Mitosis and Meiosis: Cell Division Processes

Learning Outcomes

• Understand the main events of mitosis, including prophase, prometaphase, metaphase, anaphase, and telophase, as well as cytokinesis in animal cells.
• Describe the stages of meiosis: DNA replication, homologous chromosome pairing to form bivalents, crossing over, and the first and second meiotic divisions
• Compare mitosis and meiosis and explain the genetic ramification of meiosis in creating haploid gametes and generating genetic diversity.
• Know that haploid, diploid, genotype, phenotype, gene, allele, homozygote, and heterozygote are genetic terms
• Grasp the paternal generation (P), their offspring (F1), and subsequent offspring (F2) mean
• Learn how test crosses between two individuals are useful for studying inheritance patterns.
• Understand how alleles are defined as dominant or recessive, including the term "wild type."
• Learn that a monohybrid cross study different alleles of the same gene
• Learn that a dihybrid cross study the alleles of two genes on different chromosomes, where independent segregation occurs
• Outline Mendel’s experiments and explain how they led to establishing Mendel’s laws.
• Understand Punnett squares as a way to visualise the phenotypes and genotypes of offspring
• Learn how chi-square analysis evaluates the influence of chance on genetic data
• Interpret simple human pedigrees and compare the inheritance patterns of recessive and dominant autosomal diseases to ascertain the probability of offspring being carriers or affected.
• Answer the questions in Workshop 1, Mendelian Genetics and Pedigrees.

Genetic Information

• Genetic information is organised from genome to chromosomes to genes
• DNA encodes genetic information which is translated into proteins, which perform biological functions

Genome Organisation: Diploids vs Haploids

Humans

• Have 2 matching sets of homologous chromosomes
• Possess 2 copies of every gene
• Chromosome complement is 2N
• Receive one set of chromosomes from their father and one from their mother
• Sets are generated through meiosis in parents
• Two haploid gametes fuse to form a diploid zygote

Bacteria

• Possess 1 set of chromosome(s)
• There is one copy of every gene
• Chromosome complement: 1N
• Receive one set of chromosomes from a parent cell
• Generated by mitosis
• One haploid cell divides to generate two haploid cells

What is a Gene?

• A gene is a basic heredity unit
• Genes are located on a specific part of a chromosome
• Contains DNA sequences that code for a protein

Genes Come in Different Forms

• Members of the same species have the same set of genes
• For example, all Drosophila melanogaster possess the same gene for eye colour
• A specific gene may come in different forms
• For example, one form of they eye colour gene can cause white eyes and another can cause red eyes
• The same species differ because they possess different forms of the same genes

Alleles are Different Forms of the Same Gene

• An allele refers to one of two or more alternative forms of a gene
• Alleles has specific difference in DNA sequence

Genes vs Alleles

• A gene is a functional unit of heredity, comprising a stretch of DNA that codes for one or more proteins
• An allele is one form of a gene that exhibits a specific change in its DNA sequence

What is a Genotype?

• Genotype is the genetic composition of an organism, referring to the set of alleles in the organism
• Humans are diploid, possessing two copies of each gene
• Refers to the alleles for both copies of a given gene

Different Genotypes

• Diploid organisms have a genotype for a given gene such as Homozygous/homozygote, where same alleles are present
• Or heterozygous/heterozygote containing different alleles

What is a Phenotype?

• Phenotype is the set of observable properties of an organism
• Produced by a combination of genotype and environment

Wild Type

• Wild type is the genotype or phenotype that is most prevalent in nature or in the laboratory stock of an organism

From Genotype to Phenotype: Dominant and Recessive Alleles

• In heterozygotes, if one allele phenotypically expresses over the other, it’s considered dominant
• In heterozygotes, if one allele is phenotypically expressed only in either the homozygous state, it is considered recessive
• Dominant and recessive relationships are only meaningful when comparing two specific alleles

Summary: Genes and Alleles

• Alleles are different forms of a single gene that generate diverse types of proteins.
• Some alleles are recessive and encode a non-functional protein, which can be masked by a functional protein from a wild-type allele.
• Also, some alleles are dominant, encoding a disruptive protein capable of blocking the activity of the normal protein.

Cell Division: Mitosis and Meiosis

• Mitosis includes 1 diploid cell dividing to generate 2 diploid cells
• Mitosis main purpose is to make more cells
• Meiosis includes 1 diploid cell dividing to generate 4 haploid cells
• Recombination occurs
• Meiosis main purpose is to make gametes

Mendel’s Law of Segregation

• Organisms' characteristics are controlled by "factors" (genes) carried in pairs but occur singly in gametes.
• Monohybrid crosses show loss of white phenotype (F1) and then reappearance in (F2) in a 3:1 ratio
• Genes are represented twice in diploids: one copy on each chromosome of a homologous pair
• The two chromosomes in a homologous pair are separated at meiosis to generate haploid gaemtes

Mendel’s Law of Independent Assortment

• Genes for different traits assort independently from each other during gamete formation
• 9:3:3:1 ratio of phenotype combinations, need genes with 2 alleles each to get these combinations
• Random alignment of chromosomes in meiosis I means “maternal” and “paternal" copies of genes are assigned to gametes in random combinations
• linked genes violate this law

The Basis for Mendel’s Traits

• Genes and mutations leading to various traits have been identified using modern molecular genetics
• Their normal functions in biochemical pathways explain how the traits Mendel studied come about

Summary

• Differences between genes and alleles
• Haploid vs diploid genetic states
• Genotypes (homozygotes, heterozygotes)
• Phenotypes and the relationships
• Mitosis and Meiosis
• Dominant and recessive alleles
• Working out a cross using a Punnett's Square
• Follow inheritance of one or two genes during a cross
• What types of offspring are generated from certain parents
• Mendel's Laws
• Segregation
• Independent Assortment
• Explain Mendel's Laws using knowledge of meiosis
• What types of gametes are made
• How these gametes combine to form a zygote with a specific genotype, which in turn generates a particular phenotype
• The biochemical and genetic basis for Mendel's phenotypes