Asexual and Sexual Reproduction: Mitosis and Meiosis

Reproduction

process by which living organisms create new organisms

Asexual Reproduction

Reproduction that does not involve gametes, offspring are genetic clones

Binary Fission

Form of asexual reproduction in prokaryotes where one cell divides into two identical cells.

Mitosis

Division of somatic (non-sex) cells in eukaryotes to produce identical daughter cells

Mutation

A change in the DNA sequence that can be beneficial, neutral, or harmful.

Artificial selection

Human-directed breeding for specific traits.

Sexual Reproduction

involves fusion of haploid gametes from two individuals to form a diploid zygote.

Meiosis

cell division that reduces chromosome number by half to form gametes (haploid).

Gamete

Haploid sex cell (sperm or egg)

Haploid (n)

a cell with one set of chromosomes (eg. gametes)

Diploid (2n)

a cell with two sets of chromosomes (en body cells)

Chromosome

DNA molecule with part/ all of the genetic material

Homologous Chromosomes

Chromosome pairs (one from each parent) with the same genes but possibly different alleles.

Sister Chromatids

Identical copies of a chromosome joined together

Independent Assortment

Random distribution of maternal and paternal chromosomes into gametes.

Crossing over

Exchange of genetic material between non-sister chromatids during meiosis.

Recombinant

offspring with combinations of traits not found in either parent due to crossing over (exchange of DNA -> new combinations)

Fitness

an organism's ability to survive and reproduce

Natural Selection

Process where organisms with beneficial traits leave more offspring.

Evolution

need variation, mutations + selection over generations

Steps of Binary Fission

DNA replicates, cell elongates, cells membrane pinches inward (invagination), two identical daughter cells form.

Genetic Variation from Meiosis

Independent assortment occurs when homologous chromosomes line up randomly during Metaphase I, producing a mix of maternal and paternal chromosomes in gametes.

Crossing Over (Recombination)

Occurs in Prophase I between non-sister chromatids, produces new allele combinations.

Recombination Frequency Formula

Recombination Frequency (%) = Number of Recombinants / Total Offspring × 100.

Errors in Meiosis

Nondisjunction is the failure of chromosomes to separate, e.g., Trisomy 21 (Down syndrome).

Mutation during Meiosis

Only these are heritable and subject to natural selection.

Evolutionary Implications in Asexual Populations

Mutations are the only source of variation.

Experimental Evolution

E. coli shows how random mutations can lead to adaptation.

Evolutionary Implications in Sexual Populations

Variation arises from independent assortment, crossing over, and mutation.

Main Difference between Asexual and Sexual Reproduction

Asexual creates identical offspring using one parent, sexual combines DNA from both parents creating variation.

Why Mitosis Does Not Contribute to Evolution

It affects somatic cells, not heritable gametes.

How Crossing Over and Independent Assortment Promote Variation

They shuffle alleles, creating unique gametes with new gene combinations.

Evolutionary Advantage of Sexual Reproduction

Increases genetic diversity, which is vital for adaptation via natural selection.

Why Siblings are Genetically Different

Due to meiosis (crossing over + independent assortment) and different gametes used during fertilization.

Observation of 20% Recombinant Offspring

The genes are linked but somewhat far apart, crossing over occurred.

Mutation Occurs in Mitosis

No, only meiosis mutations affect gametes and are heritable.

Why Mutations are the Raw Material for Evolution

It introduces new DNA sequences that natural selection can act on.

Recombination Frequency Calculation

Out of 1,000 offspring, 300 are recombinant. Recombination frequency = 300/1000 x 100 = 30%. Therefore, yes the genes are linked (since < 50%).

Example of Artificial Selection

Breeding large tomatoes from wild small ones by planting seeds from the largest.