Quiz 2

Chapter 18: Cell Growth and Development

Introduction to Growth and Development

• We originate from a single cell, a zygote, that undergoes multiple rounds of cell division, growth, and differentiation to form a complete organism.

• Growth involves an increase in cell number and cell size, while development encompasses the maturation and specialization of those cells.

• Cell division is essential for increasing the number of cells during growth and development.

• To maintain genetic integrity, DNA replication and equal distribution of duplicated DNA are crucial before cell division.

Cell Reproduction (Cell Division): Mitosis

• During cell division, DNA, in the form of chromosomes, is replicated to produce two identical sister chromatids.

• DNA condenses into the chromosome form for organized segregation.

• Sister chromatids separate and are distributed equally into two daughter cells.

• Cytoplasm and organelles are also divided between the two daughter cells.

• This entire process, resulting in two identical daughter cells, is called mitosis.

• Mitosis increases cell number during development and tissue repair, not for reproduction (meiosis is for that).

Cell Divisions: Mitosis vs. Meiosis

• Mitosis: Increases the number of cells in the body for growth and repair.

• Meiosis: Produces gametes (sperm and eggs).

The Cell Cycle: An Overview

The cell cycle consists of distinct phases:

• G1 phase: The cell performs its normal functions; DNA instructs cell activity.

• S phase: DNA replication (synthesis) occurs.

• G2 phase: The cell prepares for division after DNA duplication.

• M phase: Mitosis; chromosomes are separated into two daughter cells.

• Interphase: The collective term for G1, S, and G2 phases. Mitosis is a relatively short part of the cell cycle.

Cell Cycle Control and Checkpoints

The cell cycle has checkpoints to ensure proper execution:

• G1 phase: Normal cell function; DNA instructs cell functions.

• G1 checkpoint: Verifies that the environment is suitable for growth, development, and that DNA is intact before replication.

• G2 checkpoint: Ensures all DNA has been replicated accurately before mitosis.

• G0: Resting phase where cells are not actively dividing.

Triggering Mitosis: Cyclins and Cdks

• Mitosis is triggered by Cyclins and Cyclin-dependent Kinases (Cdks).

• Cyclin-dependent Kinases (Cdks): Responsible for phosphorylation events, adding phosphate groups (PO_4) to target molecules.

• Cyclins bind to Cdks, activating them and allowing phosphorylation to occur.

• M-Cdk: Mitotic Cyclin-dependent Kinase.

• Analogy: CDK is like an electronic key that needs a battery (phosphate); Cyclin supplies this battery.

Cyclin Degradation and M-Cdk Activity

• Cyclin is tagged with ubiquitin, a molecule that marks it for degradation by enzymes.

• Degradation of cyclin inactivates Cdk, turning off mitotic activity.

• When cyclin is not degraded, its concentration rises, increasing M-Cdk activity and promoting mitosis.

• The degradation of cyclin decreases M-Cdk activity.

Cell Cycle, Cell Division, and Mitosis

• Mitosis is a small part of the cell cycle.

• G1, S, and G2 phases constitute the larger portion of the cell cycle, collectively known as interphase.

Stages of M Phase (Mitosis)

Mitosis is divided into several phases, each with distinct characteristics:

• Before mitosis:

  • DNA replication during S-phase.

  • DNA condensation and packaging into chromosomes during G2 phase.

  • Microtubules anchored at opposite sides of the cell.

  • Microtubules attached to chromosomes to facilitate movement.

Interphase (Not Part of Mitosis)

• Includes G1, S, and G2 phases.

• DNA is in a loose state within the nucleus, not yet condensed into chromosomes.

• The nuclear membrane is intact.

• DNA replication occurs in the S phase.

• Chromosome condensation starts at the end of G2 phase.

Prophase

• Chromosome condensation is completed.

• Chromosomes become visible.

• The nuclear membrane is still intact, and a distinct nucleus is visible.

Prometaphase

• Chromosomes begin to align in the middle of the cell.

• Transitional stage between prophase and metaphase.

Metaphase

• Chromosomes are aligned in the middle of the cell.

• The nuclear membrane (envelope) has completely disappeared.

• Microtubules attach to chromosomes at the centromere (center of the chromosome).

• Kinetochore: Protein structure that mediates the attachment between microtubules and chromosomes.

Anaphase

• Chromosomes separate and move toward opposite poles of the cell.

• The nuclear envelope is still absent.

Telophase

• Chromosomes arrive at the poles.

• The nuclear envelope starts to re-form around the separated chromosomes.

• Cytokinesis begins, involving a contractile ring.

Cytokinesis

• The contractile ring, composed of actin and myosin, constricts to divide the cell into two daughter cells.

Summary of Mitotic Phase Features

• Interphase: G1, S, and G2 phases; DNA replicated in S-phase.

• Prophase: DNA condenses into chromosomes. Nuclear membrane starts to dissolve. Centrosome splits into two asters that move to opposite sides of the cell.

• Metaphase: Nuclear membrane is gone. Chromosomes align in the middle. Asters are at opposite poles. Microtubules attach to chromosomes.

• Anaphase: Sister chromatids split and move to opposite sides. Nuclear membrane is still absent.

• Telophase: Chromosomes arrive at opposite sides and start to decondense. New nuclear membranes reform. The contractile ring begins to form.

DNA Condensation: Condensins and Cohesins

• After replication, DNA needs to be packed into chromosomes.

• Condensin: A protein that coils DNA up tightly to form chromosomes.

• Cohesin: A protein that holds sister chromatid pairs together.

Sister Chromatid Segregation at Anaphase

• Sister chromatids are held together by cohesin.

• To start anaphase, cohesin must be cleaved to free the sister chromatids.

Centrosomes and Asters

• Centrosome: An organelle composed of bundles of microtubules, located outside the nucleus during interphase. NOT a centromere!

• During prophase, the centrosome splits into two asters, which move to opposite poles of the cell.

• Microtubules extend from the asters and attach to chromosomes.

• Plant cells do not have centrosomes.

Kinetochore Function

• Kinetochore: A protein structure located at the centromere of chromosomes that serves as an attachment point for microtubules.

• Microtubules attach to chromosomes via the kinetochore.

• Microtubules guide chromosomes to opposite poles of the cell.

• The entire microtubule-chromosome complex is called the mitotic spindle.

Telophase to Cytokinesis

• Chromosomes arrive at opposite poles during telophase.

• Cytokinesis begins with the contractile ring "snapping" the cell in half.

• The contractile ring is composed of actin and myosin.

Chapter 19

Genes and Genomes

• What is a Gene?

  • A section of DNA that codes for a protein.

  • Everything in a cell is either made of proteins or made by proteins.

  • To perform a function, the cell needs to make the protein responsible for that function.

  • To make the protein, the cell needs to use the blueprint, which is the gene.

  • Therefore, having the gene means having the function.

  • Genes are located on DNA.

  • Humans have approximately 58,000 genes and 23 pairs of chromosomes.

• Chromosomes and Genes

  • Each chromosome comes as a pair: one from the mother (maternal) and one from the father (paternal). These are homologous chromosomes.

  • Each chromosome contains many genes.

  • Each gene also comes as a pair, with one on the maternal chromosome and the other on the paternal chromosome; these are called alleles.

  • Each copy of a pair of genes is located at the same position on their chromosome, called the locus (pl. loci).

  • Therefore, gene and locus are often used as synonyms.

• Alleles

  • Examples of allele combinations: AA, Aa, aa.

Gene, Mutation, and Dominance

• Gene as a DNA Sequence

  • A gene can be seen as a section of DNA sequence that encodes for one protein.

  • The process involves:

    • Transcription: DNA sequence is transcribed into RNA sequence in the nucleus.

    • Translation: RNA sequence is translated into a polypeptide (premature protein) at the ribosome in the cytoplasm.

    • Post-translational modification: The polypeptide undergoes modification in the Golgi apparatus and cytoplasm to become a functional protein.

• Dominant vs. Recessive

  • Dominant: A gene sequence is intact and can express a functional protein, so the protein's function is visible.

  • Recessive: A gene sequence has a mutation (broken) and can only express a non-functional protein (the protein will be broken), therefore, the protein's function cannot be seen.

  • Gene expression is the process of using a gene blueprint to make a functional protein.

  • A recessive gene is not hidden behind the dominant gene; a dominant gene doesn't cover the recessive gene either. Dominant is functional, and recessive is not.

• Genotype and Phenotype

  • A diploid organism has two homologous chromosomes, each bearing a copy (allele) of a particular gene.

  • As long as one copy is still functional, the phenotype will show wild-type characteristics.

  • Two alleles of a gene can be both dominant (AA), which is called homozygous.

  • They can be one dominant and one recessive (Aa), which is called heterozygous, and the phenotype is still dominant.

  • Or alleles can be both recessive (aa), which is called homozygous, and the phenotype is recessive.

Inheritance

• Asexual Reproduction (Single-Celled Organisms)

  • Produces identical offspring.

  • Since all offspring individuals are identical, when the environment changes, the population can either all sustain the change or all be wiped out.

  • Results in low adaptation.

• Sexual Reproduction

  • Each parent contributes half of the genetic information, and the children have traits from both parents.

  • Offspring from sexual reproduction have more diversities and therefore, the population is better at adapting to a changing environment.

  • To produce cells containing half of the genetic materials, another type of cell division is needed: meiosis.

Mitosis vs. Meiosis

• DNA Replication

  • Both mitosis and meiosis require DNA replication.

  • Mitosis replicates DNA once and divides cells once, whereas meiosis replicates DNA once and divides cells twice.

  • Therefore, when meiosis produces new cells, they are haploid (have only half of the DNA/chromosomes).

• Cell Type

  • Mitosis produces somatic cells (body cells) to increase the number of cells in the body.

  • Meiosis produces gametes.

• Chromosome Alignment

  • Homologous chromosomes align at Metaphase I during meiosis, but not in mitosis.

  • Each pair of chromosomes consists of one chromosome from the father and one from the mother. These two chromosomes are homologous chromosomes to each other.

• Chromosome Structure During Interphase

  • During interphase, chromosomes are replicated (and become two chromatids).

  • During anaphase, these two chromatids will split and go to different cells.

• Chromosome Recombination/Crossover

  • Because homologous chromosomes align together, they can exchange their parts. This is called chromosome recombination or crossover.

• Mathematical Representation

  • 1M + 1P represents one maternal and one paternal chromosome.

Chromosome Recombination

• Occurence

  • Chromosome recombination (crossover) occurs at Metaphase I.

  • When homologous chromosomes align together, they can exchange their parts.

  • No recombination during metaphase II.

• Chiasma

  • Chiasma stabilize metaphase I.

• Diversity

  • Due to recombination, meiosis generates more diverse products.

Mitosis vs. Meiosis: Key Differences

• Functions

• Number of Chromosomes

• Number of Products

• Types of Products

• Crossing-over (Recombination)

• Number of Divisions

Mendel's Experiments

• Material Selection

  • Mendel's experiments utilized inheritance of seed color.

• Observations

  • Inheritance of seed color is not passed down by blending two parental traits.

  • Whether using the pollens of yellow-seeded plants or green-seeded plants, the result will be the same.

  • The green trait was not lost, just cannot be seen when the yellow trait is present.

Mendel's Laws of Inheritance

• Basic Principles

  • Traits of inheritance are governed by hereditary factors (now called genes).

  • Such factors act like particles, independent, rather than blending.

  • Two variations of each trait (e.g., green and yellow), now known as dominant and recessive alleles.

  • Plants carry two alleles (on two chromosomes, one from mother, one from father) for each gene.

  • The appearance (phenotype) of plants depends on the actual genes (genotype).

  • If two alleles of one individual are identical, it is homozygous; if different, it is heterozygous.

  • In heterozygotes, not all the alleles can be shown in the phenotype; the one that can be detected is called dominant.

• Law of Segregation

  • During gamete formation, each of the two alleles will go to two different gametes.

  • Then two of them (one from father, and one from mother) will unite randomly during fertilization.

• Considering Two Pairs of Alleles (Two Genes)

  • Example: Yy Rr, where Y and y are alleles for one gene, and R and r are alleles for another gene.

• Law of Independent Assortment

  • The segregation of each pair of alleles is independent of each other (different traits don’t affect each other).

Gametes and Linked Genes

• Unlinked Genes

  • If two genes A and B are on different chromosomes, they are not linked.

  • Example: AaBb

  • Possible gamete combinations: AB, Ab, aB, ab.

• Parental Genotype

  • AaBb x AaBb can produce offspring with genotypes: AABB, AABb, AaBB, AaBb, AAbb, Aabb, aaBB, aaBb, aabb.

• Segregation of Chromosomes and Mendel's Discovery

  • Separation of chromosomes during meiosis explains Mendel’s discovery.

Complex Traits

• Polygenic Traits

  • Traits that are controlled by more than one gene (or called polygenic).

  • Pigmentation, for example, the colors of organisms are normally controlled by multiple genes.

• Example

  • Precursor pigment is converted to Pigment B by Enzyme A (Gene A).

  • Pigment B is converted to Pigment C by Enzyme B (Gene B).

  • Pigment C is converted to Pigment D by Enzyme C (Gene C).

  • Pigment D produces Color 1, Pigment C produces Color 2, and Pigment B produces Color 3.

Single-Nucleotide Polymorphism (SNPs)

• Polymorphism

  • A highly diversified region in the genome (all DNA in the body).

• SNP Definition

  • SNP is one type of this highly diversified region, and it’s only one nucleotide different among individuals.