Cell Division and Variation Flashcards
describe the relationship
among DNA, chromatin,
and chromosomes
The relationship among DNA, chromatin, and chromosomes is crucial for understanding how genetic information is organized and transmitted within cells. Here’s an explanation of this relationship:
DNA (Deoxyribonucleic Acid):
DNA is a molecule that carries genetic information in living organisms.
It consists of two long strands arranged in a double helix structure, made up of nucleotides containing a sugar-phosphate backbone and nitrogenous bases (adenine, thymine, guanine, and cytosine).
DNA encodes the instructions for building and maintaining an organism’s cells, tissues, and organs through the genetic code.
Chromatin:
Chromatin is the complex of DNA and proteins found in the nucleus of eukaryotic cells.
It consists of DNA wrapped around histone proteins, forming nucleosomes.
The primary function of chromatin is to package DNA efficiently into a compact structure that can fit inside the nucleus and regulate gene expression.
Chromatin undergoes dynamic changes in its structure to allow DNA replication, gene transcription, and other cellular processes.
Chromosomes:
Chromosomes are condensed structures of chromatin that are visible during cell division.
They consist of tightly coiled and condensed chromatin fibers.
Chromosomes are essential for the accurate segregation of genetic material during cell division (mitosis and meiosis).
Humans typically have 46 chromosomes (23 pairs) in each cell, including 22 pairs of autosomes and one pair of sex chromosomes (XX for females, XY for males).
Relationship and Transition:
DNA is the fundamental molecule that carries genetic information in the form of a sequence of nucleotides.
Chromatin represents the complex of DNA and proteins that forms the chromosomal material within the nucleus.
During cell division (particularly during mitosis and meiosis), chromatin condenses further into visible structures called chromosomes.
Chromosomes are distinct, visible structures that contain tightly packaged DNA and are essential for the accurate distribution of genetic material to daughter cells during cell division.
describe with the aid
of diagrams, the stages
of mitotic cell division
(include interphase)
Mitotic cell division is a process by which a single cell divides to produce two genetically identical daughter cells. It consists of several stages, including interphase, prophase, prometaphase, metaphase, anaphase, and telophase. Here’s a description of each stage, including interphase, along with diagrams illustrating the process:
- Interphase:
Interphase is the phase of the cell cycle during which the cell grows, replicates its DNA, and prepares for cell division.
It consists of three subphases: G1 (Gap 1), S (Synthesis), and G2 (Gap 2).
During G1 phase, the cell grows, synthesizes proteins, and carries out its normal functions.
In the S phase, DNA replication occurs, resulting in the duplication of the cell’s genetic material.
G2 phase is characterized by further cell growth and preparation for mitosis.
2. Prophase:
In prophase, chromatin condenses into visible chromosomes, and the nuclear envelope disintegrates.
Each chromosome consists of two sister chromatids held together by a centromere.
Spindle fibers begin to form, extending from opposite poles of the cell.
3. Prometaphase:
During prometaphase, the spindle fibers further extend and attach to the centromeres of the chromosomes.
The chromosomes become more condensed, and the nuclear envelope fully dissolves.
Microtubules from the spindle apparatus attach to the kinetochores, specialized protein structures located at the centromeres.
4. Metaphase:
In metaphase, the chromosomes align along the metaphase plate, an imaginary plane equidistant between the two poles of the cell.
The spindle fibers attach to the kinetochores of the chromosomes, ensuring proper alignment.
5. Anaphase:
Anaphase begins with the separation of sister chromatids as the centromeres divide.
The spindle fibers shorten, pulling the sister chromatids toward opposite poles of the cell.
Each pole receives an equal and identical set of chromosomes.
6. Telophase:
Telophase marks the end of nuclear division.
Chromosomes decondense, returning to an extended chromatin state.
Nuclear envelopes form around the separated sets of chromosomes, and the nucleoli reappear.
Meanwhile, cytokinesis, the division of the cytoplasm, occurs, resulting in the formation of two daughter cells, each with a complete set of chromosomes.
explain the importance of
DNA replication for
maintaining genetic
stability
DNA replication is essential for maintaining genetic stability in living organisms. It ensures the faithful transmission of genetic information from parent cells to daughter cells during cell division and the preservation of genetic integrity across generations. Here are several key reasons why DNA replication is crucial for maintaining genetic stability:
Accurate Transmission of Genetic Information: DNA replication ensures that each daughter cell receives an accurate copy of the genetic material from the parent cell. Errors or mutations in DNA replication can lead to genetic abnormalities, which may cause diseases or impair cellular functions.
Cellular Growth and Development: DNA replication is necessary for cellular growth and development. During growth and tissue repair, cells divide to produce new cells, each containing a complete set of genetic instructions. Accurate DNA replication ensures that the newly formed cells have the correct genetic information to perform their specific functions.
Genetic Diversity: While DNA replication primarily aims to maintain genetic stability, it also plays a role in generating genetic diversity. DNA replication errors, such as mutations or DNA damage, can lead to variations in the genetic material, contributing to genetic diversity within populations and evolution over time.
Repair of DNA Damage: DNA replication machinery includes mechanisms for detecting and repairing errors or damage in the DNA sequence. Various DNA repair pathways, such as base excision repair and nucleotide excision repair, correct mistakes introduced during replication or caused by external factors such as radiation, chemicals, or oxidative stress. Efficient DNA repair processes help maintain genomic integrity and prevent the accumulation of mutations.
Cellular Homeostasis: Proper regulation of DNA replication is crucial for maintaining cellular homeostasis and preventing uncontrolled cell proliferation. Dysregulation of DNA replication can lead to genomic instability, chromosomal aberrations, and the development of cancerous tumors.
discuss the role and
importance of mitosis in
growth, repair and asexual
reproduction
Mitosis is a fundamental process of cell division that plays crucial roles in growth, tissue repair, and asexual reproduction in multicellular organisms. Here’s a discussion of the role and importance of mitosis in these biological processes:
Growth:
Mitosis is essential for the growth and development of multicellular organisms. During growth, cells divide to produce new cells, which subsequently increase the size and complexity of tissues and organs.
Mitotic cell division allows organisms to grow from a single fertilized egg (zygote) to a complex multicellular organism composed of trillions of cells.
Mitosis ensures that each newly formed cell receives an identical copy of the genetic material (DNA), ensuring genetic stability and the proper functioning of cells and tissues.
Tissue Repair:
Mitosis plays a critical role in tissue repair and regeneration following injury or damage. When tissues are damaged, cells undergo mitotic cell division to replace lost or damaged cells and restore tissue integrity.
Injured tissues activate signaling pathways that stimulate nearby cells to proliferate and undergo mitosis to replace the damaged cells.
Mitosis is particularly important in rapidly renewing tissues with high cell turnover rates, such as the skin, gastrointestinal tract, and blood cells.
Asexual Reproduction:
In many organisms, mitosis is the primary mechanism of asexual reproduction, allowing individuals to reproduce without the need for a mate.
During asexual reproduction, a single parent cell divides mitotically to produce genetically identical offspring, or clones.
Examples of asexual reproduction via mitosis include budding in yeast, fragmentation in certain plants and animals, and binary fission in bacteria and protists.
Mitotic cell division ensures that the offspring inherit an identical set of genetic information from the parent, maintaining genetic continuity and stability within the population.
Importance of Mitosis:
Mitosis ensures the accurate transmission of genetic material from parent cells to daughter cells, maintaining genetic stability and integrity.
It allows for the growth, development, and maintenance of tissues and organs in multicellular organisms.
Mitosis plays a vital role in tissue repair and regeneration, facilitating the healing process following injury or damage.
In organisms that reproduce asexually, mitosis enables the production of genetically identical offspring, ensuring the continuity of the species.
describe with the aid of
diagrams, the processes
involved in meiotic cell
division
(Names of the intermediate stages of meiosis not required)
Meiotic cell division is a specialized type of cell division that occurs during the formation of gametes (sperm and eggs) in sexually reproducing organisms. Meiosis involves two sequential divisions (meiosis I and meiosis II) that result in the production of haploid daughter cells with half the number of chromosomes as the parent cell. Here’s a description of the processes involved in meiotic cell division, along with diagrams illustrating the key stages:
Meiosis I:
Prophase I:
Homologous chromosomes pair up to form tetrads (bivalents), consisting of four chromatids.
Crossing over occurs between non-sister chromatids, leading to the exchange of genetic material between homologous chromosomes.
The nuclear envelope breaks down, and the spindle apparatus forms.
Metaphase I:
Tetrads line up along the metaphase plate, with homologous chromosomes oriented toward opposite poles of the cell.
Spindle fibers attach to the centromeres of homologous chromosomes.
Anaphase I:
Homologous chromosomes separate and move to opposite poles of the cell, pulled by the spindle fibers.
Sister chromatids remain attached at the centromere.
Telophase I and Cytokinesis:
Chromosomes arrive at the poles, and the nuclear envelope reforms around each set of chromosomes.
Cytokinesis divides the cell into two daughter cells, each containing half the number of chromosomes as the parent cell.
Meiosis II:
Prophase II:
The nuclear envelope breaks down, and spindle fibers reappear.
Chromosomes condense, and sister chromatids become visible.
Metaphase II:
Chromosomes line up along the metaphase plate, with sister chromatids oriented toward opposite poles of the cell.
Spindle fibers attach to the centromeres of sister chromatids.
Anaphase II:
Sister chromatids separate and move to opposite poles of the cell, pulled by the spindle fibers.
Telophase II and Cytokinesis:
Chromosomes arrive at the poles, and the nuclear envelope reforms around each set of chromatids.
Cytokinesis divides the cell into four haploid daughter cells, each containing a single set of chromosomes.
Describe terms homologous, chromosomes, haploid
diploid, inclusion of crossing over, alignment of chromosomes at
metaphase, random segregation at anaphase.
Homologous Chromosomes:
Homologous chromosomes are pairs of chromosomes that have the same genes at the same loci (positions) but may have different alleles (variants) of those genes.
One homologous chromosome in each pair is inherited from the organism’s mother, while the other is inherited from the father.
Homologous chromosomes are similar in size, shape, and gene content, and they carry genetic information for the same traits.
Chromosomes:
Chromosomes are thread-like structures made of DNA and proteins (histones) found in the nucleus of eukaryotic cells.
They contain the genetic information (genes) necessary for the growth, development, and functioning of an organism.
Chromosomes are visible under a microscope during cell division when they condense and become tightly coiled.
Haploid:
Haploid refers to a cell or organism that has a single set of chromosomes (n).
In sexually reproducing organisms, haploid cells, such as gametes (sperm and eggs), contain half the number of chromosomes as diploid cells.
Haploid cells are produced through the process of meiosis, where diploid cells undergo two rounds of division to generate haploid daughter cells.
Diploid:
Diploid refers to a cell or organism that has two sets of chromosomes (2n).
Diploid cells contain pairs of homologous chromosomes, one set inherited from each parent.
Most somatic (non-reproductive) cells in multicellular organisms are diploid.
Crossing Over:
Crossing over is a process that occurs during prophase I of meiosis, where homologous chromosomes exchange genetic material.
It results in the recombination of genetic material between homologous chromosomes, leading to the production of new combinations of alleles.
Crossing over increases genetic diversity by creating genetic variation among offspring.
Alignment of Chromosomes at Metaphase:
During metaphase of mitosis or meiosis, chromosomes align along the metaphase plate, an imaginary plane equidistant between the two poles of the cell.
In mitosis, individual chromosomes align along the metaphase plate.
In meiosis I, homologous chromosomes pair up and align along the metaphase plate as tetrads (bivalents).
Proper alignment ensures that chromosomes are evenly distributed to daughter cells during cell division.
Random Segregation at Anaphase:
During anaphase of meiosis I, homologous chromosomes separate and move to opposite poles of the cell.
During anaphase of meiosis II, sister chromatids separate and move to opposite poles of the cell.
The segregation of chromosomes is random, meaning that the arrangement of chromosomes on the metaphase plate is independent of other chromosome pairs, leading to genetic diversity in the resulting gametes.
discuss how meiosis
contributes to heritable
variation
(Comparison with the
process of mitosis.)
- Independent Assortment:
During meiosis I, homologous chromosomes randomly align along the metaphase plate, leading to the random distribution of maternal and paternal chromosomes to daughter cells.
This process results in different combinations of maternal and paternal chromosomes in the gametes produced, contributing to genetic diversity.
In contrast, mitosis results in genetically identical daughter cells, as chromosomes segregate equally to each daughter cell without random assortment.
2. Crossing Over:
Crossing over occurs during prophase I of meiosis, where homologous chromosomes exchange segments of genetic material.
This genetic exchange between homologous chromosomes creates new combinations of alleles on the chromosomes.
Crossing over increases genetic diversity by shuffling alleles between chromosomes, resulting in offspring with novel genetic combinations.
Crossing over does not occur during mitosis, as there are no homologous chromosomes to undergo genetic exchange.
3. Reduction in Chromosome Number:
Meiosis reduces the chromosome number by half, producing haploid gametes (sperm and eggs) from diploid parent cells.
This reduction in chromosome number ensures that the offspring receive the correct number of chromosomes from each parent.
In contrast, mitosis maintains the chromosome number, producing genetically identical diploid daughter cells.
4. Random Segregation:
During anaphase of meiosis I and II, homologous chromosomes and sister chromatids segregate randomly to daughter cells.
This random segregation of chromosomes leads to different combinations of alleles in the gametes produced.
In mitosis, sister chromatids segregate equally to each daughter cell, resulting in genetically identical offspring.
Comparison with Mitosis:
Mitosis results in the production of genetically identical daughter cells, as chromosomes replicate and segregate to produce two diploid daughter cells.
Meiosis, on the other hand, generates genetic diversity by reducing the chromosome number, shuffling alleles through crossing over, and randomly segregating chromosomes during cell division.
While mitosis plays a crucial role in growth, tissue repair, and asexual reproduction, meiosis is essential for sexual reproduction and the generation of genetically diverse offspring.
explain why sexually
produced organisms vary
in characteristics
Meiosis and Genetic Recombination:
Meiosis is a specialized form of cell division that produces haploid gametes (sperm and eggs) from diploid parent cells.
During meiosis, homologous chromosomes pair up and undergo crossing over, where segments of genetic material are exchanged between chromosomes.
Crossing over results in the recombination of alleles on homologous chromosomes, creating new combinations of genetic traits.
Additionally, independent assortment during meiosis leads to the random distribution of maternal and paternal chromosomes to daughter cells, further increasing genetic diversity.
Fertilization:
Fertilization is the process by which haploid gametes (sperm and eggs) fuse to form a diploid zygote.
During fertilization, genetic material from both the sperm and egg combines, resulting in a unique combination of alleles in the offspring.
Each zygote formed through fertilization inherits a different combination of genetic traits from its parents, contributing to variation among individuals within a population.
Mendelian Inheritance:
Genetic variation is also influenced by Mendelian inheritance patterns, where alleles for specific traits are inherited according to predictable ratios.
Genes may have multiple alleles, and the expression of traits can be influenced by complex interactions between alleles and environmental factors.
Variation in characteristics among sexually produced organisms can result from the combination and expression of different alleles inherited from their parents.
Environmental Factors:
Environmental factors can also contribute to variation in characteristics among individuals within a population.
Environmental conditions, such as temperature, light, nutrient availability, and social interactions, can influence gene expression and phenotypic traits.
Organisms may exhibit phenotypic plasticity, where the same genotype can produce different phenotypes in response to environmental cues, further contributing to variation.
Random Mutation:
Random mutations, changes in the DNA sequence, can also contribute to genetic variation within a population.
Mutations can arise spontaneously during DNA replication, exposure to mutagens (such as radiation or chemicals), or errors in DNA repair mechanisms.
Some mutations may be beneficial, harmful, or have no significant effect on an organism’s phenotype, leading to variation among individuals within a population
describe gene and
chromosome mutations
(Include the types of gene and chromosome mutations)
Point Mutations:
Point mutations involve changes in a single nucleotide base within a gene.
Subtypes of point mutations include:
Substitution: One nucleotide base is replaced by another, potentially leading to changes in the amino acid sequence (missense mutation) or no change (silent mutation).
Insertion: An extra nucleotide base is inserted into the DNA sequence, causing a shift in the reading frame (frameshift mutation).
Deletion: One nucleotide base is removed from the DNA sequence, also causing a frameshift mutation.
Frameshift Mutations:
Frameshift mutations occur when the addition or deletion of nucleotides alters the reading frame of the gene, leading to changes in the amino acid sequence downstream of the mutation.
Frameshift mutations can have significant effects on protein structure and function.
Missense and Nonsense Mutations:
Missense mutations result in the substitution of one amino acid for another in the protein sequence, potentially altering protein function.
Nonsense mutations introduce a premature stop codon in the mRNA sequence, leading to the premature termination of protein synthesis.
Silent Mutations:
Silent mutations do not result in changes to the amino acid sequence of a protein due to redundancy in the genetic code.
These mutations may occur in non-coding regions of the gene or in codons that specify the same amino acid.
Chromosome Mutations:
Deletion:
Deletion involves the loss of a segment of a chromosome.
Large deletions can lead to the loss of multiple genes, resulting in genetic disorders or developmental abnormalities.
Duplication:
Duplication occurs when a segment of a chromosome is duplicated, resulting in an extra copy of the genetic material.
Duplication events can lead to gene dosage effects, gene amplification, or the formation of gene families.
Inversion:
Inversion involves the reversal of the orientation of a segment of a chromosome.
Inversions may disrupt gene function or gene regulation by altering the order or orientation of genes within the inverted segment.
Translocation:
Translocation occurs when a segment of one chromosome breaks off and attaches to another non-homologous chromosome.
Translocations can result in the relocation of genes to different chromosomal locations or the fusion of genes from different chromosomes.
Insertion:
Insertion involves the insertion of an extra segment of DNA into a chromosome.
Insertions may disrupt gene function or alter gene regulation, depending on the location and size of the inserted segment.
Aneuploidy:
Aneuploidy refers to the gain or loss of whole chromosomes, leading to an abnormal chromosome number.
Examples include trisomy (three copies of a chromosome) and monosomy (one copy of a chromosome), which can result in genetic disorders such as Down syndrome and Turner syndrome.
discuss the implications
of changes in DNA
nucleotide sequence for
cell structure and function
in sickle cell anaemia
Sickle cell anemia is a genetic disorder caused by a specific mutation in the DNA sequence that affects the structure and function of hemoglobin, the protein responsible for carrying oxygen in red blood cells. The implications of changes in the DNA nucleotide sequence for cell structure and function in sickle cell anemia are significant and include:
Abnormal Hemoglobin Structure:
Sickle cell anemia is caused by a point mutation in the HBB gene, which encodes the beta-globin subunit of hemoglobin.
The mutation results in the substitution of a single nucleotide base (adenine for thymine) in the HBB gene, leading to the replacement of glutamic acid with valine at the sixth position of the beta-globin chain.
This amino acid substitution alters the structure of hemoglobin, causing it to polymerize and form long, rigid chains under low oxygen conditions.
Sickled Red Blood Cells:
The polymerization of abnormal hemoglobin molecules distorts the shape of red blood cells, causing them to assume a sickle or crescent shape.
Sickled red blood cells are less flexible and more prone to deformation and damage compared to normal, disc-shaped red blood cells.
The altered morphology of sickled red blood cells contributes to their reduced ability to flow smoothly through blood vessels, leading to blockages and impaired blood flow.
Impaired Oxygen Transport:
The presence of sickled red blood cells in circulation results in impaired oxygen transport throughout the body.
Sickled cells have a reduced capacity to carry oxygen due to their abnormal shape and decreased lifespan.
Tissues and organs may become hypoxic (oxygen-deprived), leading to tissue damage, organ dysfunction, and complications such as pain crises, organ failure, and stroke.
Vaso-occlusive Crises:
The aggregation of sickled red blood cells can lead to the formation of clumps or clusters within small blood vessels, causing blockages and vaso-occlusive crises.
Vaso-occlusive crises are characterized by episodes of severe pain, tissue ischemia (lack of blood flow), and organ damage, particularly in the bones, joints, spleen, and lungs.
These crises can be triggered by factors such as dehydration, infection, stress, or exposure to cold temperatures.
Chronic Anemia:
Sickle cell anemia is associated with chronic hemolytic anemia, where sickled red blood cells are destroyed at a faster rate than normal red blood cells.
The reduced lifespan of sickled cells leads to a shortage of red blood cells, resulting in anemia, fatigue, weakness, and pallor.
Bone marrow may compensate by increasing red blood cell production, leading to erythroid hyperplasia and skeletal deformities in severe cases.
explain how mutation
brings about genetic
variation
(Not limited to: sickle-cell
anaemia, cystic fibrosis,
PKU and Down Syndrome.)
explain why heritable
variation is important to
selection
Heritable variation is essential to the process of natural selection, which is a fundamental mechanism of evolution proposed by Charles Darwin. Here’s why heritable variation is important to selection:
Raw Material for Evolution:
Heritable variation provides the raw material upon which natural selection acts. Without variation in traits among individuals within a population, there would be no basis for selection to occur.
Adaptation to Changing Environments:
Heritable variation allows populations to adapt to changing environmental conditions over time. Individuals with traits that are better suited to their environment are more likely to survive and reproduce, passing on their advantageous traits to future generations.
Response to Selection Pressures:
Selection pressures, such as predation, competition for resources, and environmental changes, exert selective forces on populations. Heritable variation ensures that there is diversity in traits within populations, allowing some individuals to possess traits that confer a survival or reproductive advantage in the face of these pressures.
Maintenance of Genetic Diversity:
Heritable variation helps maintain genetic diversity within populations, which is important for the long-term survival and evolutionary potential of species.
Genetic diversity provides a reservoir of potentially beneficial traits that can be drawn upon in response to new selection pressures or environmental challenges.
Evolutionary Innovation:
Heritable variation allows for the emergence of new traits and adaptations through genetic mutations and recombination. Novel traits may arise through genetic changes, providing opportunities for evolutionary innovation and diversification of species.
Population Differentiation and Speciation:
Heritable variation contributes to population differentiation and the formation of new species over time. Divergent selection pressures in different environments can lead to the accumulation of genetic differences between populations, eventually resulting in reproductive isolation and speciation.
Enhanced Fitness and Reproductive Success:
Heritable variation enables individuals with advantageous traits to have higher fitness and reproductive success compared to individuals with less advantageous traits. Over time, these advantageous traits become more common in the population through natural selection.
