Exam 4: Cell Cycles, Heredity, and Central Dogma

Genome

All the DNA in a cell.

Chromatin

The complex of DNA and proteins (primarily histones) that makes up eukaryotic chromosomes.

Chromosome

A condensed, thread-like structure of nucleic acids and protein found in the nucleus, carrying genetic information.

Somatic cells

Any cell in a multicellular organism except a sperm or egg cell; diploid (2n).

Gametes

Reproductive cells (sperm and egg) that have a single set of chromosomes; haploid (n).

Functions of Mitotic Cell Division

Reproduction (in single-celled eukaryotes), Growth and development, Tissue renewal.

Histones

Proteins responsible for the first level of DNA packing in chromatin.

Nucleosome

The basic unit of DNA packing, consisting of DNA wound around a protein core of histones.

Cell cycle checkpoints

Control points in the eukaryotic cell cycle where stop and go-ahead signals can regulate the cycle.

G1 checkpoint

The primary "restriction point" of the cell cycle; checks for cell size, nutrients, and growth factors before committing to division.

Cyclins

A family of proteins whose concentration fluctuates cyclically, regulating the cell cycle.

Cyclin-dependent kinases (Cdks)

Enzymes that are activated when bound to cyclins; they phosphorylate proteins to control cell cycle progression.

MPF (Maturation-promoting factor)

A cyclin-Cdk complex that triggers a cell's passage from G2 into the M phase.

Tumor

A mass of abnormal cells that develops when cancerous cells divide uncontrollably.

Metastasis

The spread of cancer cells from their original site to other parts of the body.

Heredity

The transmission of traits from one generation to the next.

Gene

A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA.

Locus

A specific, fixed position on a chromosome where a particular gene is located.

Asexual reproduction

Reproduction involving a single parent that results in offspring that are genetically identical to the parent.

Sexual reproduction

Reproduction involving two parents, combining their genetic material to produce genetically diverse offspring.

Diploid (2n)

A cell containing two sets of chromosomes, one set inherited from each parent.

Haploid (n)

A cell containing only one set of chromosomes.

Karyotype

A display of the chromosome pairs of a cell arranged by size and shape.

Homologous chromosomes

A pair of chromosomes (one from each parent) that are the same size, shape, and carry genes for the same traits.

Autosomes

Chromosomes that are not directly involved in determining sex (in humans, pairs 1-22).

Sex chromosomes

Chromosomes that determine the sex of an individual (in humans, the X and Y chromosomes).

Purpose of Meiosis

To reduce the chromosome number from diploid (2n) to haploid (n) and create genetically diverse gametes.

Meiosis I vs. Meiosis II

Meiosis I separates homologous chromosomes; Meiosis II separates sister chromatids.

Prophase I

The stage of meiosis where synapsis and crossing over occur.

Synapsis

The pairing and physical connection of homologous chromosomes during Prophase I of meiosis.

Crossing over

The exchange of genetic material between nonsister chromatids during Prophase I, creating recombinant chromosomes.

Chiasmata

The X-shaped sites where crossing over has occurred between homologous chromosomes.

Metaphase I

The stage of meiosis where pairs of homologous chromosomes (tetrads) line up at the metaphase plate.

Anaphase I

The stage of meiosis where homologous chromosomes separate and move toward opposite poles.

Independent Assortment

The random orientation of homologous pairs at Metaphase I, leading to varied combinations of chromosomes in gametes.

Recombinant chromosomes

Chromosomes that carry genetic material derived from both parents as a result of crossing over.

Sources of Genetic Variation

Independent assortment of chromosomes, crossing over, and random fertilization.

Mitosis vs. Meiosis (Daughter Cells)

Mitosis produces 2 diploid, genetically identical cells. Meiosis produces 4 haploid, genetically different cells.

True-breeding

Organisms that produce offspring of the same variety when they self-pollinate.

P Generation

The true-breeding (parental) generation in a genetic cross.

F1 Generation

The first filial (hybrid) offspring of a P generation cross.

F2 Generation

The offspring resulting from interbreeding the F1 generation.

Dominant allele

An allele that is fully expressed in the phenotype of a heterozygote.

Recessive allele

An allele whose phenotypic effect is not observed in a heterozygote.

Homozygous

Having two identical alleles for a particular gene.

Heterozygous

Having two different alleles for a particular gene.

Phenotype

An organism's observable physical or biochemical traits.

Genotype

An organism's genetic makeup, or the set of alleles it possesses.

Law of Segregation

Mendel's first law, stating that the two alleles for a heritable character separate (segregate) from each other during gamete formation.

Law of Independent Assortment

Mendel's second law, stating that each pair of alleles segregates independently of each other pair during gamete formation.

Monohybrid cross

A genetic cross involving a single pair of contrasting traits.

Dihybrid cross

A genetic cross involving two pairs of contrasting traits.

Testcross

Breeding an organism of unknown genotype with a homozygous recessive individual to determine the unknown genotype.

Incomplete Dominance

A pattern of inheritance where the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes (e.g., pink snapdragons).

Codominance

A pattern of inheritance where both alleles are fully and separately expressed in the heterozygous phenotype (e.g., ABO blood groups).

Multiple alleles

When a gene exists in more than two allelic forms in a population (e.g., ABO blood groups).

Pleiotropy

The ability of a single gene to have multiple phenotypic effects.

Epistasis

A type of gene interaction in which the phenotypic expression of one gene is affected by another gene at a different locus (e.g., Labrador coat color).

Polygenic inheritance

An additive effect of two or more genes on a single phenotypic character (e.g., skin color, height).

Pedigree

A diagram that shows the occurrence and appearance of a particular genetic trait in a family across generations.

Carrier

A heterozygous individual who carries a recessive allele for a genetic disorder but does not express the disorder.

Heterozygote advantage

The case in which the heterozygous genotype has a higher relative fitness than either the homozygous dominant or homozygous recessive genotype (e.g., sickle-cell trait and malaria).

Central Dogma

The concept that genetic information flows from DNA → RNA → Protein.

Griffith's Experiment

Demonstrated that a heritable "transforming principle" could be transferred between bacteria.

Hershey-Chase Experiment

Used bacteriophages with radioactive labels to confirm that DNA, not protein, is the genetic material.

Chargaff's rules

The observation that in DNA, the amount of Adenine (A) always equals Thymine (T), and Guanine (G) always equals Cytosine (C).

DNA Structure Discoverers

James Watson and Francis Crick, based on X-ray diffraction images by Rosalind Franklin and Maurice Wilkins.

DNA Structure

A double helix with two antiparallel sugar-phosphate backbones and complementary nitrogenous bases (A-T, G-C) joined by hydrogen bonds.

Antiparallel (DNA)

The two strands of the double helix run in opposite 5' to 3' directions.

Semiconservative model

The model of DNA replication where each new DNA molecule consists of one parental ("old") strand and one newly synthesized strand.

Origin of replication

Site where the replication of a DNA molecule begins.

Replication fork

A Y-shaped region where the parental DNA strands are being unwound for replication.

Helicase

An enzyme that unwinds and separates the parental DNA strands.

Single-strand binding proteins (SSBPs)

Proteins that bind to the unpaired DNA strands to keep them from re-pairing.

Topoisomerase

An enzyme that relieves the strain and "overwinding" of the DNA helix ahead of the replication fork.

Primase

An enzyme that synthesizes a short RNA primer to provide a 3' end for DNA polymerase to begin synthesis.

RNA primer

A short stretch of RNA with a free 3' end, required for DNA polymerase to initiate synthesis.

DNA Polymerase III

The main enzyme that synthesizes new DNA by adding nucleotides to the 3' end of the growing strand.

DNA Polymerase I

An enzyme that removes the RNA primers and replaces them with DNA nucleotides.

DNA ligase

An enzyme that joins the Okazaki fragments on the lagging strand into a continuous strand.

Leading strand

The new DNA strand that is synthesized continuously in the 5' to 3' direction, toward the replication fork.

Lagging strand

The new DNA strand that is synthesized discontinuously in short segments (Okazaki fragments) away from the replication fork.

Okazaki fragments

The short segments of DNA synthesized on the lagging strand.

Telomeres

Repetitive DNA sequences at the ends of eukaryotic chromosomes that protect against gene erosion.

Telomerase

An enzyme that lengthens telomeres in germ cells, compensating for the shortening that occurs during replication.

Transcription

The synthesis of RNA using information in DNA; occurs in the nucleus of eukaryotes.

Translation

The synthesis of a polypeptide (protein) using the information in mRNA; occurs at the ribosome in the cytoplasm.

RNA polymerase

The enzyme that separates DNA strands and synthesizes an RNA transcript complementary to the DNA template strand.

Template strand (DNA)

The DNA strand that is transcribed to create an RNA molecule.

RNA vs. DNA Differences

RNA is single-stranded, contains Uracil (U) instead of Thymine (T), and has ribose sugar.

pre-mRNA (primary transcript)

The initial RNA transcript in eukaryotes before any processing.

Introns

Noncoding, intervening sequences of RNA that are removed during RNA processing.

Exons

The coding sequences of RNA that are spliced together and expressed.

RNA splicing

The process in eukaryotes of removing introns and joining exons to create a mature mRNA molecule.

5' cap

A modified guanine nucleotide added to the 5' end of a pre-mRNA, vital for ribosome binding and protection.

Poly-A tail

A sequence of 50-250 adenine nucleotides added to the 3' end of a pre-mRNA, important for stability and export from the nucleus.

Genetic code

The set of rules by which information encoded in nucleotide sequences (codons) is translated into amino acid sequences.

Codon

A three-nucleotide sequence in mRNA that specifies a particular amino acid or a termination signal.

Redundant/Degenerate Code

The characteristic of the genetic code where multiple codons can specify the same amino acid.

transfer RNA (tRNA)

An RNA molecule that acts as an interpreter, matching specific codons in mRNA with specific amino acids.