Meiosis and Genetics

Organization of genetic materialProkaryotes: 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 (replicatedchromosomes)

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

MeiosisOccurs 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:http://genetics.gsk.com/graphics/meiosis-big.gif

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 DiseasesChromosomal Diseases• Gametes have abnormal Gametes have abnormal

chromosome numbers and chromosome numbers and mutationsmutations

• Offspring inherit extra Offspring inherit extra chromosome or are missing a chromosome or are missing a chromosomechromosome

• Caused by problems with Caused by problems with meiosismeiosis

Based on: Brooks/Cole – Thomson Learning

Nondisjunction of chromosomes during meiosis

Nondisjunction and Trisomy 21

Some Characteristics of Trisomy 21 (Down Syndrome)

Genetics:

IntroductionEach 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 SegregationFor 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.