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Reproduction: Asexual vs. Sexual

• Asexual Reproduction: Examples include budding, binary fission, and regeneration.

• Sexual Reproduction: Involves processes like external fertilization, where sperm and egg meet outside the body.

Genetic Diversity in Sexual Reproduction

• Sexual reproduction increases genetic diversity through processes like independent assortment, crossing over during meiosis, and random fertilization.

• This genetic diversity contributes to evolutionary adaptation rates by providing a broader range of traits that can be favorable in changing environments.

Sea Urchin Fertilization Steps

1. Sperm Contact: Sperm contacts the egg membrane.

2. Acrosomal Reaction: Release of digestive enzymes from the acrosome of the sperm, allowing it to penetrate the egg's protective layers.

3. Cortical Reaction: Changes in the egg's membrane prevent polyspermy (multiple sperm entering).

4. Egg Activation: Initiates metabolic processes in the egg.

5. Nuclei Fusion: Fusion of sperm and egg nuclei.

6. Zygote Formation: The fertilized egg begins development.

• In mammals, the acrosomal reaction includes a different mechanism for penetration compared to sea urchins.

Cleavage Divisions and Blastula Formation

• Cleavage Divisions: Rapid mitotic divisions of the zygote that produce smaller cell sizes without growth, leading to the formation of the blastula.

• Differences from Normal Cell Divisions: Cleavage divisions are characterized by a lack of growth between divisions, focusing solely on increasing cell number.

Gastrulation in Diploblasts and Triploblasts

• Gastrulation Process: Involves the reorganization of the blastula into a multi-layered structure.

• Germ Layers:

  • Diploblasts (e.g., cnidarians) have two layers: ectoderm and endoderm.

  • Triploblasts (e.g., vertebrates) have three layers: ectoderm, mesoderm, and endoderm.

• Anatomical Examples: Recognize that diploblasts have simpler body plans compared to triploblasts.

Germ Layers and Adult Structures in Humans

• Ectoderm: Forms skin and nervous system.

• Mesoderm: Gives rise to muscles, bones, and circulatory system.

• Endoderm: Develops into gut lining and organs like the liver and pancreas.

Organogenesis

• Definition: The process by which specific organs and structures develop from the germ layers.

Related Concepts

• Induction: Interaction between cells to influence development; e.g., signaling molecules guiding differentiation.

• Cell Migration: Movement of cells to their destined location during development.

• Determination: The point where a cell's fate is fixed; it will become a specific type of cell.

• Differentiation: The process a cell undergoes to develop its specialized function, influenced by genetic expression and environmental signals.

• Cell Fate: The final role or type a cell will become as dictated by genetic and environmental factors.

Extraembryonic Membranes

• Four Membranes in Humans and Reptiles:

  1. Amnion: Surrounds the embryo, providing a fluid cushion.

  2. Chorion: Facilitates gas exchange and contributes to the placenta in mammals.

  3. Allantois: Involved in waste storage and gas exchange; becomes part of the umbilical cord in mammals.

  4. Yolk Sac: Provides nutrients early in development.

• Differences: Mammals utilize these membranes differently, particularly in how the chorion forms part of the placenta.

Evolutionary Pressures and Amniote Development

• Amniote Evolution: Evolved to lay eggs that could survive on land due to protective membranes. Extraembryonic membranes allowed for better support and nourishment in terrestrial environments.

Derived Structures in Development

• Blastocyst: Derived from the morula; gives rise to the embryo.

• Inner Cell Mass: Forms the embryo.

• Trophoblast: Becomes part of the placenta.

• Epiblast: Gives rise to the ectoderm and mesoderm.

• Hypoblast: Gives rise to the endoderm and is involved in embryo development.

• Extraembryonic Membranes: Derived from trophoblast and epiblast; support embryo development.

• Placenta: Derived from trophoblast and endometrial tissues; supports fetal growth and nutrient exchange.

Structure of a DNA Molecule

• Nucleotides: DNA is composed of nucleotides, which are the building blocks of DNA.

Components of a DNA Nucleotide

• Uniform Parts: All DNA nucleotides have a phosphate group and a deoxyribose sugar.

• Variable Part: The nitrogenous base (adenine, thymine, guanine, cytosine) varies among nucleotides.

Phosphodiester Backbone

• Description: The phosphodiester backbone is formed by the covalent bonding of the phosphate group of one nucleotide to the sugar of the next nucleotide, creating a long chain of sugar-phosphate units.

Hydrogen Bonds in DNA

• Role: Hydrogen bonds form between the nitrogenous bases of the two strands. These bonds are crucial for the stability of the double-stranded DNA molecule, holding the two strands together.

Directionality of DNA Strand

• 5’ and 3’ Directionality: Each DNA strand has a directionality defined by the ends of the molecule. The 5’ end has a phosphate group, while the 3’ end has a hydroxyl group. DNA strands are oriented antiparallel, meaning one strand runs 5’ to 3’ and the other runs 3’ to 5’.

Complementary Base Pair Interactions

• Application: Complementary base pairing allows for the prediction of complementary DNA sequences. For example, if a DNA sequence on one strand is ACGT, the complementary sequence on the other strand will be TGCA.

Enzymes in the Replisome

• Functions and Purposes of the 7 Enzymes:

  • Helicase: Unwinds the DNA double helix.

  • Single-strand binding proteins: Stabilize single-stranded DNA.

  • Primase: Synthesizes RNA primers to initiate replication.

  • DNA polymerase: Synthesizes new DNA strands complementary to the template strand.

  • Exonuclease: Removes RNA primers during replication.

  • Ligase: Joins Okazaki fragments on the lagging strand.

  • Topoisomerase: Relieves tension ahead of the replication fork by cutting and rejoining the DNA strands.

5’-3’ Directionality in DNA Replication

• Understanding: During DNA replication, new DNA strands are synthesized in the 5’ to 3’ direction, meaning nucleotides are added to the 3’ end.

Okazaki Fragments

• Role and Orientation: Okazaki fragments are short sequences of DNA synthesized on the lagging strand during replication. They are formed in the opposite direction of the overall replication fork movement and are related to RNA primers produced by primase, which initiate each fragment's synthesis.