Reproduction & Inheritance in Biology

Reproduction

Contents

  • Gene Locus

  • Meiosis & Variation

  • Mammalian Gametes

  • Fertilisation - Mammals

  • Fertilisation - Flowering Plants

  • The Cell Cycle & Mitosis

Gene Locus

Gene Loci

  • Definition: A gene is a segment of DNA that codes for a single polypeptide or protein. A gene’s position on a chromosome is known as its locus (plural: loci). Every chromosome consists of long DNA molecules, containing hundreds to thousands of genes.

  • Gene Mapping: Through experiments and genetic mapping techniques, scientists have identified the specific physical locations of genes on different chromosomes.

  • Alleles: Variants of a gene that exist in two or more forms are referred to as alleles. Different alleles of a gene have slightly different nucleotide sequences but occupy the same locus on a chromosome.

  • Example: The presence of five different genes at five different loci would showcase the diversity of genes on a chromosome.

Gene Linkage

  • Linked Genes: Gene loci are said to be linked if they are located on the same chromosome. Autosomal Linkage refers to genes located on human autosomes (chromosomes 1 to 22, which are not sex chromosomes) and represents a situation where they are inherited together.

  • Sex Linkage: Involves genes located on sex chromosomes. Females have two X chromosomes (XX), while males have one X and one shorter Y chromosome (XY). Sex-linked genes often prominently exhibit different inheritance patterns across sexes.

Autosomal Linkage

  • Autosomal linkage implies that two or more genes do not assort independently during meiosis because they are on the same autosome, retaining their original parental combinations.

Sex-Linked Genes

  • Genes that are only present on one sex chromosome are dependent on the sex of the individual for expression and inheritance.

  • Example: Many sex-linked genes are associated with the X chromosome. Thus, males, having only one X chromosome, are more likely to express recessive conditions inherited from their mothers, while females, with two X chromosomes, may be carriers if they possess one dominant allele.

Pedigree Diagrams and Punnett Squares

  • Identification: The presence of sex linkage can be discerned using genetic tools like pedigree diagrams and Punnett squares. They allow predicting the likelihood of sex-linked traits being expressed across generations. For instance, in a cross between a normal male and a carrier female for color blindness, different probabilities for color blindness in offspring are determined:

    • 25% chance of a colorblind male

    • 25% chance of a female carrier

    • 25% chance of a normal female

    • 25% chance of a normal male.

Meiosis & Variation

Independent Assortment

  • Definition: Meiosis is a type of cell division that produces gametes and leads to genetic diversity among them. It consists of two main divisions:

    1. Meiosis I

    2. Meiosis II

Meiosis I

  • The parent cell is diploid (2n), containing two sets of chromosomes which replicate before this stage. The homologous chromosomes are separated, resulting in two haploid nuclei. During this phase, the chromosome number halves from 2n to n.

Meiosis II

  • The chromatids separate during Meiosis II, resulting in four genetically diverse haploid nuclei, with each chromosome now consisting of a single chromatid.

Mechanisms Increasing Genetic Diversity
Independent Assortment

  • During metaphase I of meiosis I, homologous pairs of chromosomes align randomly along the spindle’s equatorial plane, contributing to the variation in genetic combinations.

  • Formula for Combinations: The number of different chromosome combinations resulting from independent assortment can be calculated using the formula 2^n, where n is the number of chromosomes in a haploid cell. For humans, this number leads to approximately 8,324,608 combinations.

Crossing Over

  • Definition: Crossing over occurs during prophase I when non-sister chromatids exchange genetic material. This process results in new allele combinations.

  • Mechanism: Homologous chromosomes form bivalents, allowing segments of chromatids to break and reconnect. Points of entanglement are called chiasmata. Genetic diversity is heightened because of the swapping of alleles during crossing over, usually occurring at multiple chiasmata in each bivalent, and is more frequent toward the distal ends of chromosomes.

Genetic Variation Beyond Chromosomes

  • Mutations: While meiosis contributes significantly to genetic variation, mutations can also occur at the level of genes during DNA replication. Such random mutations can lead to the formation of new alleles enhancing variation.

Mammalian Gametes

Definition and Role

  • Gametes: The sex cells (sperm and egg) of an organism, which are haploid cells containing one of each chromosome. Through fertilisation, these gametes combine to create a diploid zygote containing genetic material from both parents.

Specialisation of Mammalian Gametes

  • Sperm Cells:

    • Have a flagellum (tail) for mobility towards the egg.

    • Rich in mitochondria for energy production.

    • Possess an acrosome, containing enzymes to penetrate the zona pellucida of the egg cell.

  • Egg Cells:

    • Are larger than sperm and contain nutrients for the embryo.

    • Surrounded by follicle cells for protection and have a zona pellucida which prevents polyspermy (entry of multiple sperm).

Adaptations Summary

  • Visual diagrams illustrate the structure and adaptations of both sperm and egg cells to enhance their respective functions in fertilisation.

Fertilisation - Mammals

Process Overview

  • Fertilisation: Occurs when sperm and egg nuclei fuse, resulting in a zygote. The sperm travels through the female reproductive system, reaching the oviduct where fertilisation is likely to occur shortly after ovulation.

Mechanisms in Fertilisation

  • Acrosome Reaction: Upon reaching the egg cell, the sperm’s nucleus releases enzymes to facilitate penetration through the zona pellucida.

  • Cortical Reaction: After sperm entry, the egg releases cortical granules, leading to thickening of the zona pellucida, preventing further sperm entry.

  • The fusion of the sperm and egg nuclei creates a zygote, which will undergo mitotic divisions to develop into an embryo.

Fertilisation - Flowering Plants

Introduction to Plant Fertilisation

  • Sexual reproduction in flowering plants involves the transfer of pollen from the male to female flower structures.

  • Key flower components include:

    • Anther: Producing pollen

    • Stigma: Receiving pollen

    • Ovary: Housing the female gametes

Pollination and Double Fertilisation
Pollination

  • The process through which pollen moves from the anther to the stigma, enabling fertilisation.

Double Fertilisation Mechanism

  • Once pollen reaches the ovule through the pollen tube, double fertilisation occurs:

    1. One male nucleus fuses with the egg cell nucleus to form a diploid zygote.

    2. The second male nucleus fuses with two polar nuclei in the ovule, forming a triploid endosperm, which will nourish the developing plant embryo.

Distinction Between Pollination and Fertilisation

  • It is critical to distinguish between pollination (the transfer of pollen) and fertilisation (the union of gamete nuclei). Pollination can be compared to the process of sexual intercourse, while fertilisation is akin to the fusion of gametic nuclei.

The Cell Cycle & Mitosis

Overview of the Cell Cycle

  • Mitosis occurs within the cell cycle - a regulated series of events leading to cell division.

  • Phases of the cell cycle: 1. Interphase 2. Nuclear Division (Mitosis) 3. Cell Division (Cytokinesis).

  • The length of the cycle varies across different cell types and organisms, governed by chemical signals called cyclins.

Interphase Breakdown

  • Interphase encompasses growth and preparation for division and consists of:

    • G Phase: Cells grow and carry out functions; interim between previous division and S phase.

    • S Phase: DNA synthesises and replicates, leading to sister chromatids formation.

Mitosis Overview

  • Phases of Mitosis: Mitosis, although continuous, is categorized into four main stages:

    1. Prophase: Chromosomes condense, the nuclear envelope breaks down, and spindle fibers emerge.

    2. Metaphase: Chromosomes align at the metaphase plate; spindle fibers attach to centromeres.

    3. Anaphase: Sister chromatids separate and are pulled to opposite poles of the cell.

    4. Telophase: Chromosomes arrive at poles, nuclear envelopes reform, and cell division progresses.

The Significance of Mitosis

  • Mitosis is crucial for:

    • Growth of multicellular organisms: As daughter cells produced are genetically identical to the parent.

    • Repair of tissues: Cells are replaced continually, especially in areas of rapid turnover, such as skin and gut lining.

    • Asexual reproduction: Offspring are clones of the parent organism.

  • Understanding the phases (PMAT: Prophase, Metaphase, Anaphase, Telophase) is essential for applying knowledge in various biological contexts, especially regarding chromosome numbers during the cell cycle, such as humans having 46 chromosomes per diploid cell prior to mitosis.