The chromosomal basis of inheritance

Point: Key differences between mitosis and meiosis
Mitosis and meiosis

I. A chromosome is a single DNA molecule complexed with histone proteins.

A. DNA is highly compacted: DNA + protein = nucleosome structure
Point: chromatin = DNA + histone proteins; coiling

B. : By convention, numbering starts with the largest chromosome. 
Point: Biggest (per microscopy) is first

C. Chromosome landmarks: Via light microscopy, with appropriate preparation, certain regions of the chromosome, are visible via light microscopy and have specific functions. 

1. Centromere: _______________
   
   
2. Telomere: _______________
   
   

Nucleolus: _______________ "The most conspicuous nuclear structure observed by light and electron microscopy" Raska et al. 2004. The nucleolous and transcription of ribosomal genes. Biology of theCell. 96: 579-594. The nucleolus organizer re gion (NOR) is part of the chromosome and consists of many tandemly-repeated copies of ribosomal genes. A given species will have one or more NORs.

D.  Heterochromatin and euchromatin: terms used to describe the compactness of DNA. 

1. Heterochromatin: _______________
   
   
2. Euchromatin: _______________

II. Mitosis and meiosis

Mitosis: illustrated; animated ; very animated
Point: What happens at key stages

Mitosis results in an equal qualitative and quantitative transfer of chromosomes and their genes from one cell generation to the next.

A. Cell cycle  
Point: Names and key events at each stage.

• A period of RNA and protein synthesis, but no DNA replication.
• Period of DNA synthesis. The total DNA content goes from 2n to 4n
• Period after DNA synthesis and before Prophase. At this point a diploid cell contains two complete diploid sets of chromosomes. 
• Mitosis. Stages 1 - 3 = Interphase 

B. Mechanics of mitosis: Example: Maize; 2n=2x=20

• Prophase: Chromatin contracts. May be able to visualize that each chromosome consists of two sister chromatids. One is "original" chromosome, other is copy made during S phase. Chromatids held together at centromere. At late Prophase, attachment of spindle fibers to centromeric region and dissolution of nuclear membrane. Continued contraction of paired chromatids. Nucleolus disappears. 20 sets of 2 sister chromatids = 40 chromatids.
• Metaphase: Orientation of paired chromatids on Metaphase "plate". Stage of choice for chromosome counts. 20 sets of sister chromatids aligned on plate = 40 chromatids.
• Anaphase: Sister chromatids migrate to opposite poles. Appears as though centromeres are being pulled to opposite poles. This separation of sister chromatids accomplishes the equal distribution of chromosomal material. 20 chromatids migrate to each pole. 
• Telophase: Nuclear membrane reforms, and single chromatids, or at this point, chromosomes, return to the relaxed Interphase state. Nucleolus reappears. Cytokinesis divides original cell into two daughter cells. Two cells, each with 10 pairs of chromosomes = 20 chromosomes per cell.  Maize :  2n=2x=20

C. Telomere shortening during mitosis.

Chromosomes are capped at the ends with repetitive DNA sequences - the telomeres. Due to considerations in DNA replication, each time the cell divides, the telomere is shortened.  Eventually, the chromosomes become "so frayed that the cell senesces". In some cells- eggs, sperm, and cancer cells - an enzyme known as telomerase allows for "reconstruction" of the telomere, thus prolonging cell life.   Thus, there is indeed a fine line between life (eggs and sperm) and death (cancer).

2009 Nobel Prize in Physiology/Medicine:
Understand what telomeres are, what telomerase is, and why this research merited a Nobel Prize.

There will be more details on this issue when we delve into DNA replication.

 

III. Meiosis: illustrated ; animated ; more animated
Point: Names and key events at each stage.

Meiosis results in the reduction of the genome from the 2n to the n level.

A. Meiotic cell cycle

• Archesporial cell differentiation
• Pre-meiotic S phase: 99.7% of DNA replicated

First meiotic division

Prophase I (5 stages)

1.  Leptonema: 40% of meiotic cycle. Longitudinal duality of chromosomes not discernible.
2.  Zygonema: 20% of meiotic cycle. Pairing of homologous chromosomes. Formation of synaptonemal complex and zygotene DNA synthesis.
3.  Pachynema: 16% of meiotic cycle: Pairing persists: synaptonemal complex + crossovers. Bivalent = 2 homologous chromosomes = 2 sets of 2 chromatids. Crossing over occurs (chiasma; chiasmata).
Point: s.c. between paired homologs
4.  Diplonema: 10% of meiotic cycle. Synaptonemal complex dissolves; visualize longitudinal duality.
5.  Diakinesis: 1% of meiotic cycle; continued bivalent contraction; dissolution of nuclear envelope.

Metaphase I: 4% of meiotic cycle. Bivalents  appear on the Metaphase plate.

The random alignment of non-homologous chromosomes is the basis of independent assortment.

Anaphase I: 2% of meiotic cycle.

The physical separation of homologous chromosomes is the basis of segregation.

Telophase I. Each pole receives one-half of the original chromosome number of the meiocyte,i.e. one set of chromosomes in the case of diploidy.

Second Meiotic Division: 7% of total meiotic cycle.

Prophase II. Chromatin condensation.

Metaphase II. Chromosomes align on Metaphase plate

Anaphase II. Sister chromatids go to opposite poles.

Telophase II. Cytokinesis: Tetrad.

OWB meiosis
The VVWWNNLL x vvnnll cross
Point: Consequences of random alignment of non-homologs; Consequences of crossovers; Consequences of separation of sister chromatids
More detail in linkage section

B. Summary
Point: Key differences between mitosis and meiosis

• Homologous chromosomes pair and separate at Anaphase I; sister chromatids separate at Anaphase II.
• Crossing over may generate new configurations at alleles at linked loci. 
• Mitosis produces identical daughter cells; meiosis produces 4 daughter cells, which may be genetically different.
• Mitosis is 2n to 2n ; Meiosis is 2n to n.
• Meiosis takes longer than mitosis.
• No S phase between meiotic divisions.
• Mitosis occurs from the zygote stage onward through the life of the organism: meiosis occurs in specialized cells. 

 

Required Readings: Molecular Biology: Directing the Centromere Guardian. Jean-Paul Javerzat Science 8 January 2010: Vol. 327. no. 5962, pp. 150 - 151. DOI: 10.1126/science.1184770

Useful Readings: How telomeres solve the end-protection problem. Titia de Lange. Science. November 2009:
Vol. 326, pp. 948 - 952. DOI: 10.1126/science.1170633

Text: 31 - 57 (mitosis); 102 - 109 (meiosis)