D2.1 Cell and Nuclear Division Flashcards
List implications of the idea that new cells are only produced from a pre-existing cell.
- Supports the concept of biological continuity, since it’s saying that life continues only from existing life
- It supports cell theory “all cells come from pre-existing cells”
- ensures genetic continuity
- limits spontaneous generation
- explains cell-based diseases like cancer
- explains how reproduction at the cellular level supports reproduction at the organismal level
Define cytokinesis
Cytokinesis is the physical division of the cytoplasm and the cell itself into two daughter cells, following nuclear division
prokaryotic cells divide by?
binary fission
eukaryotic cells divide by?
a type of nuclear division called mitosis
(most eukaryotes can also carry out meiosis, which creates gametes)
what is the difference between mitosis and cytokinesis?
- Mitosis = the division/reproduction of nucleus
–> it ensures that each daughter cell receives an identical set of chromosomes
- Cytokinesis = the physical division of the cytoplasm and the cell into two separate daughter cells, following nuclear division
nuclear division can be mitosis/meiosis
Compare and contrast cytokinesis in plant and animal cells (SIMILARITIES)
1. Goal: both aim to result in two daughter cells after nuclear division
2. Occurs After Nuclear Division: both occur after the nucleus has divided
3. Membrane Alteration: both processes involve changes to the cell membrane to separate the two daughter cells
4. Cytoskeletal Involvement: both involve components of the cytoskeleton to facilitate the division (actin and myosin in animal cells + microtubules guiding vesicles to form the cell plate in plant cells)
5. Equal Distribution of Organelles: both ensure organelles are properly distributed between daughter cells
Compare and contrast cytokinesis in plant and animal cells (DIFFERENCES)
PLANT:
- cell plate is assembled from the fusion of vesicles
- cell plate grows outwards until it reaches the existing cell wall
- cell plate fuses with the cell wall, splitting the parent cell into the two daughter cells
ANIMAL:
- a network of actin and myosin proteins form a contractile ring that pinches the cell membrane together
- this forms a cleavage furrow, which gradually deepens and eventually splits the cytoplasm to form the two separate daughter cells
Actin and myosin are also involved in muscle contraction!
What are diploid cells?
Cells that have 2 sets of chromosomes (46)
Diploid cells are typically somatic (body) cells
What are haploid cells?
Cells that have 1 set of chromosomes (23)
Haploid cells are produced during meiosis
What are somatic cells?
Cells that only undergo mitosis (no meisos)
Before cell division occurs, the cell ________, so each daughter cell gets a ________
Before cell division occurs, the cell replicates all of its DNA, so each daughter cell gets a complete set of genetic information from its parent cell
Describe the formation of the cleavage furrow in animal cell cytokinesis (talk in more detail about this in specific)
cleavage furrow = is a pinching in of the cell membrane to separate the two daughter cells
- A network of actin and myosin proteins form a contractile ring
–> these proteins interact to generate the contractile force needed to pinch the membrane inward
- the contractile ring tightens and pulls the membrane inward, forming a cleavage furrow
- the cleavage furrow gradually deepens until the membrane completely pinches together, physically separating the two daughter cells
Describe the formation of the cell wall in plant cell cytokinesis (talk in more detail about this in specific)
- Vesicles containing cell wall materials are produced by the Golgi apparatus
- These vesicles are transported to the center of the cell, where they begin to fuse together
–> this creates the cell plate in the center of the cell
- the cell plate grows outwards, towards the existing cell wall + eventually fuses with it
–> this divides the cytoplasm and creates two daughter cells, each surrounded by its own cell wall
Cytokinesis usually, but not always, results in _______
Cytokinesis usually, but not always, results in equal division of the cytoplasm
Why must daughter cells receive at least one mitochondrion during cytokinesis?
- Mitochondria are essential for aerobic cellular respiration, which produces ATP (energy)
–> ATP = required for cellular functions
- Mitochondria cannot be made from scratch, so it must be passed down by dividing pre-existing structures
–> same goes for other organelles like peroxisomes
Equal cytoplasmic division results in ________, which is important to ________
Equal cytoplasmic division results in both daughter cells being the same size, which is important to ensure they have the same structure and function
What is oogenesis?
the process of producing mature egg cells or ova in humans
What is budding?
a type of asexual reproduction that involves the outgrowth of a genetically identical daughter cell (“bud”) from the parent cell
budding in yeast = an example of unequal cytokinesis
Outline unequal cytokinesis in yeast budding
the cell is unequal because the daughter cell is smaller and receives less than half of the cytoplasm and organelles from the parent cell during budding
What happens after budding?
the parent cell is left with a small round mark, called a budding scar, at the point where the daughter cell detached
–> scars can be used to determine the cell’s age and the number of divisions
Outline unequal cytokinesis during human oogenesis
First Division:
Primary oocyte divides into:
- Secondary oocyte (larger, receives most cytoplasm, organelles, ribosomes, and energy stores)
- First polar body (smaller, receives very little cytoplasm)
Second Division (if fertilization occurs):
Secondary oocyte divides into:
- Mature ovum (receives most of the cytoplasm, essential for early development)
- Second polar body (smaller, typically disintegrates)
Importance of Unequal Division
- the larger ovum has necessary nutrients and organelles for fertilization and early development
- smaller polar bodies ensure proper haploid chromosome count and provide energy for the developing embryo
what is mitosis?
mitosis = nuclear division resulting in continuity of the chromosome number and genome
- done produce cells for growth, or to replace cells that are lost or damaged
–> cells produced = genetically identical to the parent cell
- can occur as a form of asexual reproduction
- only happens in eukaryotes! (somatic cells in specific)
what is meiosis?
meiosis = nuclear division that results in reduction of the chromosome number and diversity between genomes
- produces four haploid genetically unique daughter nuclei, which will form gametes
- cells produced are genetically unique because of crossing over and independent assortment
- (the same as mitosis) meiosis begins with a diploid cell
- BUT (unlike mitosis), meiosis involves two rounds of nuclear division, producing four haploid daughter cells that contain only half the normal number of chromosomes
–> this is why meiosis is referred to as a reduction division
Outline the cause of anucleate cells
Enucleation: the process by which certain cells lose their nucleus after differentiation
Naturally Anucleate Cells: cells such as red blood cells and sieve tube elements undergo enucleation as part of their maturation process
Outline the consequence of anucleate cells.
Lack of Nucleus: anucleate cells cannot divide and do not contain DNA for replication or transcription
Functional Role:
- Red blood cells transport oxygen but cannot reproduce or repair themselves
- Sieve tube elements in plants transport nutrients but rely on adjacent cells for support
Developmental Limitation: without a nucleus, anucleate cells cannot develop or undergo cellular division, limiting their ability to adapt or respond to changes in the organism
DNA replication occurs before?
it occurs before both mitosis AND meiosis, in the S-phase of interphase
How are replicated DNA molecules are held together?
- After DNA replication, each replicated chromosome consists of two identical sister chromatids
–> Sister Chromatids = two identical DNA molecules produced by the replication of a single chromosome
- The sister chromatids are held together by the centromere, which acts as a “connector” that ensures they stay together until it’s time for cell division
a protein complex called cohesin is responsible for keeping the sister chromatids tightly bound at the centromere
What is chromatin made of?
made of DNA and histone proteins
(+ it forms the relaxed structure of chromosomes when the cell is not dividing)
What are nucleosomes, and what is their structure?
- Nucleosomes = the basic units of chromatin, made of 8 histone proteins with DNA coiled around them
- A 9th histone helps stabilize the DNA structure
Why do histones bind to DNA?
Histones have a positive charge, which allows them to interact with the negatively charged DNA, helping to compact and organize it
What happens to chromatin during nuclear division?
At the start of nuclear division, chromatin is supercoiled and condensed by histones into tightly packed chromosomes
Explain how and why chromosomes condense during mitosis and meiosis
HOW:
- In interphase, DNA is loosely packed as chromatin, wrapped around histone proteins to form nucleosomes
- At the start of mitosis or meiosis (nuclear division), histone proteins help to supercoil and condense the chromatin
–> chromatin = DNA + proteins
- This condensation forms the tightly coiled chromosomes that are visible under a microscope
WHY:
- Chromosome condensation is essential to allow efficient and accurate separation of genetic material
- Supercoiling prevents tangling or breakage and ensures each daughter cell receives the correct number of chromosomes during nuclear division
State the role of microtubules and kinetochore motor proteins
Microtubules:
- Form the spindle apparatus
–> spindle apparatus = a structure that organizes and pulls chromosomes apart during mitosis and meiosis
- Attach to chromosomes at the kinetochore
Kinetochore Motor Proteins:
- Specialised proteins located at the kinetochore
- Bind to microtubules of the spindle apparatus
- Hydrolyse ATP to generate energy, allowing them to “walk” chromosomes along microtubules toward either pole of the cell
- Ensure proper and accurate chromosome segregation during mitosis and meiosis
State the names of the four phases of mitosis
prophase, metaphase, anaphase and telophase
Draw typical eukaryotic cells as they would appear during the interphase and the four phases of mitosis
outline 4 events that occur during prophase
Chromosome Condensation:
- Chromatin coils, shortens, and thickens, making chromosomes distinct and more visible under a microscope
–> each chromosome consists of two genetically identical sister chromatids joined at the centromere
Nucleolus Disappearance:
- The nucleolus becomes less prominent and eventually disappears as the cell prepares for division
Nuclear Envelope Breakdown:
- The nuclear membrane disintegrates, allowing spindle fibers to access the chromosomes
Spindle Fiber Formation:
- Microtubule organizing centers (MTOCs), such as centrioles in animal cells, begin to form the mitotic spindle
- Spindle fibers extend from MTOCs, which migrate to opposite poles of the cell
In animal cells, centrioles divide and move to opposite poles of the cell to organize the spindle apparatus
Outline the process of metaphase
(make sure to talk about microtubules and the kinetochore)
1. Chromosome Alignment:
- Sister chromatids align at the metaphase plate (cell equator)
–> this ensures each daughter cell will receive one copy of each chromosome
2. Spindle Formation & Attachment:
- (in animal cells) centrioles are positioned at opposite poles + produce spindle fibres (made of microtubules)
–> plant cells use MTOCs to assemble the spindle apparatus by organizing spindle fibers
3. Kinetochore Interaction:
- Spindle fibres attach to kinetochores (protein structures located at the centromeres of each chromosome)
- Each kinetochore is connected to microtubules from both poles, ensuring balanced pulling forces
4. Tension Ensures Proper Alignment:
- Microtubules exert tension on the kinetochores to align chromosomes precisely at the equator
–> this proper attachment is crucial for accurate chromosome segregation during the next phase
Outline the process of anaphase (6 steps)
- Spindle fibres contract by shortening
- This shortening generates the pulling force needed to move chromosomes
- The centromere splits, separating the sister chromatids
- Each chromatid becomes an individual daughter chromosome
- The spindle fibres pull the daughter chromosomes toward opposite poles
- Each pole receives one copy of each chromosome, ensuring equal genetic distribution
Outline four events that occur during telophase
1. Chromatids reach the poles and become daughter chromosomes:
- The separated chromatids are now called daughter chromosomes as they reach opposite poles of the spindle
2. Chromosomes begin to decondense:
- The daughter chromosomes start to uncoil and become less distinct as they return to their chromatin form
3. Nuclear envelope reforms:
- The nuclear membrane starts to form around each set of chromosomes at both poles of the cell, marking the re-establishment of the nucleus
4. Spindle fibres disintegrate and cell elongates:
- The spindle apparatus breaks down, and the cell begins to elongate, preparing for cytokinesis
Determine the phase of mitosis of a cell viewed in a diagram, micrograph or with a microscope
Determine the phase of mitosis of a cell viewed in a diagram, micrograph or with a microscope
Determine the phase of mitosis of a cell viewed in a diagram, micrograph or with a microscope
Determine the phase of mitosis of a cell viewed in a diagram, micrograph or with a microscope
What does it mean for chromosomes to be “homologous”?
Chromosomes are considered homologous if they:
- Are similar in size, shape, and structure.
- Contain the same genes at the same loci (locations)
- Have one homologous chromosome coming from the mother, and the other coming from the father
- May carry different alleles for the same trait
State the human cell diploid number
46 chromosomes
(23 pairs of chromosomes, with one chromosome in each pair inherited from the mother and the other from the father)
State the human cell haploid number
23 chromosomes
This represents the number of chromosomes in a gamete, with one chromosome from each pair of the diploid set
examples of haploid cells?
- Human sperm cells
- Human egg cells (ova)
- Pollen cells in plants
- Spore cells in fungi
- Egg cells in animals
Meiosis begins with _______ and produces _______
Meiosis begins with diploid cells and produces haploid cells
what happens during fertilisation?
why is it important that the gametes are haploid during fertilisation?
- During fertilisation, male and female gametes fuse, joining their nuclei to produce a zygote
- It is important that the gametes are haploid so that the zygote +the cells that form from the zygote by mitosis are diploid
As in mitosis, ______ must occur prior to meiosis
As in mitosis, DNA replication must occur prior to meiosis
+ so at the start of meiosis, each chromosome will consist of two sister chromatids, joined together at a centromere
Meiosis consists of two rounds of division: _____ and ______
meiosis I and meiosis II
each round of division consists of a prophase, metaphase, anaphase and telophase, and will end with cytokinesis
Given a diploid number outline the movement and structure of DNA through the stages of meiosis
Practice with the number 2n=4
REMEMBER: The process of meiosis is the same regardless of the diploid number! The only difference is the number of chromosomes involved!
(ex: if it was 2n = 16, there will be 16 chromosomes at the start, and 32 chromatids in total)
- 2n = 4 means the organism has 4 chromosomes, 2 from each parent
Before Meiosis (Interphase):
DNA Replication:
- Chromosomes replicate, resulting in 4 chromosomes, each with two sister chromatids (total of 8 chromatids)
Meiosis I (Reduction Division):
Prophase I:
- Chromosomes condense and homologous chromosomes pair up to form bivalents (2 pairs in this case)
- Crossing over occurs, exchanging genetic material between non-sister chromatids
Metaphase I:
- Bivalents align along the metaphase plate
Anaphase I:
- Homologous chromosomes are separated to opposite poles; sister chromatids stay attached
Telaphase I:
- Two haploid cells are formed, each with 2 chromosomes, still consisting of two sister chromatids
Meiosis II (Equational Division):
Prophase II:
- Chromosomes condense again, and new spindle fibers form
Metaphase II:
- Chromosomes align at the metaphase plate of both haploid cells
Anaphase II:
- Sister chromatids are separated to opposite poles
Telaphase II:
- Four haploid cells are produced, each with 2 chromosomes, now consisting of single chromatids
SUMMARY:
- Before meiosis: DNA replication results in chromosomes with two sister chromatids.
- Meiosis I: Homologous chromosomes separate, and two haploid cells form.
- Meiosis II: Sister chromatids separate, leading to four haploid cells.
why is meiosis I a reductive division?
It is considered a reductive division because it reduces the chromosome number by half
reductive / reduction = interchangeable
cells are _____ at the end of meiosis I
cells are haploid at the end of meiosis I
compare meiosis with mitosis (“compare” = only similarities)
- DNA Replication Occurs Before Division
- Involve the Same Basic Phases
- Chromatin Condenses into Chromosomes
- Spindle Fibres Play a Role
- Chromosome Movement to Opposite Poles
- Cytokinesis Follows Nuclear Division
- Essential for Life Processes
Prophase I?
- Chromatin condenses into chromosomes (supercoil)
- Homologous chromosomes pair up to form bivalents (tetrads).
- Crossing over occurs at chiasmata
- Nuclear envelope breaks down.
- Spindle fibres begin to form
Metaphase I?
- Bivalents align along the metaphase plate.
- Spindle fibres attach to centromeres of homologous chromosomes.
- Random orientation of chromosomes
Anaphase I?
- Spindle fibres shorten, separating homologous chromosomes.
- Sister chromatids remain attached at centromeres.
- Homologous chromosomes move toward opposite poles.
- Chromosome number is halved (reduction division)
Telophase I?
- Homologous chromosomes reach the poles.
- Chromosomes may decondense slightly.
- Nuclear membranes reform (optional, depends on species).
- Cytokinesis follows → two haploid daughter cells form.
Prophase II?
- Chromosomes (still 2 sister chromatids) re-condense.
- Nuclear envelope breaks down (if it was reformed in Telophase I).
- Spindle fibres form from MTOCs or centrosomes at opposite poles.
- No crossing over occurs here.
Metaphase II?
- Chromosomes align along the metaphase plate.
- Spindle fibres attach to each chromatid at the centromere.
- Random orientation again occurs (genetic variation).
Anaphase II?
- Centromeres divide, separating sister chromatids.
- Spindle fibres pull chromatids (now individual chromosomes) to opposite poles.
- Chromosome number remains haploid.
Telophase II?
- Chromosomes reach poles and decondense.
- Nuclear membranes reform around each set.
- Spindle fibres disintegrate.
- Cytokinesis occurs + results in four genetically unique haploid cells.
Define nondisjunction. What does it lead to?
- nondisjunction = a genetic error that can occur during meiosis
- can be either the failure of pairs of homologous chromosomes to separate during anaphase I, or the failure of sister chromatids to separate during anaphase II
- leads to gametes with one extra or one missing chromosome
it can also occur during anaphase of mitosis, but it usually only impacts a few cells, and therefore the effects are rarely noticeable
State the result of nondisjunction during anaphase I and anaphase II
anaphase I:
- homologous chromosomes fail to separate
- this results in all gametes having the wrong number of chromosomes (half with one extra and the other half missing one)
anaphase II:
- sister chromatids fail to separate
- this leads to two normal gametes, one gamete with an extra chromosome, and one gamete missing a chromosome
What is the result of a gamete with an extra chromosome fusing with a normal gamete?
The resulting zygote will have three copies of one chromosome instead of two.
–> This is called trisomy and can lead to genetic conditions like Down syndrome.
What causes Down syndrome, and what are the symptoms?
CAUSE:
an extra copy of chromosome 21 (trisomy 21), usually due to non-disjunction in meiosis
SYMPTOMS:
intellectual disability, heart defects, delayed development, and characteristic facial features
Explain how meiosis leads to genetic variation in gametes
meiosis can lead to genetic variation in gametes due to:
- Crossing over in prophase I, where sections of DNA are exchanged between non-sister chromatids
- Random orientation and independent assortment of homologous chromosomes in metaphase I
- Random orientation of sister chromatids in metaphase II
further impact on genetic diversity:
- The random fusion of genetically unique gametes during sexual reproduction results in genetically diverse offspring
Define bivalent
*AKA a tetrad
- it is a pair of homologous chromosomes that come together during prophase I of meiosis
What is the process of crossing over?
What are chiasmata?
- Crossing over = the exchange of equivalent sections of DNA between non-sister chromatids of homologous chromosomes (only occurs in meiosis)
- Chiasmata = the physical points where crossing over occurs between non-sister chromatids
How often does crossing over happen?
Crossing over can occur multiple times within the same bivalent and at random locations along the chromosome
Describe the process of crossing over during prophase I of meiosis
1. Homologous Chromosomes Pair Up:
- Homologous chromosomes come together and align side by side
–> These chromosome pairs are called bivalents or tetrads
2. Formation of Chiasmata:
- Non-sister chromatids physically come close to each other, and at points of contact, they may exchange sections of DNA
3. Exchange of Genetic Material:
- The exchange of genetic material between non-sister chromatids occurs through the breaking and rejoining of the chromatids
–> This process swaps equivalent sections of DNA, leading to a reshuffling of genetic material
4. Multiple Crossovers:
- Multiple crossovers can occur along the same bivalent, increasing the genetic diversity of the chromatids
Describe the result of crossing over during prophase I of meiosis
Recombinant Chromatids:
After crossing over, chromatids that have exchanged DNA are called recombinant chromatids because they now carry a combination of alleles
Non-Recombinant Chromatids:
Chromatids that do not undergo crossing over are called non-recombinant chromatids because they retain the original combination of alleles
Genetic Diversity:
Crossing over results in chromatids that are no longer identical, even though they are sister chromatids.
–> This shuffling of alleles increases genetic variation among the gametes produced during meiosis, leading to genetically diverse offspring.
Draw a diagram to illustrate the formation of new allele combinations as a result of crossing over
Describe the process and result of random orientation of bivalents during metaphase I of meiosis
Process of Random Orientation:
- During metaphase I of meiosis, homologous chromosomes (which are paired as bivalents) align along the equator of the cell.
- The orientation of each homologous chromosome pair is random.
–> This means that each pair of homologous chromosomes has an equal chance of being oriented toward either pole of the cell.
- The orientation of one chromosome pair is independent of the orientation of other pairs, so there’s no fixed relationship between the maternal and paternal chromosomes in the bivalent.
Result of Random Orientation:
Independent Assortment:
- Because the orientation is random, the combination of maternal and paternal chromosomes that will end up in each daughter cell is random.
–> This results in different possible genetic combinations for each gamete.
Increased Genetic Variation:
- This randomness in chromosome arrangement during metaphase I is a key source of genetic variation in the gametes.
Draw a diagram to illustrate the formation of different chromosome combinations that result from random orientation during meiosis
what is the number of chromosome combinations possible due to random orientation?
2^n!
*n = the haploid number (the number of chromosome pairs in a cell)
(ex: if a cell has two pairs of chromosomes (2n = 4), the possible number of combinations after random orientation is 2^2 = 4)
(ex: in humans, where the diploid number is 46, the haploid number is 23, so the number of possible combinations is 2^23, which is approximately 8,388,608 possible combinations)
mitosis VS meiosis
