DNA Structure and Chromosomes

• DNA is the genetic material determining species characteristics, passed to offspring for life's continuity.

Human Chromosomes

• Can be imaged, matched, and ordered by size into karyotypes.
• The first 22 pairs are autosomes; the 23rd pair (X and Y) are heterosomes/sex chromosomes.

Basic DNA Structure

• Composed of two helical strands winding around a central axis.
• Individual subunits are nucleotides, featuring a sugar-phosphate backbone and a base facing the double helix center.

DNA Organization

• Long DNA strands wrap around proteins called histones to fit within the nucleus of eukaryotic cells.
• Coiled DNA is stored as chromatin in non-dividing cells and further coiled into chromosomes in dividing cells.
• DNA loops around eight histone proteins twice to form a nucleosome, which coils into chromatin, then chromosomes.

Terminology

• Histones are proteins around which DNA wraps, aiding chromatin condensation.
• A nucleosome consists of eight histone proteins wrapped by DNA.
• Chromatin is a complex of DNA and proteins in the nucleus.

Chromatin

• Heterochromatin is tightly coiled and stains darker under a microscope.
• Euchromatin is less tightly coiled and appears lighter.

Chromosome Structure

• Telomeres are DNA lengths at chromosome ends that prevent damage during cell division.
  • Telomeres shorten with each cell division, leading to accumulated damage and aging.
• Centromeres are chromosome constrictions where chromatids join to form a double chromosome, influencing chromosome shape.

Loci

• A locus is a genetic location, such as a gene's position or a single nucleotide on a chromosome.
• Somatic cells contain two copies of each chromosome, thus two copies of each gene.
• Individual gene copies are called alleles (e.g., D and d).

Double vs. Single Chromosomes

• A double chromosome comprises two chromatids.
• During cell division, chromatids separate to form single chromosomes.

Types of Chromosomes

• Autosomes/somatic chromosomes are identical in males and females, occurring in homologous pairs inherited from each parent.
• Heterosomes are non-identical chromosomes that pair in meiosis, typically sex chromosomes influencing sexual traits.

Human Chromosomes

• Ploidy indicates the number of chromosome pairs in a cell.
• Human somatic cells have 46 chromosomes, the diploid number (2n).
• Human gametes have 23 chromosomes, the haploid number (n).

Location of Chromosomes: Eukaryotes

• Human chromosomes exist as single lengths in eukaryotes.
• DNA is in the nucleus, and circular DNA is in mitochondria and chloroplasts.

Location of Chromosomes: Prokaryotes

• Prokaryotes have a single, unbound, circular chromosome in the cytosol and are haploid.
• Plasmids are smaller, free-floating DNA sections in bacteria that carry non-essential genes for survival, replicating independently.

Eukaryotes vs. Prokaryotes

Components of DNA

• DNA comprises a five-carbon sugar (deoxyribose), a negatively charged phosphate group, and a nitrogenous base.
• These components form a nucleotide.

Double Helix

• DNA has two strands twisted to form a double helix, held by base pair bonds.

Backbone

• The backbone is deoxyribose sugars connected by phosphate groups.
• Deoxyribose sugar has five carbon atoms: the 1st bonds to the nitrogen base, the 3rd and 5th to phosphate groups.
• DNA strands run 3' to 5' and 5' to 3' in opposite directions.

Nitrogenous Bases

• DNA's double helix is held together by nitrogenous bases: Adenine, Cytosine, Guanine, and Thymine.
• Bases are classified as purines or pyrimidines.
• Chargaff's rule states the ratio of purines to pyrimidines is 1:1, so A=T and C=G.

Nucleotide

• A nucleotide has one sugar, one phosphate, and one nitrogen base.
• The phosphate joins to the next nucleotide's sugar via a phosphodiester bond.
• Nitrogen bases are complementary, joined by weak hydrogen bonds.
• Adenine pairs with thymine, and cytosine bonds with guanine, creating a genetic code.

Hydrogen Bonding

• The weak hydrogen bonds allow the DNA molecule to split easily without breaking chemical bonds.
• Heating DNA to 90°C breaks hydrogen bonds and denatures the DNA.

DNA Replication

• DNA copies itself before cell division during the S phase of the cell cycle.
• Essential for generational continuity and organism growth.

Part 1

• Stage 1: Representative portion of DNA about to undergo replication.
• Stage 2: DNA helicase breaks hydrogen bonds, separating DNA strands to create a replication fork.
• Stage 3: Free nucleotides are attracted to complementary bases.

Part 2

• Stage 4: DNA polymerase joins new nucleotides.
• Stage 5: DNA polymerase forms a complete polynucleotide chain, resulting in two identical DNA strands.

Enzymes

• DNA helicase unzips the DNA molecule during replication.
• DNA polymerase replicates DNA molecules to build a new strand.
• Primase makes the primer for DNA polymerase to start.
• DNA ligase glues DNA (Okazaki) fragments together.

Replicating 5' to 3'

• DNA polymerase adds nucleotides in the 5' to 3' direction (reads 3' to 5').
• One strand replicates continuously, while the other replicates in Okazaki fragments.

Leading and Lagging Strands

• The leading strand (3' to 5') replicates continuously.
• The lagging strand (5' to 3') replicates discontinuously in sections.

Okazaki Fragments

• Short DNA nucleotide sequences are synthesized discontinuously and linked by DNA ligase on the lagging strand.

DNA Replication

• After replication, the new strands re-coil, each containing half of the original DNA and half new material.
• DNA replication is semi-conservative.

Heredity

• The study of inheritance and genetic material transmission to the next generation.

Types of Reproduction

• Asexual reproduction occurs without interaction between different members of a species, including binary fission, mitosis, budding, vegetative propagation, and parthenogenesis.
• Sexual reproduction requires male and female contributions.

Asexual Reproduction: Binary Fission

• Occurs primarily in prokaryotes.
• The single chromosome replicates, attaches to different membrane parts, and segregates as the cell divides via cytokinesis.
• Bacteria increase genetic variation through horizontal gene transfer (HGT): conjugation and transformation.

Asexual Reproduction: Mitosis

• Occurs primarily in eukaryotes.
• Somatic cells divide, chromosomes are copied, and each new cell receives identical genetic information.
• Involved in identical twin creation when a zygote splits into two identical copies.

Binary Fission vs. Mitosis

• Binary fission differs from mitosis with its single chromosome and no centromere.

Asexual Reproduction Advantages

• Suited for organisms in stable environments that cannot find mates.

Asexual Reproduction Disadvantages

• Lacks variation, making the group susceptible to disease or environmental changes.

Sexual Reproduction

• Requires genetic material from male and female of the same species.
• Gametes produced through meiosis halve the chromosome number.

Sexual Reproduction

• Fertilization combines gametes to produce a diploid zygote with a unique genetic makeup.

Sexual Reproduction Advantages

• Allows for variation, fundamental for evolution.
• Creates species that can adapt to new environments and resist disease.

Sexual Reproduction Disadvantages

• Requires significant energy to find a mate.
• Unsuitable for isolated organisms.

Summary: Asexual vs. Sexual

The Cell Cycle

• Continuous: G1 phase, S phase, G2 phase, M phase and C phase
• Mitosis is only a small part; most time spent in interphase.

• The first three phases (G1, S, and G2) are collectively the interphase.

G1 Phase (Interphase)

• First growth stage after cell division.
• Cells mature through cytoplasm and organelle production.
• Normal metabolic activities occur.

S Phase (Interphase)

• Synthesis stage; DNA is copied/replicated.

G2 Phase (Interphase)

• Second growth stage after DNA replication.
• Cell structures needed for division are made, such as centrioles.
• Organelles and proteins are synthesized.

Mitosis

• Cell division in eukaryotic cell nuclei (except sex cells).
• Accurate chromosome replication to produce two identical diploid daughter cells with the same chromosome number as the parent.

Prophase (Pro = First)

• Chromatin condenses, chromatids become visible as chromosomes.
• Nucleoli disappear, nuclear membrane breaks down.
• Centrioles migrate to opposite poles, spindle forms.

Metaphase (Meta = Between Two States)

• Chromosomes align on the cell equator, attached to spindle fibers.

Anaphase (Ana = To Happen Twice)

• Spindle fibers attach to chromosomes, dividing at the centromere.
• Daughter chromatids (now chromosomes) move to opposite poles.

Telophase (Telo = At the End)

• Spindle disappears.
• Nucleoli and nuclear membranes reform.
• Centrioles divide.
• Chromosomes uncoil into chromatin.
• Cytokinesis begins.

Cytokinesis

• Cytoplasm division.
• Two identical daughter cells are created with the same chromosome number and type as the parent cell.
• After telophase, daughter cells enter interphase (G1) to grow.
• In plant cells, a cell plate forms at the equator for cell wall formation.
• In animal cells, a cleavage furrow forms to split the cell.

G0 Phase

• The cell exits the cell cycle, not preparing for division.
• Terminally differentiated cells (e.g., nerve cells) are usually in G0 phase.
• Cells can re-enter the cell cycle with the correct stimuli.

Cell Life Span

Uncontrolled Mitosis

• Leads to unlimited cell division, resulting in tumors and cancer due to mutation.

Apoptosis

• Programmed cell death triggered when a cell detects issues.
• Cancer or tumors occur due to the body’s inability to trigger apoptosis.

Summary

Meiosis vs. Mitosis

• Mitosis produces two daughter cells identical to the parent.
• Meiosis produces gametes with half the parent's chromosome number.

Meiosis Summary

• Meiosis is required for haploid gamete formation in sexual reproduction.
• Occurs in sex organs (ovaries/testicles).
• Two meiotic division phases: Meiosis 1 (chromosome reduction) and Meiosis 2 (chromatid separation).

Meiosis Phases

• Interphase: DNA replication; replicated copies are sister chromatids held at the centromere.

Prophase 1

• DNA coils, chromosomes become visible.
• Genetic material is exchanged between chromosomes (crossing over).

Metaphase 1

• Recombined homologous chromosomes line up along the cell equator.
• Homologues orient towards either pole, creating variation.

Anaphase 1

• Spindle fibers pull one chromosome from each pair towards opposite poles.
• Each pole ends with one copy of each chromosome, halving the number (haploid).

Telophase 1

• Nuclear membrane forms.

Prophase 2

• Spindle fibers form and attach to chromatids.

Metaphase 2

• Chromosomes line up along the equator of the cell.

Anaphase 2

• Spindle fibers contract and pull one chromatid from each chromosome to cell ends.

Telophase 2

• The cell divides, forming four nuclei with half the parent's chromosome number.

Gametogenesis

• Gamete development (includes meiosis).
• Two types: spermatogenesis and oogenesis.

Fertilization

• Two haploid (1n) gametes combine to form a diploid (2n) zygote.

Genotype Variation

• Meiosis and fertilization create genotype variations in offspring.
• Genetic variation in meiosis is through crossing over, independent assortment, random fertilization, and mutation.