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Biology

Biology. Meiosis. How are chromosomes classified?. Autosomes - 1 st 22 pairs of chromosomes Sex chromosomes- last set of chromosomes that determine gender Females only carry X chromosomes in egg Males carry X or Y in sperm If XX- girl If Xy- boy.

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Biology

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  1. Biology Meiosis

  2. How are chromosomes classified? • Autosomes- 1st 22 pairs of chromosomes • Sex chromosomes- last set of chromosomes that determine gender • Females only carry X chromosomes in egg • Males carry X or Y in sperm • If XX- girl • If Xy- boy

  3. What are homologous chromosomes? • Two chromosomes that both carry genes controlling the same inherited trait. • One comes from father, one from mother • Have same banding pattern • Matching bands = matching traits

  4. What is the difference between dominant & recessive alleles? • You have 2 alleles for each gene- one from mom & one from dad • Dominant allele (trait) = capital letter • Recessive allele (trait) = lowercase letter • EX: Brown eyes (B) dominant over blue (b) • Possible gene combinations could be • BB = brown eyes (homozygous dominant) • Bb = brown eyes (heterozygous) • bb = blue eyes (homozygous recessive)

  5. What is the difference between DIPLOID & HAPLOID? • Diploid (2n) = 2 sets of homologous chromosomes. • Human somatic cells = 46 • Haploid (n) = 1 set of homologous chromosomes • Human gametes = 23

  6. What is meiosis? • Produces gametes. • Reduces the number of chromosomes in the cell so that when gametes combine they have the correct number of chromosomes. • 2 major steps: • Meiosis I- homologous chromosomes separate • Meiosis II- sister chromatids separate (like mitosis)

  7. MEIOSIS I • Interphase- DNA replicates • Prophase I- • Chromatin coils up • Homologous chromosomes pair up and form a tetrad (4 sister chromatids) • Nucleus and nucleolus disappear • Spindle forms and captures tetrad of chromosomes • Metaphase I- • Tetrad lines up at middle of cell • Anaphase I- • Tetrad splits. One chromosome goes to one pole, other goes to other pole • Telophase I- • Cell splits, nucleus and other parts reform.

  8. MEIOSIS II • This is essentially the same as mitosis • The two cells from meiosis I go through these steps at the same time. • Prophase II • Spindle forms, chromosomes attach • Metaphase II • Chromosomes line up at the middle of cell • Anaphase II • Sister chromatids are pulled apart • Telophase II • Each cell splits into 2 more cells creating a total of 4 cells. http://www.johnkyrk.com/meiosis.html

  9. In females: In ovaries Produces: 1 mature egg, 3 polar bodies which break down Women born with all eggs they will have. Egg is much larger Have all X chromosomes Has no method of movement In males: In testes Produces:4 mature sperm Males begin to produce sperm after puberty, produced constantly until death Much smaller than egg May have X or Y chromosomes Have flagella to move How is meiosis different in males & females?

  10. What is INDEPENDENT ASSORTMENT? • Chromosomes can line up in any order before separating during metaphase I. • This leads to some gametes getting only mother genes and some only father genes. • This can also lead to some gametes getting mixture of mother and father’s genes. • See figure 8.16 p. 143

  11. What is CROSSING OVER? • Sometimes sister chromatids can exchange pieces of corresponding chromosome creating a new combination of traits. • Figure 8.18B p. 145 http://www.kean.edu/~fglazer/PPT/ANIMS/lw13_meiosis.swf

  12. Describe karyotyping • Picture of chromosomes are taken from a cell sample, cut out and matched up in pairs • Humans have 23 pairs of chromosomes • Karyotypes can be used to determine if genetic disorder is present • If too many are present can indicate Down’s syndrome • If some are missing can indicate Turner’s syndrome

  13. ERRORS IN MEIOSIS • Nondisjunction: • Members of a chromosome pair fail to separate. Resulting daughter cells have too many chromosomes. • Examples: • Trisomy 21- extra #21: down’s syndrome • Klinefelter’s syndrome- XXY • XYY male- taller than average, increased testosterone levels • Turner’s syndrome- XO, females with only one sex chromosomes. • See figure 8.21 A, B, C p. 148

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