Meiosis and Genetics

Organization of genetic material Prokaryotes: DNA is circular, not associated with proteins Eukaryotes: DNA is linear, associated with proteins

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

Chromosome Structure Has two chromatids (replicated chromosomes) Chromatids held together by a region of DNA called a centromere

Stages of Mitosis

Karyotype 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

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

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).

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 GlaxoSmithKline:

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.

Meiosis I

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)

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 haploid—no homologous chromosome pairs

Mitosis/Meiosis

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).

Chromosomal Diseases Gametes have abnormal chromosome numbers and mutationsGametes have abnormal chromosome numbers and mutations Offspring inherit extra chromosome or are missing a chromosomeOffspring inherit extra chromosome or are missing a chromosome Caused by problems with meiosisCaused by problems with meiosis

Based on: Brooks/Cole – Thomson Learning Nondisjunction of chromosomes during meiosis

Nondisjunction and Trisomy 21

Some Characteristics of Trisomy 21 (Down Syndrome)

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.

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.

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

Examples of Inheritance

Dominant and Recessive Traits Brown eyes are dominant Tongue rolling ability is recessive

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.