Biology Chapter: Cell Cycle and Mitosis

The Cell Cycle

• 90% of a cell's life is spent in interphase
• Interphase is comprised of three phases:
  • G1: gathering energy and proteins for DNA replication
  • S: DNA replication occurs, resulting in two copies of the chromosomes
  • G2: storing more energy and proteins, some organelles are duplicated

Mitosis

• Prophase: nuclear membrane breaks down, mitotic spindle begins to form
• Metaphase: chromosomes align on the mitotic spindle
• Anaphase: sister chromatids separate and move to opposite poles
• Telophase: nuclear membrane begins to form, a cleavage furrow develops

Cytokinesis

• At the equator of the cell, a cleavage furrow pulls inward
• Microfilaments force the cell membrane to contract and separate the cell into two daughter cells

Dinofuzz

• "Dinofuzz" is the fluff that surrounded a dinosaur fossil (simple structure)
• Sinosauropteryx likely used feathers for attracting mates or keeping warm

Sexual Reproduction

• Sexual reproduction requires a lot of energy
• Finding and attracting mates is an essential part of sexual reproduction
• Forming haploid (sex) cells requires energy
• Less offspring are produced in sexual reproduction compared to asexual reproduction

Meiosis

• Meiosis results in four genetically unique haploid daughter cells for reproduction
• Meiosis reduces the number of chromosomes by half
• Mitosis results in two genetically similar diploid daughter cells for growth and repair
• Mitosis maintains the number of chromosomes

Meiosis I

• Prophase I: membrane around the nucleus breaks down, spindle fibers begin to form
• Crossing-over: exchange of chromosome segments between non-sister homologous chromatids
• Metaphase I: the meiotic spindle is fully formed, pairs of homologous chromosomes align along the "equator," chromosomes attach to spindle tracks to prepare for separation
• Independent Assortment: many arrangements of homologous chromosomes during Metaphase I, different combinations of chromosomes in the final daughter cells
• Anaphase I: homologous chromosomes are pulled away from the equator, sister chromatids stay together
• Telophase I: chromosomes reach the poles, the nuclear membrane forms
• Cytokinesis: two daughter cells form

Meiosis II

• Sister chromatids separate

Genetic Variation During Meiosis

• Crossing over occurs after Prophase I
• Independent Assortment occurs after Metaphase I

Nondisjunction

• Error in chromosome distribution during Anaphase I or Anaphase II
• In Anaphase I: chromosome pairs fail to separate, all daughter cells have the incorrect number of chromosomes
• In Anaphase II: sister chromatids fail to separate, some daughter cells have the incorrect number of chromosomes

Patterns of Inheritance

• Mendel observed traits such as pea color and plant height in his experiments
• Hybrids are the result of two parents that differ in a given trait
• In the F1 generation of a cross between purebred purple and white flowers, all flowers are purple, not blended colors
• F2 generation shows a 3:1 ratio of purple to white flowers, regardless of which parent was purple
• Alleles are versions of genes
• Homozygous: identical alleles
• Heterozygous: different alleles
• Dominant alleles are expressed in a hybrid, while recessive alleles have no noticeable effect

Genotype vs. Phenotype

• Genotype is the genetic makeup of an individual
• Phenotype is the outward expression of those genes

Punnett Squares

• Punnett squares can be used to determine the possible genotypes of offspring from two parents with known genotypes

Chickenosaurus

• Jack Horner proposed that a small "pet dinosaur" could be created by manipulating the genes of a chicken embryo and reversing evolution to bring dinosaur-like features

Variations on Dominance

• Incomplete dominance: heterozygotes appear as a mix between parent appearances
• Codominance: both parent alleles are expressed in the phenotype of the offspring

Pleiotropy

• One gene that affects many traits
• Sickle cell anemia can lead to:
  • Affects the hemoglobin protein and RBC shape
  • Retinal detachment, liver enlargement, renal failure, stroke, heart attack

Structure of DNA

• Double helix composed of nucleotides
• Nucleotides are composed of: nitrogen base, phosphate group, and deoxyribose sugar
• Bases form pairs between one purine (adenine, guanine) and one pyrimidine (thymine, cytosine)

DNA Replication

• Helicase splits the helix, forming a replication fork
• DNA polymerase adds base pairs to the 3' end of the leading strand, and in sections to the lagging strand
• DNA ligase binds Okazaki fragments together
• The end result of DNA replication is two new DNA strands that are exact copies of the original

RNA

• RNA is a single helix
• Uracil replaces thymine in RNA

Transcription

• Initiation: RNA polymerase binds to the promoter region of the gene
• Elongation: RNA polymerase moves along the DNA, adding RNA nucleotides
• Termination: RNA polymerase reaches the termination sequence and detaches

mRNA Processing

• In eukaryotes, mRNA undergoes post-processing before it is ready for translation
• Stabilizing proteins are added to prevent degradation
• Caps are added to the ends of mRNA
• Introns are removed
• Exons remain

Translation

• The process of building proteins from mRNA
• Ribosomes and mRNA are involved
• tRNA interacts with the start codon to begin translation

Mutations

• A codon is a sequence of three consecutive nucleotides in a DNA or RNA molecule that codes for a specific amino acid
• Mutations are permanent changes in the genetic information of a cell or virus
• Substitutions: replacement of one nucleotide base for another
• Silent mutation: codon for the same amino acid
• Missense mutation: codon for different amino acid
• Nonsense mutation: stop codon, resulting in a prematurely ended protein

Gene Control

• Cells containing the same DNA become specialized by regulating gene expression
• Gene expression is regulated by activating and deactivating genes
• Proteins are created at specific times

Gene Regulation in Prokaryotes

• Transcription and translation occur in the same location (cytoplasm) almost simultaneously in prokaryotes
• Prokaryotes control gene expression by regulating the transcription step
• Example: lactose and lactase
• Operon: related genes and sequences, such as lac operon

How Prokaryotes Regulate Gene Expression

• Prokaryotes regulate gene expression by regulating the transcription step