BIOL 411 Lecture 33: Meiosis Notes
Lecture Objectives
Understand Gamete formation in human cells
Contrast the steps of Meiosis I and Meiosis II
Describe Crossing over events during Prophase I
Explain Nondisjunction events and how they effect fertilization
Understand the ways Meiosis influences genetic diversity and evolution
Prophase I (Meiosis I) : Steps 4–5
At the end of prophase I:
The chromosomes are fully condensed and have formed chiasmata.
The nuclear envelope has begun to disappear.
The meiotic spindle is forming.
Crossing Over
Recombinant chromosomes bring alleles together in new combinations in gametes
Random fertilization increases even further the number of variant combinations that can be produced
This abundance of genetic variation is the raw material upon which natural selection works
The Stages of Meiosis Metaphase I
In metaphase I, homologous pairs line up at the metaphase plate, with one chromosome facing each pole
Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad
Microtubules from the other pole are attached to the kinetochore of the other chromosome
The Stages of Meiosis Anaphase I
In anaphase I, pairs of homologous chromosomes separate
One chromosome moves toward each pole, guided by the spindle apparatus
Sister chromatids remain attached at the centromere and move as one unit toward the pole
The Stages of Meiosis Telophase I and Cytokinesis
In the beginning of telophase I, each half of the cell has a haploid set of chromosomes
Each chromosome still consists of two sister chromatids
Cytokinesis usually occurs simultaneously, forming two haploid daughter cells
In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms
No chromosome duplication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated
The Stages of Meiosis II
Division in meiosis II also occurs in five phases
Prophase II
Prometaphase II
Metaphase II
Anaphase II
Telophase II and cytokinesis
Meiosis II is very similar to mitosis
The Stages of Meiosis Prophase II
In prophase II, a spindle apparatus forms
In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate
The Stages of Meiosis Metaphase II
The sister chromatids are arranged at the metaphase plate
Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical
The kinetochores of sister chromatids attach to microtubules extending from opposite poles
The Stages of Meiosis Anaphase II
In anaphase II, the sister chromatids separate
The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles
The Stages of Meiosis Telophase II and Cytokinesis
Nuclei form, and the chromosomes begin decondensing
At the end of meiosis, there are four daughter cells, each with a haploid set of unduplicated chromosomes
Each daughter cell is genetically distinct from the others and from the parent cell
A Comparison of Mitosis and Meiosis
Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell
Meiosis reduces the number of chromosome sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell
Meiosis includes two divisions after replication, each with specific stages
Three events are unique to meiosis, and all three occur in meiosis l
Crossover in prophase I: Homologous chromosomes physically connect and exchange genetic information
Alignment of homologous pairs at the metaphase plate: Homologous pairs of chromosomes are positioned there in metaphase I
Separation of homologs during anaphase I
Review for Final Exam
Grading Breakdown
Homework Assignments 15% Lowest 2 dropped
Exams 30% Exams 1-4- Lowest 1 dropped
Final Exam 15% Not dropped
Discussion Boards/PLTL 10% Lowest 2 dropped
iClicker 05% adjusted- 75% = full credit
Lab 25%
Extra Credit Assignment for the Final
SEL Surveys
Reminders
Monday, May 5
HW10 closes at 1159pm
Extra Credit Assignment for Final Exam closes at 1159pm
Blue SEL Surveys close at 1159pm
Final Exam is on Thursday 8 May, 10:30am-12:30pm
Last Names A-L will take the Final in HORTON B04
Last Names M-Z will take the Final in MURKLAND 115
Final Exam
Cumulative- questions from every week
85 Multiple Choice Questions
Bring UNH ID
Coverage- Lectures 1-33
Helpful Tools
Previous Exams 1-4; Homework Assignments
Make flashcards/quizlets
Recordings and Clicker questions
Use exam reviews (weeks 4, 7, 11, 14, 15 in Modules/Canvas)
Other resources in Canvas
Reach out for help (Office hours May 7)
Find the BIG Picture
Exam 1
Organization of Life- Molecules to Genetic material to macromolecules to cells
Eukaryotic cell
Membrane
Cytoplasm
Membrane- enclosed organelles
Nucleus (membrane- enclosed)
DNA (throughout nucleus) 1 mm
Prokaryotic cell
DNA (no nucleus)
Membrane
DNA (part of the crystallin gene)The crystallin gene is a section of DNA in a chromosome.
Crystallin gene
A lens cell uses information in DNA to make crystallin proteins.
Using the information in the sequence of DNA nucleotides, the cell makes (transcribes) a specific RNA molecule called mRNA.
The cell translates the information in the sequence of mRNA nucleotides to make a protein, a series of linked amino acids.
Chain of amino acids
mRNA
The chain of amino acids folds into the specific shape of a crystallin protein.
Crystallin proteins can then pack together and focus light, allowing the eye to see.
Protein
Crystallin protein
TRANSCRIPTION
TRANSLATION
PROTEIN FOLDING
Exam 1
Element and Atoms
Chemical Bonds
Covalent
Ionic
Hydrogen
pH Scale
Covalent Bonds
A single bond, the sharing of one pair of electrons, is indicated by a single line between the atoms – For example,
A double bond, the sharing of two pairs of electrons, is indicated by a double line between atoms – For example,
Each atom that can share valence electrons has a bonding capacity, the number of bonds that the atom can form
Bonding capacity, or valence, usually corresponds to the number of electrons required to complete the atom
Ionic Bonds
Formed between two oppositely charged ions.
Hydrogen Bonding
Hydrogen Bonding Gives Water Properties That Help Make Life Possible on Earth
All organisms are made mostly of water and live in an environment dominated by water
Water molecules are polar molecules, with the oxygen region having a partial negative charge and the hydrogen region a partial positive charge
Two water molecules are held together by a hydrogen bond
At any instant, most of the water molecules are hydrogen-bonded to their neighbors
Acids and Bases pH Scale
We use a pH scale to describe how acidic or basic a solution is.
It tends to range from 0-14.
It is a logarithmic scale.
Each increase by one represents a 10x increase.
Chapter 2
Carbon-Hydrocarbons are organic molecules consisting of only carbon and hydrogen
Macromolecules and their functions
ATP: An Important Source of Energy for Cellular Processes
An organic phosphate molecule, adenosine triphosphate (ATP), has an important function in the cell
ATP stores the potential to react with water, releasing energy that can be used by the cell
The Synthesis and Breakdown of Polymers
Cells make and break down polymers by the same mechanisms
A dehydration reaction occurs when two monomers bond together through the loss of a water molecule
Polymers are disassembled to monomers by hydrolysis, a reaction that is essentially the reverse of the dehydration reaction
These processes are facilitated by enzymes, which speed up chemical reactions
Carbohydrates Serve as Fuel and Building Material
Carbohydrates include sugars and the polymers of sugars
The simplest carbohydrates are monosaccharides, or simple sugars
Fats
Fats made from saturated fatty acids are called saturated fats and are solid at room temperature
Most animal fats are saturated
Plant fats and fish fats are usually unsaturated
Fats made from unsaturated fatty acids, called unsaturated fats or oils, are liquid at room temperature
Proteins Include a Diversity of Structures, Resulting in a Wide Range of Functions
Proteins account for more than 50% of the dry mass of most cells
Protein functions include defense, storage, transport, cellular communication, movement, and structural support
Levels of Protein Structure
Primary Structure Linear chain of amino acids
Secondary Structure Regions stabilized by hydrogen bonds between atoms of the polypeptide backbone
Tertiary Structure Three-dimensional shape stabilized by interactions between side chains
Quaternary Structure Association of two or more polypeptides (some proteins only)
Nucleic Acids Store, Transmit, and Help Express Hereditary Information
The amino acid sequence of a polypeptide is programmed by a unit of inheritance called a gene
Genes are made of DNA, a nucleic acid made of monomers called nucleotides
The Components of Nucleic Acids
The sugar in DNA is deoxyribose; in RNA it is ribose
A prime (′) is used to identify the carbon atoms in the ribose, such as the 2′ carbon or 5′ carbon
A nucleoside with at least one phosphate attached is a nucleotide
Chapter 5
Cells: Eukaryotic vs Prokaryotic
Organelle- Functions
Comparing Prokaryotic and Eukaryotic Cells
In a eukaryotic cell most of the DNA is in the nucleus, an organelle that is bounded by a double membrane
Prokaryotic cells are characterized by having
No nucleus
DNA in an unbound region called the nucleoid
No membrane-bound organelles
Both types of cells contain cytoplasm bound by the plasma membrane
A Panoramic View of the Eukaryotic Cell
A eukaryotic cell has extensive internal membranes that divide the cell into compartments—organelles
The plasma membrane and organelle membranes participate directly in the cell’s metabolism
The Evolutionary Origins of Mitochondria and Chloroplasts
Mitochondria and chloroplasts display the following similarities with bacteria that led to the endosymbiont theory:
Enveloped by a double membrane
Contain ribosomes and multiple circular D N A molecules
Grow and reproduce somewhat independently in cells.
The endosymbiont theory is widely accepted:
An early ancestor of eukaryotic cells engulfed a nonphotosynthetic prokaryotic cell, which formed an relationship with its host
The host cell and endosymbiont merged into a single organism, a eukaryotic cell with a mitochondrion
At least one of these cells may have then taken up a photosynthetic prokaryote, becoming the ancestor of cells that contain chloroplasts
Chapter 5
Membrane Transport
Membrane Proteins and Their Functions
Six major functions of membrane proteins
Transport
Enzymatic activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton and extracellular matrix (ECM)
Passive Transport Is Diffusion of a Substance Across a Membrane with No Energy Investment
Diffusion is the tendency for molecules to spread out evenly into the available space
Substances diffuse down their concentration gradient, from where it is more concentrated to where it is less concentrated
Substances move down their own concentration gradient, unaffected by concentration gradients of other substances
The diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell to make it happen
Review: Passive and Active Transport
Chapter 6
Metabolic Pathways
Catabolic pathways release energy by breaking down complex molecules into simpler compounds
The energy is then available to do cellular work
For example, in cellular respiration glucose and other organic fuels are broken down into carbon dioxide and water
Anabolic pathways, called biosynthetic pathways, consume energy to build complex molecules from simpler ones
For example, proteins are synthesized from simpler molecules called amino acids
Energy is fundamental to all metabolic processes
Bioenergetics is the study of how energy flows through living organisms
The Second Law of Thermodynamics
According to the second law of thermodynamics
Every energy transfer or transformation increases the entropy of the universe
Entropy is a measure of molecular disorder
Scientists use the term “disorder” to describe how dispersed energy is in a system and how many energy levels are present
Free Energy and Metabolism
The concept of free energy can be applied to the chemistry of life’s processes Exergonic and Endergonic Reactions in Metabolism
An exergonic reaction proceeds with a net release of free energy and is spontaneous; \\Delta G is negative
The magnitude of \\Delta G represents the maximum amount of work the reaction can perform
The Activation Energy Barrier
Enzyme Inhibitors
Enzyme activity is often regulated by molecules that selectively inhibit enzyme function
Competitive inhibitors bind to the active site of an enzyme and prevent the substrate from binding
Noncompetitive inhibitors bind to an alternate site on the enzyme, causing the active site to change shape and become less effective
Reversible enzyme inhibitors bind to enzymes by weak interactions; irreversible inhibitors form covalent bonds
Toxins and poisons are often irreversible enzyme inhibitors
Chapter 7
Cellular Respiration
Glycolysis
Citric Acid Cycle
Electron Transport Chain-Oxidative Phosphorylation
Fermentation
An Overview of Cellular Respiration: Electron Transport Chain
Stepwise Energy Harvest via NAD+ and the Electron Transport Chain
In cellular respiration, glucose and other organic molecules are broken down in a series of steps
Electrons from organic compounds are usually first transferred to NAD+, a coenzyme
As an electron acceptor, NAD+, functions as an oxidizing agent during cellular respiration
Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP
One hydrogen ion H+ is released in this process
The Stages of Cellular Respiration: A Preview
Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration
This process involves the transfer of inorganic phosphates to ADP
A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation
In this process, an enzyme transfers a phosphate group directly from a substrate molecule to ADP
For each molecule of glucose degraded to CO2 and water by respiration, the cell makes up to 32 molecules of ATP
Glycolysis is a Two-Phase Process
Set of 10 enzymatic reactions that convert Glucose into Pyruvate
Phase 1: Energy Investment
Two ATP are used to breakdown Glucose
Phase 2: Energy Generation
Four ATP are generated in the process of making 2 pyruvate
After Pyruvate Is Oxidized, the Citric Acid Cycle Completes the Energy-Yielding Oxidation of Organic Molecules
The citric acid cycle has eight steps, each catalyzed by a specific enzyme
The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate
The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle
The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain
Chemiosmosis Couples the Electron Transport Chain to ATP Synthesis
ATP Yield per Molecule of Glucose at Each Stage of Cellular Respiration
Photosynthesis
Light Reaction
Calvin Cycle
The Two Stages of Photosynthesis: A Preview
The light reactions convert solar energy into chemical energy
H2O is split to provide electrons and protons
O2 is released as a waste product
The electron acceptor is reduced to NADPH
ATP is generated by adding a phosphate group to ADP in a process called photophosphorylation
The Calvin cycle produces sugar from CO2 with the help of the NADP H and ATP produced by the light reactions
CO2 is initially incorporated into an organic molecule through a process called carbon fixation
ATP provides the necessary chemical energy, and NADPH provides electrons needed to reduce CO2
. The Calvin cycle uses the chemical energy of ATP and NADPH to reduce C O2 to sugar
Phase 3, regeneration, involves the rearrangement of the five remaining molecules of G3P to regenerate the initial CO2 acceptor, RuBP
Three additional ATP are required to power this step
Life Depends on Photosynthesis
Chapters 3 and 12
DNA- the Genetic Material
Replication of the genome
DNA Replication: Getting Started
Replication begins at sites called origins of replication, where the two DNA strands are separated, opening up a replication “bubble”
At each end of a bubble is a replication fork, a Y- shaped region where the parental strands of DNA are being unwound
For the long DNA molecules in eukaryotes, multiple replication bubbles form and eventually fuse, speeding up the copying of DNA
The DNA Replication Complex
The proteins that participate in DNA replication form a large complex, a “DNA replication machine”
The DNA replication machine may be stationary during the replication process
Recent studies support a model in which two D NA polymerase molecules “reel in” parental DNA and “extrude” newly made daughter DNA molecules
A Chromosome Consists of a DNA Molecule Packed Together with Proteins
Proteins called histones are responsible for the first level of DNA packing in chromatin
Four types of histones are most common in chromatin
Histones proteins are positively charged and DNA carries a negative charge
A nucleosome consists of DNA wound twice around a protein core of eight histones, two of each of the main histone types
Histones can undergo chemical modifications that result in changes in chromatin organization
TRANSCRIPTION
Transcription- Ch3
Translation- Ch4
Elongation of the RNA Strand
As RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at a time
Transcription progresses at a rate of 40 nucleotides per second in eukaryotes
A gene can be transcribed simultaneously by several RNA polymerases
Codons: Triplets of Nucleotides
The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words
The words of a gene are transcribed into complementary nonoverlapping three-nucleotide words of mRNA
These words are then translated into a chain of amino acids, forming a polypeptide
Molecular Components of Translation
A cell translates an mRNA message into protein with the help of transfer RNA (tRNA)
tRNA’s transfer amino acids to the growing polypeptide in a ribosome
Translation is a complex process in terms of its biochemistry and mechanics
A Summary of Transcription and Translation in a Eukaryotic Cell
Substitutions
A nucleotide-pair substitution replaces one nucleotide and its partner with another pair of nucleotides
Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the genetic code
Missense mutations change one amino acid to another
Substitution mutations are usually missense mutations
Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein
Organization of a Typical Eukaryotic Gene and Its Transcript
Associated with most eukaryotic genes are multiple control elements
These segments of noncoding DNA serve as binding sites for transcription factors that help regulate transcription
Control elements and the transcription factors they bind are critical for precise regulation of gene expression in different cell types
Enhancers and Specific Transcription Factors
Ch9-The Three Stages of Cell Signaling: A Preview
Cells receiving signals undergo three processes
Reception, detection of the signal
Transduction, conversion of the signal to a cellular response, via a signal transduction pathway
Response, a cellular activity in response to the signal
Chapter 11- Phases of the Cell Cycle
The cell cycle consists of
Mitotic (M) phase, including mitosis and cytokinesis
Interphase, including cell growth and copying of chromosomes in preparation for cell division
Interphase (about 90% of the cell cycle) can be divided into subphases
G1 phase (“first gap”)
S phase (“synthesis”)
G2 phase (“second gap”)
The cell grows during all three phases, but chromosomes are duplicated only during the S phase
Phases of the Cell Cycle
Binary Fission in Bacteria
Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission
In bacteria, the single chromosome replicates, beginning at the origin of replication
The two daughter chromosomes actively move apart while the cell elongates
The plasma membrane pinches inward, dividing the cell into two
Chapter 11
Meiosis
The Stages of Meiosis
For a single pair of homologous chromosomes in a diploid cell, both members of the pair are duplicated
The resulting sister chromatids are closely associated all along their lengths; this is called sister chromatid cohesion
Homologs may have different versions of genes, each called an allele
Homologs are not associated in any obvious way except during meiosis
The Stages of Meiosis
A Comparison of Mitosis and Meiosis
Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell
Meiosis reduces the number of chromosome sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell
Meiosis includes two divisions after replication, each with specific stages