Asexual & Sexual Reproduction: Cycles, Structures & Functions
Asexual Reproduction vs. Sexual Reproduction
Asexual Reproduction
Origin: 3500 million years ago
Cell Division: Mitosis
Parents: 1 progenitor
Cell Type: Normal cells
Characteristics:
- Simple and profitable reproduction
- Genetically identical individuals
- No recombination of genes or characters
- Rapid growth in the number of individuals
Sexual Reproduction
Origin: 1500 million years ago
Cell Division: Meiosis
Parents: 2 (almost always)
Cell Type: Specialized cells
Characteristics:
- Constructive and complex reproductive process
- High material expenditure
- Genetically different individuals
- Easy gene and character recombination
Parthenogenesis
A variant of sexual reproduction where an egg develops into an embryo without fertilization.
Bivitelline Twins
Two or more eggs fertilized simultaneously by different sperm.
Univitelline Twins
One egg fertilized by one sperm, where the cell group forms two separate, genetically identical individuals.
Biological Cycles
Haplont Cycle
The zygote undergoes meiosis upon division. One set of chromosomes. Found in fungi and algae.
Diplont Cycle
Meiosis occurs immediately before gamete formation (gametogenesis). Found in humans and animals. Sperm and ova are diploid.
Diplohaplont Cycle
Combines advantages of both diplont and haplont cycles.
Example: Mosses
Meiosis occurs in the sporophyte, leading to the sporangium where spores are produced. Spores germinate and give rise to the protonema, which develops into the gametophyte (male or female, antheridium or archegonium). The antheridium releases antherozoids that travel through water to fertilize the archegonium. This results in a sporophyte, where the cycle restarts with meiosis.
Example: Ferns
Spores are produced in the sporangium. They germinate into a prothallus, which is the gametophyte (antheridium or archegonium). The antheridium releases antherozoids that travel through water to fertilize the archegonium. This leads to the development of the sporophyte, where meiosis occurs, and the cycle begins again.
Germ Layers
Ectoderm
- Epidermis
- Nails
- Nervous system
- Sweat glands
- Enamel
Mesoderm
- Dermis
- Connective tissue
- Cartilage
- Bone
- Muscle
- Blood
- Gonads
- Kidneys
Endoderm
- Mucous membranes of the digestive tract
- Bronchi
- Lungs
- Urethra
- Pancreas
- Liver
- Thymus
- Thyroid
Metamorphosis
Incomplete Metamorphosis
Found in insects and other animals. The larva resembles the adult, does not go through a period of inactivity, and continues to feed during development.
Complete Metamorphosis
Larvae (like caterpillars) undergo a pupal or chrysalis stage before reaching adulthood.
Proteins
Structure and Function
Proteins, also known as proteids, are polypeptides formed by chains of amino acids linked by peptide bonds. They can undergo structural, property, and chemical function alterations, a process known as denaturation.
Holoproteins
Formed solely by amino acids.
Conjugated Proteins
Contain a non-protein prosthetic group.
Amino Acids
Organic compounds with a basic amino group and a carboxyl group. They are amphoteric. Each amino acid has an alpha carbon atom linked to four groups: a carboxyl group, an amino group, a hydrogen atom, and a side chain or radical.
Biological Functions of Proteins
- Structural and protective
- Catalytic (enzymes)
- Transport
- Nutrient storage and supply
- Hormonal regulation
- Muscle contraction
- Defense and recognition (immune system)
- Homeostatic
Protein Structure
Primary Structure
The sequence of amino acids in a polypeptide chain, indicating the order and type of amino acids linked by peptide bonds between the amino and carboxyl groups.
Secondary Structure
The spatial arrangement of the primary structure. Includes alpha helix and beta sheet structures, resembling a coiled phone cord.
Tertiary Structure
The three-dimensional folding of the secondary structure. The position of amino acids in space determines the protein’s function. Conformations include:
- Fibrous: Rope-like, stable, hard, and insoluble in the cellular environment (e.g., keratin, elastin, collagen, gelatin).
- Globular: Compact, soluble, biologically active, and have high nutritional value (e.g., histones, prolamins, albumins, globulins).
Quaternary Structure
Proteins formed by two or more polypeptide chains (e.g., hemoglobin).
Holoproteins or Simple Proteins
Formed only by polypeptide chains.
Fibrous Proteins
Very stable, hard, and insoluble. Form sheets or fibers. Found in animal tissues.
- Alpha Keratin: High mechanical resistance, rich in sulfur.
- Elastin: Elastic, found in connective tissues.
- Collagen: Most abundant protein in vertebrates. Found in tissues and hard organs. Forms jelly when cooked.
Globular Proteins
Polypeptide chains folded into compact, roughly spherical conformations. Soluble, biologically active, and have high nutritional value.
- Histones: Basic proteins associated with DNA in chromatin.
- Prolamins: Glutenins, found in cereal grains.
- Albumins: Reserve or nutrient transport functions.
- Globulins: High molecular weight.
Heteroproteins or Conjugated Proteins
Formed by a protein fraction and a prosthetic group.
- Glycoproteins: Prosthetic group is a carbohydrate. Found in cell membranes, coatings, hormones, immunoglobulins, and certain enzymes.
- Lipoproteins: Prosthetic group is a lipid. Transport fats in the blood.
- Phosphoproteins: Prosthetic group is a phosphate.
- Nucleoproteins: Prosthetic group is a nucleic acid.
- Chromoproteins: Prosthetic group is a pigment (porphyrin or non-porphyrin).
Nucleic Acids
Long, linear or branched chains formed by the polymerization of nucleotides.
Nucleoside
The union of a pentose sugar and a nitrogenous base.
Nucleotide
The basic unit of nucleic acids. Formed by a pentose sugar (ribose or deoxyribose), a nitrogenous base (adenine, guanine, cytosine, thymine, or uracil), and a phosphate group.
ATP (Adenosine Triphosphate)
A nucleotide of adenine esterified with three phosphate groups. Acts as an intermediary between energy-releasing and energy-consuming reactions.
Nucleases
Enzymes that break the bonds between nucleotides. Useful for research and study.
DNA (Deoxyribonucleic Acid)
Function
The carrier of genetic information. Programs the coordinated biosynthesis of all cellular components. Information is stored in the sequence of nucleotides.
Sequence and Composition
High molecular mass. Composed of two chains of nucleotides arranged in a double helix. Prokaryotes have a single large DNA molecule forming the chromosome, while eukaryotes have multiple DNA molecules associated with chromatin proteins, forming chromosomes during cell division.
Three-Dimensional Structure
Except for some viruses with single-stranded DNA, DNA is a double helix, as described by Watson and Crick in 1953. Each strand coils around an imaginary central axis. The two strands are antiparallel. Base pairing occurs between adenine and thymine, and cytosine and guanine. Due to this specific pairing, the two strands are complementary, and the sequence of one strand determines the sequence of the other.
Prokaryotic and Viral DNA
Viruses with DNA as genetic material replicate using DNA polymerase. The nucleic acid can be single-stranded or double-stranded DNA.
Eukaryotic DNA
Much larger in quantity than bacterial DNA. Located in the cell nucleus as chromatin, associated with histone proteins. During cell division, chromatin condenses into chromosomes.
Nucleosome
The fundamental unit of chromatin organization in eukaryotes. Formed by a core of eight histone proteins (two each of H2A, H2B, H3, and H4) and DNA wrapped around it. Resembles a”beads-on-a-strin” structure under an electron microscope. Each histone octamer is surrounded by approximately two turns of DNA. Another histone (H1) is located outside the nucleosome core.
C-Value Paradox
The DNA content of a species’ gametes varies greatly between species. Within a group of organisms, the C-value tends to increase with evolutionary complexity.
RNA (Ribonucleic Acid)
Formed by long chains of ribonucleotides linked by phosphodiester bonds.
Differences from DNA
- The pentose sugar is ribose, not deoxyribose.
- Uracil replaces thymine as a nitrogenous base.
- RNA molecules are typically single-stranded, although some can form double-stranded structures similar to DNA.
Types of RNA
Messenger RNA (mRNA)
Carries genetic information from DNA to ribosomes for protein synthesis.
Ribosomal RNA (rRNA)
The main component of ribosomes.
Transfer RNA (tRNA)
Carries amino acids to the growing polypeptide chain during protein synthesis.
Other Types of RNA in Eukaryotes
Heterogeneous Nuclear RNA (hnRNA)
The precursor molecule of mRNA.
Small Nuclear RNA (snRNA)
Involved in mRNA processing.