Mitosis and Meiosis

Variation Transmission and Meiosis

Introduction

• This lecture focuses on how genetic variation, originating from mutations, is transmitted and amplified through independent assortment and recombination.

• It centers around mitosis (cell division producing identical copies) and meiosis (diploid germline cell division producing haploid gametes).

Learning Objectives:

• List three differences between mitosis and meiosis.

• Explain how meiotic segregation transmits alleles to offspring, contributing to population variation.

• List two processes during meiosis that increase variation among gametes.

Phenotypic Diversity and Biological Explanation

• Biologists explain phenotypic diversity using a downward-looking (functional) approach, studying smaller scales.

• Coding and noncoding differences explain antigen diversity, but the origin of these differences needs further explanation.

The Variation Factory

• The "variation factory" is a metaphor for the engine of variation, causing differences in coding and noncoding DNA.

• It includes mutation (previous lecture), recombination, and independent assortment (this lecture).

• Recombination and independent assortment occur during meiosis.

The process of mutation occurs mainly during DNA replication prior to mitosis and meiosis, whereas the processes of recombination and independent assortment only occur during meiosis.

Mitosis and Genetic Similarity

• Somatic cells in a person's body are genetically identical due to mitosis.

• Fertilization: Union of sperm and egg cells.

• Zygote: Diploid cell formed by fertilization of a haploid egg by a haploid sperm.

• The zygote undergoes mitosis to form the embryo, followed by further mitotic divisions and differentiation during development.

• Mitosis produces two identical daughter cells from a starting cell; these cells are identical to each other and to the original cell.

• Mitosis produces all somatic cells in the body, except gametes.

• Mitosis explains why all somatic cells have the same genetic material.

Mitosis and the Variation Factory

• Mitosis contributes less to the variation factory compared to meiosis.

• Genetic variation during mitosis in somatic cells is not heritable (though important for health, e.g., cancer).

Meiosis and Genetic Variation

• Meiosis is closely involved with the variation factory.

• Mutations in meiotic cells (leading to gametes) are heritable.

• Genetic recombination and independent assortment greatly increase variation in gametes.

The Process of Meiosis

• Gametes from an individual have different DNA sequences due to meiosis.

• Meiosis produces gametes in humans and most animal species.

• It starts with a diploid premiotic cell (originally produced by mitosis) set aside as a germline cell.

• Diploid cells have two copies of each chromosome and each chromosome pair are undergoing meiosis.

• DNA is replicated before meiosis, and each chromosome consists of two sister chromatids.

• Duplicated chromosomes pair with each other, allowing recombination to occur.

• Pairing of homologous chromosomes and recombination occur in meiosis but not mitosis.

• The first cell division of meiosis takes place, with sister chromatid pairs remaining together.

• The second meiotic cell division results in four haploid cells, which may mature into gametes.

• In human female meiosis, only one cell develops into an egg cell, with other cells becoming support cells.

• In human male meiosis and most animals, all four cells mature into sperm cells.

• Gametes have one copy of each chromosome (23 in humans).

Alleles and Meiosis

• Consider an individual with genotype Aa (one chromosome with allele A, the other with allele a).

• The two copies of the chromosomes have different alleles because the sperm and egg that united to make the zygote had different alleles of the genes.

• Mitosis explains why all cells of an Aa individual also have genotype Aa.

• During meiosis, alleles segregate, with half the daughter cells having allele A and the other half having allele a.

• Segregation: Alleles that were together in the pre-meiotic cell separate.

• Each individual produces a large number of gametes with approximately 50% having one allele and 50% having the other.

• Healthy gametes are haploid, having only one allele.

Key Points About Meiotic Segregation:

• Daughter cells are haploid, with only one of the two alleles.

• Daughter cells develop into gametes.

• About 50% of gametes have one allele and 50% have the other allele.

Human Allele Packaging

• Humans package alleles from 20,000-25,000 genes into gametes during meiosis.

• Each sperm and egg contain one copy/allele of each gene.

• Chromosomes are transmitted as single units, with one of each chromosome in each gamete.

Genetic Recombination and Independent Assortment

• Two processes increase genetic variation among gametes: genetic recombination and independent assortment.

• Genetic recombination creates new allele combinations by physically exchanging DNA between copies of the same chromosome.

• Independent assortment creates diverse combinations of chromosome copies (and alleles) among gametes.

• Both processes occur in meiosis but not mitosis.

• These processes greatly increase variation among gametes, making it extremely unlikely for two gametes from the same individual to be genetically identical.

Comparing Mitosis and Meiosis

Similarities:

• The starting cell is diploid.

• DNA replication occurs prior to both processes.

• Cell division takes place to produce new cells.

Differences:

Big Ideas of Genetics Unit

• Genetics involves functional explanations of phenotypic diversity.

• Genetic variation underlies phenotypic diversity, occurring in both coding and noncoding DNA sequences.

• Three causes of genetic variation: mutation, recombination, and independent assortment.

Review:

• Be familiar with the main events in meiosis and how it differs from mitosis.

Evolution

• Understanding the variation factory prepares us to discuss how heritable traits change in populations over time (evolution).

Variation Transmission and Meiosis

Introduction

• This lecture focuses on how genetic variation, originating from mutations, is transmitted and amplified through independent assortment and recombination. The understanding of these mechanisms is crucial for grasping the diversity observed in biological systems.

• It centers around mitosis (cell division producing identical copies) and meiosis (diploid germline cell division producing haploid gametes). These processes are fundamental to life, with mitosis ensuring genetic consistency in somatic cells and meiosis generating genetic diversity in gametes.

Learning Objectives:

• List three differences between mitosis and meiosis. Understanding these differences is key to appreciating how each process contributes to genetic stability and variation.

• Explain how meiotic segregation transmits alleles to offspring, contributing to population variation. This segregation is a direct result of how chromosomes align and separate during meiosis.

• List two processes during meiosis that increase variation among gametes. These processes—recombination and independent assortment—are primary drivers of genetic diversity.

Phenotypic Diversity and Biological Explanation

• Biologists explain phenotypic diversity using a downward-looking (functional) approach, studying smaller scales. This approach involves breaking down complex traits into their underlying genetic and molecular components.

• Coding and noncoding differences explain antigen diversity, but the origin of these differences needs further explanation. Understanding these differences is crucial for developing effective vaccines and immunotherapies.

The Variation Factory

• The "variation factory" is a metaphor for the engine of variation, causing differences in coding and noncoding DNA. This engine drives the continuous generation of new genetic combinations.

• It includes mutation (previous lecture), recombination, and independent assortment (this lecture). These three processes are the main sources of genetic variation.

• Recombination and independent assortment occur during meiosis, emphasizing meiosis's role in generating genetic diversity.

The process of mutation occurs mainly during DNA replication prior to mitosis and meiosis, whereas the processes of recombination and independent assortment only occur during meiosis.

Mitosis and Genetic Similarity

• Somatic cells in a person's body are genetically identical due to mitosis. This genetic consistency ensures that each cell performs its designated function correctly.

• Fertilization: Union of sperm and egg cells. Fertilization restores the diploid number of chromosomes and initiates the development of a new organism.

• Zygote: Diploid cell formed by fertilization of a haploid egg by a haploid sperm. The zygote is the first cell of a new organism, containing genetic material from both parents.

• The zygote undergoes mitosis to form the embryo, followed by further mitotic divisions and differentiation during development. These mitotic divisions ensure that all cells in the developing organism have the same genetic information.

• Mitosis produces two identical daughter cells from a starting cell; these cells are identical to each other and to the original cell. This identity is crucial for maintaining tissue integrity and function.

• Mitosis produces all somatic cells in the body, except gametes. All cells that are not involved in sexual reproduction are produced by mitosis.

• Mitosis explains why all somatic cells have the same genetic material. This ensures that all cells can perform their specific functions based on the genetic instructions.

Mitosis and the Variation Factory

• Mitosis contributes less to the variation factory compared to meiosis. Mitosis primarily maintains genetic consistency.

• Genetic variation during mitosis in somatic cells is not heritable (though important for health, e.g., cancer). Somatic mutations can lead to diseases like cancer but are not passed on to future generations.

Meiosis and Genetic Variation

• Meiosis is closely involved with the variation factory. Meiosis is a key source of genetic diversity.

• Mutations in meiotic cells (leading to gametes) are heritable. These mutations can be passed on to future generations, influencing their traits.

• Genetic recombination and independent assortment greatly increase variation in gametes. These processes ensure that each gamete has a unique combination of genetic material.

The Process of Meiosis

• Gametes from an individual have different DNA sequences due to meiosis. This diversity is essential for evolution and adaptation.

• Meiosis produces gametes in humans and most animal species. Gametes are specialized cells used for sexual reproduction.

• It starts with a diploid premiotic cell (originally produced by mitosis) set aside as a germline cell. This cell undergoes meiosis to produce haploid gametes.

• Diploid cells have two copies of each chromosome and each chromosome pair are undergoing meiosis. These chromosome pairs are called homologous chromosomes.

• DNA is replicated before meiosis, and each chromosome consists of two sister chromatids. Each sister chromatid is an identical copy of the chromosome.

• Duplicated chromosomes pair with each other, allowing recombination to occur. This pairing ensures that homologous chromosomes are aligned correctly for recombination.

• Pairing of homologous chromosomes and recombination occur in meiosis but not mitosis. This is a key difference between the two processes.

• The first cell division of meiosis takes place, with sister chromatid pairs remaining together. This division separates homologous chromosomes.

• The second meiotic cell division results in four haploid cells, which may mature into gametes. Each of these cells has a unique combination of genetic material.

• In human female meiosis, only one cell develops into an egg cell, with other cells becoming support cells. This ensures that the egg cell has enough resources for development.

• In human male meiosis and most animals, all four cells mature into sperm cells. This maximizes the number of potential offspring.

• Gametes have one copy of each chromosome (23 in humans). This is the haploid number, which is half the number of chromosomes in somatic cells.

Alleles and Meiosis

• Consider an individual with genotype Aa (one chromosome with allele A, the other with allele a). This individual is heterozygous for this particular gene.

• The two copies of the chromosomes have different alleles because the sperm and egg that united to make the zygote had different alleles of the genes. This is why individuals have two alleles for each gene.

• Mitosis explains why all cells of an Aa individual also have genotype Aa. Mitosis ensures that all cells in the body have the same genetic information.

• During meiosis, alleles segregate, with half the daughter cells having allele A and the other half having allele a. This segregation is due to the way chromosomes line up and separate during meiosis.

• Segregation: Alleles that were together in the pre-meiotic cell separate. This ensures that each gamete gets only one allele for each gene.

• Each individual produces a large number of gametes with approximately 50% having one allele and 50% having the other. This is due to the random segregation of chromosomes during meiosis.

• Healthy gametes are haploid, having only one allele. This is necessary for maintaining the correct chromosome number after fertilization.

Key Points About Meiotic Segregation:

• Daughter cells are haploid, with only one of the two alleles. This is crucial for sexual reproduction.

• Daughter cells develop into gametes. Gametes are specialized cells used for sexual reproduction.

• About 50% of gametes have one allele and 50% have the other allele. This ensures genetic diversity in offspring.

Human Allele Packaging

• Humans package alleles from 20,000-25,000 genes into gametes during meiosis. This vast number of genes contributes to the complexity of human traits.

• Each sperm and egg contain one copy/allele of each gene. This ensures that offspring inherit a complete set of genes from each parent.

• Chromosomes are transmitted as single units, with one of each chromosome in each gamete. This ensures that each gamete has the correct number of chromosomes.

Genetic Recombination and Independent Assortment

• Two processes increase genetic variation among gametes: genetic recombination and independent assortment. These processes are unique to meiosis and greatly enhance genetic diversity.

• Genetic recombination creates new allele combinations by physically exchanging DNA between copies of the same chromosome. This exchange results in chromosomes with new combinations of alleles.

• Independent assortment creates diverse combinations of chromosome copies (and alleles) among gametes. This occurs because chromosomes align and separate randomly during meiosis.

• Both processes occur in meiosis but not mitosis. This is a key difference between the two types of cell division.

• These processes greatly increase variation among gametes, making it extremely unlikely for two gametes from the same individual to be genetically identical. This diversity is essential for adaptation and evolution.

Comparing Mitosis and Meiosis

Similarities:

• The starting cell is diploid. Both processes start with a cell that has two sets of chromosomes.

• DNA replication occurs prior to both processes. This ensures that each daughter cell receives a complete copy of the genetic material.

• Cell division takes place to produce new cells. Both processes involve the division of a cell into two or more daughter cells.

Differences:

Big Ideas of Genetics Unit

• Genetics involves functional explanations of phenotypic diversity. Understanding genetics helps explain why individuals have different traits.

• Genetic variation underlies phenotypic diversity, occurring in both coding and noncoding DNA sequences. Both types of DNA sequences contribute to individual differences.

• Three causes of genetic variation: mutation, recombination, and independent assortment. These processes are the main sources of genetic diversity.

Review:

• Be familiar with the main events in meiosis and how it differs from mitosis. This knowledge is crucial for understanding genetics and inheritance.

Evolution

• Understanding the variation factory prepares us to discuss how heritable traits change in populations over time (evolution). This is the foundation for understanding how species adapt and change over generations.