risks increase with mother’s age Normal division with the wrong chromosome number Examples: Patau Syndrome - The result of an extra copy of chromosome 13. These people have serious eye, brain, circulatory defects as well as cleft palate. Children rarely live more than a few months. Edward's Syndrome - The result of an extra copy of chromosome 18. The child has almost every organ system affected. Children rarely live more than a few months. Nondisjunction Of The Sex Chromosomes - These disorders can be fatal, but many people are fine. The Y chromosome carries few genes Only one X chromosome functions in each cell Klinefelter Syndrome (XXY) – Small testes, sterile, some female body characteristics (ex. breast enlargement) Also XX+Y+: XXYY, XXXY, XXXXY XYY – No defined syndrome, taller than average Trisomy X (XXX) – Function normally, identified only by karyotype Turner Syndrome (Monosomy X) The result of only a single X karyotype in women. Women with Turner's have only 45 chromosomes (normal is 46). These people are genetically female but they do not mature sexually during puberty and are sterile. 98% percent of these fetuses die before birth. Abnormal Chromosomes Breaks in chromosomes can alter structure/function and cause disorders Deletion – loss of a chromosomal fragment Duplication – second copy of a section of chromosome from a sister chromatid. Inversion – reversal of fragment to original chromosome Translocation – attachment of a chromosomal fragment to a nonhomologous chromosome. o Reciprocal – exchange segments Which might be least harmful? Inversion? Growth and cell size