Lecture 1: Mitosis and Meiosis

Lecture 1: Mitosis and Meiosis

• Genetics impacts many areas of biology, including disease, the link between phenotype and genotype, development, immunity, and food production.
• Most diseases are polygenic or quantitative in their inheritance.
• Geneticists use model organisms, such as Drosophila melanogaster, Escherichia coli, Mus musculus, and Arabidopsis thaliana.
• These organisms excel as models due to short generation times, controlled crosses, large numbers of offspring from those crosses, and the ability to be raised in lab settings with associated benefits for agriculture and human consumption.
• Today, whole genomes can be sequenced within a day and cost less than $180 per completed genome. This is a stark contrast to when the human genome was sequenced in 2003.

Tools and Organisms

• Key model organisms are used in genetics research due to their short generation times, allowing for the examination of multiple generations rapidly.
• Controlled crosses (where the parental genotypes are known and carefully controlled) allow for tracing patterns of inheritance.
• Model organisms can produce a large number of offspring, providing ample material for study.
• Researching in lab settings is useful as it allows for a controlled and consistent environment.
• Model organisms are useful for understanding genetic processes in general as well as their impact on agriculture and human consumption.

Human Health: Determining the Genetic Basis of Disease

• Normal and mutant beta-globin DNA and mRNA sequences, along with their corresponding amino acid sequences, are presented for comparative analysis.

Chromosomes

• Genes are located on chromosomes.
• Alleles are variations of genes.
• A diploid organism has two sets of chromosomes organized as homologous pairs.
• Human somatic cells have 22 pairs of autosomes and one pair of sex chromosomes.
• There are different types of chromosomes based on centromere position, including metacentric, submetacentric, acrocentric, and telocentric.
• A chromosome consists of two sister chromatids that are joined together at the centromere region.

Nomenclature of Chromosomes and Chromatids

• A chromosome can consist of a single chromatid at some points during the cell cycle or two sister chromatids.
• Telomeres are the stable ends of chromosomes.
• A kinetochore is a protein structure associated with the centromere and is used for spindle fiber attachment.

Point of Confusion: Homologous Chromosomes vs. Chromatids

• Homologous pairs constitute a complete set of chromosomes.
• Homologous chromosomes are not identical, but rather one chromosome from each parent.
• Diploid species have each chromosome present twice.
• Chromatids are copies of a chromosome, created during the S-phase of the cell cycle.
• Chromatids are genetically identical.

The Cell Cycle

• The cell cycle has distinct phases and checkpoints that regulate progression through the cycle.
• Interphase, mitosis, and cytokinesis are central to the cell cycle.

Mitotic Phases

• Interphase, Prophase, Prometaphase, Metaphase, Anaphase, Telophase and Cytokinesis.
• These are distinct phases within the process of mitosis, which, along with cytokinesis, is part of the general cell cycle.

Moving Chromosomes

• Chromosomes move due to the disassembly of tubulin-based structures.
• Gene families of proteins maintaining the cytoskeleton are involved in chromosome movements.
• Motor proteins disassemble tubulin via ATP hydrolysis.

The Kinetochore

• The movement of chromosomes during anaphase is tightly controlled.
• The kinetochore is composed of 90 proteins.

Errors in Cell Division

• Errors in cell division can lead to tumor formation and abnormal chromosome structures, such as the presentation of marker chromosomes with abnormal or atypical banding patterns.

Meiosis: Essential for Sexual Reproduction

• Meiosis is essential for sexual reproduction and increases variation.
• Meiosis has two rounds of division to produce gametes with one copy of each chromosome.
• Meiosis differs from mitosis by having two consecutive divisions leading to genetic variation.

Meiosis is preceded by interphase, G1, S, and G2; Two rounds of division; Leads to four haploid daughter cells

• Meiosis is preceded by interphase (G1, S, G2) and involves two rounds of division, creating four haploid daughter cells.

Prophase I is a lengthy process: Split into 5 sub stages.

• Prophase I is a complex phase in meiosis with five identifiable stages, namely Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.

Homologous pairs form tetrads. Synapsis begins in zygotene; Formation of chiasmata initiated in diplotene

• Homologous paired chromosomes form tetrads. Synapsis (connecting) begins in zygotene. At diplotene, the chiasmata (points of crossover) start formation.

Synaptonemal complex

• The synaptonemal complex is a protein structure facilitating the process of synapsis between homologous chromosomes.

MLH1 and initiation of crossing over

• MLH1, a MutL homolog protein, plays a role in controlling crossing over and localizes to areas of chiasma.

Other stages of meiosis I very similar to mitosis.

• The stages of meiosis I (metaphase, anaphase, telophase) are similar, yet distinct, to their counterparts in mitosis.

Meiosis II: Second round of division very like mitosis; End result is four haploid daughter cells.

• Meiosis II is very similar to mitosis and produces four haploid daughter cells.

Sources of genetic variation in meiosis

• Variation in genetic material and separation of chromosomes during meiosis is a substantial factor in variation.

Separation of homologous chromosomes

• Whether a gamete inherits a maternal or paternal chromosome copy is random during meiosis.

How does this generate variation? Random separation of chromosomes gives 2ⁿ number of combinations; In humans 2²³= 8,388,608.

• The random assortment of chromosomes during meiosis leads to a large number of combinations, which contributes enormously to genetic variation.
• Human cells have 23 pairs of chromosomes, allowing 8.4 million possible combinations.

Separation of genetic material

• Cohesin proteins are critical for holding chromatids together, particularly during S phase and G2.
• At the onset of anaphase and during meiosis I, cohesin proteins are degraded, allowing chromatids to separate
• Meiosis-specific cohesin holds the duplicated chromosomes together during the first meiotic division, but cohesin at the centromere is protected by shugoshin to prevent separation before meiosis II.

Initiation of meiosis

• The precise mechanism for initiating meiosis depends on the species and can be very different between mammals, for instance.
• In mammals, there are hormonal regulatory elements and signaling pathways that trigger meiosis.

Lecture Summary

• Model organisms provide a valuable system for genetic research.
• The stages of mitosis and meiosis are detailed in terms of their importance in cellular division and genetic variation generation.
• Recombination and random separation of homologous chromosomes during meiosis play a major role in generating variation in offspring from sexual reproduction.

This quiz explores the foundational concepts of mitosis and meiosis as they relate to genetics. It highlights the importance of model organisms in genetic research and discusses advances in genome sequencing. Understand how genetics influences various biological fields, including disease and food production.