Lecture 4 (Discussion) - Mitosis, Meiosis, and Gene Mapping

Meiosis

Genetic information copied and physically set aside in gametes (eggs and sperm)

Mitosis

Genetic information copied so each daughter cell contains full copy of parental DNA

Mitosis occurs in both….

diploid and haploid cells

Mitosis consists of

Interphase, Prophase, Metaphase, Anaphase, Telophase (IPAT)

Early Prophase

• Chromosomes and centrioles replicate in G2

• Centriole pairs move apart

• Chromosomes shorten and thicken (start becoming visible)

• Nucleolus disappears

Late Prophase

• Mitotic spindle assembles between centrioles (outside nucleus)

• Structure made of microtubules composed of tubulins

• Each duplicated chromosome becomes visible as two sister chromatids

• Nuclear envelope breaks down

Metaphase

• Kinetochore microtubules orient duplicated chromosomes on the metaphase plate

• Metaphase plate = plane halfway between 2 spindle poles

• Long axes of chromosomes oriented at 90 degrees to the spindle axis 

Anaphase

• Joined centromeres of sister chromatids separate, giving rise to 2 daughter chromosomes

  • Paired kinetochores on each chromosome separate

  • Sister chromatids separate (disjunction)

  • Daughter chromosomes move to opposite poles

• Shortening kinetochore microtubules pull chromosomes to poles

Early Anaphase

• Two centromeres of each sister chromatid pair separates

• Resulting daughter chromosomes begin migration to opposite poles

Late Anaphase

• 2 sets of daughter chromosomes approach the poles

• Cytokinesis usually begins

Telophase

• Nuclear envelopes begin forming

• Chromosomes begin to become extended and less visible

• Nucleolus reforms

• Cytokinesis continues

• Compartmentalizes the 2 nuclei into 2 daughter cells

  • Completes mitosis and cell division

• Constriction forms in the middle of the cell continuing until the 2 cells are produced

Cytokinesis

division of cytoplasm completed by the end of telophase

Meiosis occurs in…

ONLY in diploid cells

Meiosis

two successive divisions of a diploid nucleus after one DNA replication cycle

• Diploid to haploid to haploid

Meiosis I (first meiotic division)

• Chromosome number is reduced from diploid to haploid

• Consists of five stages

  • Prophase I, Metaphase I, Anaphase I, and Telophase I

Meiosis II (second meiotic division)

• Like mitotic division

• Consists of five stages as well

• Prophase II, Metaphase II, Anaphase II, and Telophase II

Prophase I

chromosomes are already duplicated, they shorten and thicken

• Crossing over occurs, causing a reciprocal physical exchange of chromosome segments

• Positions where crossing over occurs are largely random

Metaphase I

kinetochore microtubules align on each chromosome pair on metaphase plate

Anaphase I

chromosomes in each pair separate and begin migrating toward opposite poles

Telophase I

chromosomes complete migration to the poles and new nuclear envelopes may form

• Cytokinesis occurs dividing the cytoplasm to create two haploid cells

Prophase II

chromosomes condense and a spindle forms

Metaphase II

movement of kinetochore microtubules aligns chromosomes on metaphase plate

Anaphase II

centromeres separate causing daughter chromosomes to be pulled to opposite poles of spindle

Telophase II

chromosomes decondense and nuclear envelope forms around each set of chromosomes

• Cytokinesis occurs again, dividing the cytoplasm and producing four haploid cells (gametes in animals)

• Due to crossover, each chromosome will not be exact copies of each other

Mitosis (Recap)

• Somatic cells

  • Every cell except germ line

• Produces 2 haploid daughter cells

• 1 round of replication and 1 round of
  division

• Purpose = growth

Meiosis (Recap)

• Germ line cells

  • Cells that create gametes (eggs and sperm)

• Produces 4 recombinant haploid cells

• 1 round of replication and 2 rounds of division 

• Purpose = reproduction 

Parentals

• Original genotypes of 2 chromosomes

• Can describe phenotypes

  • “parental genotypes” or “parental classes”

Recombinants

• Nonparental combinations of linked alleles

• 50% recombinant phenotypes expected with independent assortment

  • Lower percentage = more evidence of gene linkage

Gene Linkage Maps

a map which determines the distance between genes 

• Based on recombination frequency 

Frequency of recombination

how often two genes have a recombination event occur between them

• used to create a gene linkage map

Coupling arrangement

one homolog has two wild-type alleles, other has two recessibe mutant alleles

• AB//ab or AB/ab

Repulsion arrangement

each homolog contains wild-type and recessive mutant allele

• Ab//aB or Ab/aB

Map units (mu)

genetic distance between genes (1 map unit = 1% crossing-over)

• also called a centimorgan (cM

Recombination Frequencies - In a doubly heterozygous individual, alleles can be arranged in two ways:

w+ m+ / w m OR w+ m / w m+