Meiosis and Genetics

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Meiosis and Genetics
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Organization of genetic material

• Prokaryotes DNA is circular, not associated
  with proteins

• Eukaryotes DNA is linear, associated with
  proteins

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Somatic Cell Cycles

• Somatic (body) cells have a distinct cell cycle
  with four phases
• G1 growth
• S DNA is replicated
• G2 growth
• M phase cells divide by mitosis

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Chromosome Structure
Has two chromatids (replicated chromosomes) Chrom
atids held together by a region of DNA called a
centromere
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Stages of Mitosis
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Most eukaryotic cells are diploid --two sets of
chromosomes, one from each parent --chromosomes
from parents are similar, but not identical
homologous chromosomes --Humans have 46
chromosomes, or 23 pairs of chromosomes
Karyotype
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Mitosis summary

• Daughter cells have the same number of
  chromosomes as parental cells
• Daughter cells have the same DNA content as
  parental cells
• Daughter cells have identical DNA structure as
  the parental cells
• Mitosis starts with diploid cells and produces
  diploid cells

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Meiosis

• Occurs only in organisms that use sexual
  reproduction
• Specialized cell division that only occurs in
  germ cells.
• The product cells of meiosis are the gametes
  (egg, sperm).

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Overview of Meiosis

• Germ cells duplicate their DNA ONCE--prior to
  meiosis
• Two stages of meiosis, each with cell division
• Germ cells are diploid gametes are haploid

From GlaxoSmithKlinehttp//genetics.gsk.com/graph
ics/meiosis-big.gif
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Prophase I

• During meiosis I (prophase I), there is exchange
  of genetic material between chromosomes genetic
  recombination.
• Genetic recombination may allow for a competitive
  advantage by rearranging genetic material from
  generation to generation.

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Meiosis I
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Key points about Meiosis I

• Germ cells start out diploid
• Germ cells duplicate their DNA
• Homologous chromosomes exchange genetic material
  during prophase I
• Meiosis I ends with the separation of the
  homologs and the physical division of the cells
• Products of meiosis I are not diploid because
  they do not have homologous chromosome pairs
• Products of meiosis I are not haploid (yet)

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Key points about Meiosis II

• No DNA duplication prior to meiosis II
• Separation of the attached chromatids (replicated
  chromosomes)
• Four products (gametes) are genetically NOT
  identical to each other!
• Four products (gametes) are haploidno homologous
  chromosome pairs

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Mitosis/Meiosis
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Meiosis to Genetics....

• Meiosis produces gametes with a haploid
  chromosome number.
• During fertilization, these gametes unite to form
  a diploid zygote, which then develops by
  successive cell divisions into an organism.
• Thus, organisms inherit two sets of genetic
  information one from each gamete (parent).

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Chromosomal Diseases

• Gametes have abnormal chromosome numbers and
  mutations
• Offspring inherit extra chromosome or are missing
  a chromosome
• Caused by problems with meiosis

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Nondisjunction of chromosomes during meiosis
Based on Brooks/Cole Thomson Learning
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Nondisjunction and Trisomy 21
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Some Characteristics of Trisomy 21 (Down Syndrome)
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Genetics Introduction

• Each organism displays certain traits which are
  inherited from previous generations.
• The monk Gregor Mendel, through his studies of
  pea plants, discovered a mechanism for the
  inheritance of specific traits.

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Genes and Alleles

• Gene unit of information about a specific trait,
  passed from parent to offspring
• Allele all of the different forms of the gene,
  in diploid organisms, each gene has at least two
  alleles.

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Alleles Continued

• Allele combinations
• homozygous when both alleles are identical
• heterozygous when each allele is different
• Types of alleles
• Dominant alleles capital letter (D)
• Recessive alleles lowercase letter (d)
• When paired, the dominant allele will mask the
  effect of the recessive allele

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Examples of Inheritance
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Dominant and Recessive Traits

• Brown eyes are dominant

• Tongue rolling ability is recessive

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Mendel's Law of Segregation

• For a given trait (gene), the pair of alleles in
  each parent separate such that the offspring only
  inherits one allele.
• Separation of alleles occurs during the meiotic
  divisions that produce the gametes.