202 Part 1 Flashcards
Where are the instructions that govern cell division
During the first part of the organism’s life, the genes don’t instruct all cells to just divide. They start dividing to do specific things. True or false?
Can human cells be immortal?
What are the only immortal cells that can establish a self perpetuating lineage
cell divisions and the instructions governing cell divisions are in the hereditary material(the gene)
4.During the first part of the organism’s life, the genes instruct all cells to just divide. Upon attaining maturity, only specific cells are instructed to continue to divide daily.
5.Human cells can be cultured in the laboratory, but they survive for a few generations and then they die off.
6.The only immortal cells that can establish a self-perpetuating lineage are cancer cells.
HeLa(not Hela) Cells – Brief Overview:
HeLa cells are the first immortal human cells grown in a lab. They were taken from Henrietta Lacks’ cervical cancer tumor in 1951 without her consent.
Key Features:
• Immortal: They keep dividing indefinitely under proper lab conditions.
• Fast-growing: Easy to grow in large amounts.
• Consistent: Useful for repeatable scientific experiments.
How They Were Used:
1. Polio vaccine – Tested virus behavior and vaccine safety.
2. Cancer research – Helped understand cancer cell growth.
3. HIV/AIDS research – Used to study virus infection in cells.
4. Gene mapping & cloning – Helped map the human genome.
5. Drug testing – Tested safety and effects of new drugs.
6. COVID-19 vaccines – Used in early research and development.
HeLa cells have contributed to thousands of studies, but their use raised ethical questions about consent, privacy, and compensation.
Here are more key features of HeLa cells:
1. Immortality – They divide endlessly, unlike normal human cells that stop after a few divisions (called the Hayflick limit: The Hayflick Limit is the number of times a normal human cell can divide before it stops)
2. Rapid Growth – HeLa cells double in number about every 20–24 hours.
3. Durability – They can survive in harsh lab conditions and even cross-contaminate other cell cultures.
4. Versatility – Can be infected with viruses, exposed to radiation, or used to test chemicals and still grow — making them ideal for research.
5. Easy to transport – They stay alive longer during shipping than most cells.
6. Human origin – Even though they act differently from normal cells, they are human cells, so results are more relevant to human biology.
7. Cancerous – They are cancer cells, so they grow abnormally and don’t follow normal cell rules — useful for studying how cancer behaves.
Let me know if you want this turned into a quick table or flashcard format for easier study.
Explain 4 reasons why cells divide after birth
State four ways cells know when to divide
Cells divide because very few cells last an individual’s lifetime and therefore need to be replaced when they become too old to function well or when they die off.
2.Once the cell attains a certain size, the speed of diffusion to supply the necessary nutrients reduces making cells less efficient.
3.An individual develops from childhood to adult. New cells to needed to maintain this development, hence cells need to divide to create more cells for growth and development
4.The need for continual existence and therefore the need to produce sex cells for reproduction (fertilization).
5.In (4) above, the need to ensure normalcy in reproduction ensures haploid production of sex cells
How do cells know when to divide
1. When neighbouring cells die opening up space in the tissue, the space needs to be occupied by new cells.
2. When they are stimulated by growth factors, which are chemical signals secreted by blood platelets, kidney cells and other sources, cells divide in response.
3. When diffusion alone cannot ensure adequate exchange of materials between cells and environment because they’ve grown bigger
4. When they grow large enough and are capable of producing enough cytoplasmic material to distribute to two daughter cells.
5. When they have everything in place to ensure successful replication of their DNA successfully so they can give each daughter cell a duplicate set of genes.
Why do cells divided generally at first and then divide more specifically later on
Great question!
Simple Answer:
In early development, cells divide generally to make more cells fast. Cells divide generally just to make more cells quickly.
Later, they divide specifically to become special cell types (like muscle, nerve, or skin cells).
Why This Happens:
1. Early Stage (General Division):
• The body needs to grow quickly from a single cell (zygote) into a ball of cells.
• These early cells are unspecialized (stem cells) and can become anything.
2. Later Stage (Specific Division):
• As the body forms, cells receive signals that tell them what to become.
• They start differentiating — becoming specific types based on location and needs.
• Now, they divide in a controlled way to maintain or repair tissues (e.g., skin cells replacing old skin).
Example:
• In the embryo: cells are like clay — can become anything.
• In the adult: cells are like bricks — already shaped for a purpose.
Let me know if you’d like a diagram or short version for easy recall.
Great follow-up!
What kind of signals tell cells what to become?
Cells receive four main types of signals that guide their specialization (differentiation):
- Chemical Signals (Molecules)
• Growth factors, hormones, and cytokines.
• Example: Nerve growth factor tells a stem cell to become a nerve cell. - Cell-to-Cell Contact
• Neighboring cells send signals through direct contact.
• Helps organize tissues and tell a cell what it should become. - Position in the Body (Spatial Signals)
• A cell’s location in the embryo affects which genes turn on.
• Example: Cells near the spine may become bone or spinal cord tissue. - Environmental Signals
• Oxygen levels, nutrients, or even pressure can affect cell behavior.
Summary (Quick Recall):
Signals = Chemicals + Contact + Location + Environment
Let me know if you want examples from real tissues (like skin or blood).
When adulthood is reached, which cells continue to divide daily?
Which tissues stop routinely producing new cells after birth?
Why does this happen?
What’s the evidence that cell division isn’t a random event?
Daily: rbcs and skin cells
Nervous tissue stop routinely producing new cells after birth
Is cell division a random event
1. When adulthood is reached, specific cells such as skin and blood cells continue to divide daily.
2. Others such as nervous tissue after birth no longer routinely produce new cells.
3. Liver cells has a remarkable ability to regenerate and can do so from a third of the original cell within a short time.
4. Some cells can also move to a state where growth can be halted for a period before exiting that state and initiating cell division
This tells us that cell division is not just a random event but a well controlled and coordinated event.
Some cells can halt growth temporarily before dividing – what does this mean?
This refers to a phase called G₀ phase (G-zero) in the cell cycle.
• Normally, cells go through stages to grow and divide (G1 → S → G2 → M).
• But some cells pause in G₀, which is like a resting state.
• In G₀, the cell doesn’t divide but can stay alive and do its normal work.
• Later, some cells can exit G₀ and re-enter the cycle to divide again if needed (e.g., liver cells during injury).
Why is this useful?
It allows the body to control when and where cells divide, avoiding too much or unnecessary cell growth.
Why don’t nervous tissues routinely produce new cells after birth?
Most nerve cells (neurons) exit the cell cycle permanently and stay in G₀ forever after birth.
Why?
• Nerve cells are highly specialized and don’t regenerate easily.
• Dividing might disrupt the brain’s connections and memory.
• The nervous system values stability and function over regeneration.
However, a small number of stem cells in certain brain areas (like the hippocampus) can still produce new neurons, but this is limited.
After Birth:
• After birth, synaptogenesis continues at a rapid pace, especially during the first few years of life. The brain is highly plastic, meaning it’s very adaptable and continues to form new connections and strengthen existing ones based on experiences and learning.
• However, once neurons mature and the brain reaches certain stages of development, the rate of neurogenesis and synaptogenesis slows down.
In summary, neurons do form connections before birth, and these early connections are crucial for brain development and the proper functioning of the nervous system as the baby grows. After birth, this process continues, but at a slower pace.
Great follow-up!
Here’s a simple explanation:
Why dividing would disrupt the brain’s connections:
1. Neurons are highly connected
• Each neuron forms thousands of connections (synapses) with other neurons.
• These connections help with memory, learning, movement, and more. The nerve cells move into the Gzero stage sometime after birth. So the nerve cells form connections which contain memories. If the cells divide, it can lead to breakage of those connections and loss of memories.
G0 is a particular phase within the cell cycle where a cell is not dividing, while quiescence is a general term for cells that are inactive but can potentially resume division when needed.
2. Division would break those connections • For a cell to divide, it has to stop working, copy its DNA, and split into two new cells. • A dividing neuron would have to disconnect from its network, losing the signals and memories it stores. 3. Risk of mistakes • The brain’s wiring is very precise. If neurons started dividing, there’s a risk of errors, like: • Wrong connections • Signal confusion • Potential for diseases (like brain tumors)
So in summary:
Neurons don’t divide because it would disconnect important brain networks and risk damaging how the brain works. Stability is more important than regrowth in this case.
- If cell division is not random,
→ Then there must be a control system regulating how and when cells divide.- This control system must monitor both internal and external conditions,
→ To ensure the cell only divides under the right circumstances (e.g., enough nutrients, no DNA damage, space available, etc.). - The system must be receptive and responsive to signals,
→ So it can react appropriately to environmental cues or signals from other cells (e.g., growth factors, stress signals). - This control system has been identified as specific regulatory enzymes,
→ Mainly cyclins and cyclin-dependent kinases (CDKs), which work together to control progression through the cell cycle.
- This control system must monitor both internal and external conditions,
This is the foundation of how cell cycle regulation prevents uncontrolled division, like what happens in cancer when this system fails.
The cell must duplicate the dna in the chromosomes properly and then segregate those copies into two daughters cells
Why isn’t interphase entirely a resting stage as believed or assumed at first?
The process of cell division was believed to be a series of events that must take place in a coordinated manner thus the cell cycle concept was proposed.
4. The cell cycle is defined as the life of a cell from the first time it is formed from a parent until its own division into two cells.
5. Cell division was also found not to be occurring all the time and that in between divisions, there was a resting phase where no visible activity was assumed to take place. This phase was given the name interphase.
6. Then, it was discovered that DNA replication occurs during interphase, and the interphase was after all not a resting phase.
Originally the cell cycle was divided into two phases and later into four stages. Name the two phases and the four stages
State what happens in each of the four stages of the cell cycle
Which phase is the shortest
Which is the longest
The cell cycle concept
1. The cell cycle proposed either a two-phase or 4 stage cycle and the latter is more acceptable.
Two phase is just interphase(non dividing phase)
Mitosis phase or M phase (dividing phase)
But now we know that interphase isn’t entirely a resting phase and other things go on there so the four stage was brought up.
So now interphase has G1, S, G2 and the final stage is M
The “G” in G1 and G2 originally stood for “Gap”, because scientists first thought these were simply “gaps” between DNA replication and cell division.
However, we now know that active growth and preparation happen during these phases, so it’s also correct to describe them as “Growth phases.”
During the G1 the cell increases in mass and doubles the number of cytoplasmic components and accumulate all materials needed to replicate
DNA. Chromosomes are not duplicated here.
Then there is the S-stage (synthesis) of the cycle. This is the period of DNA synthesis or replication takes place. The proteins associated with DNA are also synthesized during this stage. So chromosomes are duplicated here
During G2, various metabolic events including replication of centrioles and synthesis of enzymes that control cell division occur. It makes sure the cell is ready for division and checks if there were errors with replication of the DNA. Cell prepares to divide
G1 checkpoint (before S phase) checks if cell DNA is ready to replicate(If failed: The cell may enter a resting state (G0) or undergo apoptosis (programmed cell death).)
G2 checkpoint (before M phase) checks if cell is ready to divide and if replication was done correctly
There is the M-stage (mitosis) which is the main division stage and it includes both mitosis and cytokinesis
S phase = Initiation of centrosome duplication.
• G2 phase = Completion of centrosome duplication.
S phase takes longer to do than all the phases.(I’m seeing something else)
Phases of the cell cycle
The cell cycle starts with the interphase which has three stages defined as the
G1, S and G2 phases. Next is the real process of cell division - prophase, metaphase, anaphase and telophase.
The length of the cell cycle varies from one cell type to the other. Stomach and skin cells divide rapidly, bone and cartilage cells divide slowly and nerve cells not at all.
After division, some cells leave the cycle for a rest in the G,, and others cease to divide for days, years or forever. Such cells are said to be in the Go phase.
The balance between actively dividing cells and those in the Go phase is important in determining the number of cells in the body.
G, (first growth) phases (6-9 hours)
This phase is very active because the cells grow in size and cellular organelles and proteins are doubled. Late in G1, the cells prepare to duplicate their chromosome.
Checkpoint 1
• The first checkpoint of cell division occurs is at the G1 stage.
• All the conditions needed for DNA replication must be met. Thus, a control system must respond to information received from the processes that it controls.
• At this stage the sensors must detect the complete synthesis (doubling) of the cellular components.
• If these conditions are not met the system is forced back to the Go stage for the process to stop entirely or the needed corrections are made before being allowed to proceed.
• This is to prevent disaster if the cycle proceeds prematurely to the next stage.
• Checkpoints can also be regulated by extracellular signals from other cells for promotion or inhibition of proliferation.
S (synthesis) phase (6 - 8 hours)
The synthesis of histones and other proteins associated with DNA is markedly increased. DNA replicates doubling the chromosomes so each of the chromosomes becomes a complex of two identical sister chromatids.
G2 phase (2-4 hours)
All remaining proteins needed for cell division are synthesized now that the cells prepares to divide including synthesizing tubulin for construction of the microtubules of the spindle apparatus.
The second checkpoint is here. A number of proteins scrutinize the cell’s DNA, making sure it is structurally intact and properly replicated. The cell may pause at this point to allow time for DNA repair, if necessary.
Mitosis(hour or two.)
Mitotic cell division now occurs. This is the shortest phase an only about an hour or two
Points to note:
> Some cells such as liver cells divide less frequently. These cells can spend months in interphase before going through a brief mitotic phase.
> Other cells such as hair follicles and skin cells cycle very frequently.
• Some cells such as red blood cells divide a number of times, then lose their nuclei and leave the cell cycle never to divide again.
• After mitosis, some cells re-enter G1, to go through the cell cycle phases again, others enter and wait, and others enter never to divide or enter an extended phase called Go.
• For the Go cells, an appropriate signal stimulates them to re-enter the cycle and divide.
Why do skin cells and gut cells divide continuously?
Where will you put an arrested nerve cell in the cell cycle for it to finish it?
What about an arrested cardiac cell
What’s the function of kinases and cyclins in the cell cycle?
How do cells control cell division?
Ah, got it — you meant skin and gut, not “hit and skin.” Thanks for the clarification!
Why do skin and gut cells divide continually?
Because both are high-turnover tissues exposed to constant damage, friction, or chemical stress, so they need to renew themselves rapidly to maintain function and protection.
- Skin
• It acts as a barrier against the environment.
• Cells in the outer layer (epidermis) constantly die and shed, so new ones must replace them.
• Stem cells at the base of the skin keep dividing to replenish lost cells. - Gut (intestinal lining)
• The gut lining faces digestive acids, enzymes, and food movement, which can damage cells.
• Cells in the lining are replaced every few days.
• Stem cells in the intestinal crypts divide rapidly to keep the lining intact and functioning.
In short:
Skin and gut cells divide continually because they are exposed to constant damage and need fast, regular renewal to stay healthy.
Let me know if you want a simple diagram or memory tip for this!
Regulation of the cell cycle
1. Some cells such as skin cells divide continuously throughout the life of the organism, whilst other arrest at certain stages of the cycle.
2. For example, if the nucleus of an arrested nerve cell is placed in the cytoplasm of an S-stage cell, it finishes the cell cycle. It is thus obvious that nerve cells are arrested in the G1 stage. There must therefore be a checkpoint at this stage.
3. Likewise, if an arrested cardiac cell is fused with a cell in the M-stage, it finishes the cell cycle. Therefore, cardiac muscle cells must be arrested in the G2 phase. Again, there must be another checkpoint at this stage.
4. There must thus be both repressive and stimulatory sensors that causes the cell to proceed through these two checkpoints.
5. Two types of proteins - kinases and cyclins have been identified as playing major roles in G, to S and G, to M stages.
6. A kinase removes a phosphate group from ATP and adds it to a protein.
You’re thinking deeply — I love that! Let’s break both parts down clearly:
- What are the “sensors” that regulate the checkpoints?
The sensors are mainly proteins and protein complexes that monitor conditions in the cell and decide whether to allow or block progression through the cell cycle.
These include:
a. Stimulatory sensors (promote progression):
• Cyclins – Regulatory proteins that increase and decrease at different cell cycle stages.
• CDKs (Cyclin-Dependent Kinases) – Enzymes that become active when bound to cyclins; they phosphorylate target proteins to move the cycle forward.
b. Repressive sensors (block progression if there’s a problem):
• p53 – A tumor suppressor that halts the cycle if there’s DNA damage, especially at the G1 checkpoint.
• p21 – Activated by p53, it inhibits CDKs, blocking progression.
• ATM/ATR proteins – Detect DNA damage and activate repair pathways or stop the cycle.
• Chk1/Chk2 – Kinases that stop the cycle by inhibiting CDC25, a phosphatase needed for CDK activation.
So in short:
• Sensors = Cyclins/CDKs (stimulatory) and p53, ATM, ATR, Chk1/2 (repressive)
- What will a cyclin do?
Let’s clear this up:
Kinase:
• A kinase (like CDK) adds a phosphate group from ATP to a target protein — this often activates or inactivates that protein.
Cyclin:
• A cyclin does NOT phosphorylate anything on its own.
• Instead, a cyclin binds to a CDK (cyclin-dependent kinase) and activates it.
So:
A cyclin’s job is to bind and activate a CDK. The cyclin-CDK complex then adds phosphate groups to specific proteins to push the cell through a checkpoint.
Summary:
• Sensors = Cyclins/CDKs (stimulatory), p53, p21, ATM/ATR (repressive)
• Cyclin itself doesn’t phosphorylate — it activates CDKs, which do the phosphorylation.
Let me know if you want a chart or analogy to remember this better!
Exactly — you’ve got the idea! Let’s make it super clear:
- Phosphorylation (by CDK-cyclin complexes) = “Go” signal
• When a cyclin binds to its CDK, it forms an active cyclin-CDK complex.
• This complex phosphorylates specific proteins.
• Those phosphorylated proteins activate processes like:
• DNA replication (in S phase),
• Spindle formation (in M phase),
• Chromosome separation, etc.
• So, phosphorylation = activating the next step in the cycle. - Removal or breakdown of the cyclin = “Stop” signal
• Cyclins are not permanent — they are destroyed at the right time.
• When the cyclin is degraded, the CDK becomes inactive.
• This stops the activity that was pushing the cell forward.
Example:
In S phase:
• Cyclin A + CDK2 → phosphorylates replication proteins → DNA replication begins.
• Later, Cyclin A is degraded → CDK2 turns off → replication ends.
So yes:
• Phosphorylation = Activation
• Cyclin removal = Inactivation
That’s how the cell cycle is tightly controlled — like a switch that turns on and off at the right times.
Want a quick analogy?
How do cells control division?
So, cells control division by
1. The use of check points which monitor and control the progression from one stage to the other.
2. At each check point, specialized proteins ensure that the required conditions have been met before the cell can enter the next phase.
3. Kinases which becomes activated by combining with cyclins at two critical checkpoints in the cycle, phosphorylates an inhibitory protein which in turn triggers the beginning of S and M stages.
4. Afterwards the cyclins and kinases dislodge, the cyclins are destroyed and the kinase reverts to the inactive state.
5. Typically, a cyclin should combine with a kinase only when growth factors are present. But a cyclin gone awry combines in the absence of growth factors and the result is uncontrollable cell growth as seen in cancers.
6. Tumour suppressors e.g. p53 controls production of proteins/growth factors that can combine with cyclins, rendering them inactive, so that it does not get the chance to bind to a kinase.
So, cells control division by
The use of check points which monitor and control the progression from one stage to the other.
At each check point, specialized proteins ensure that the required conditions have been met before the cell can enter the next phase.
what are the specialized proteins?
Perfect setup! Let’s now name the specialized proteins that regulate the checkpoints and control cell division:
Specialized Proteins Involved in Cell Cycle Checkpoints:
- Cyclins
• Regulate the timing of the cell cycle.
• Different types (e.g., Cyclin D, E, A, B) are active at different stages.
• They activate CDKs. - CDKs (Cyclin-Dependent Kinases)
• Enzymes that phosphorylate target proteins to move the cell into the next phase.
• Only active when bound to a cyclin. - CDK Inhibitors (CKIs)
• Examples: p21, p27, p16
• These block CDK activity to pause the cycle if conditions are not right. - p53
• A “guardian of the genome.”
• Detects DNA damage and activates p21 to block the cycle.
• Can also trigger apoptosis if damage is too severe. - ATM and ATR
• Sensors for DNA damage.
• Activate checkpoint kinases when problems are found. - Chk1 and Chk2 (Checkpoint kinases)
• Activated by ATM/ATR.
• Prevent progression by inhibiting CDC25, which is needed to activate CDKs. - APC/C (Anaphase-Promoting Complex/Cyclosome)
• Triggers transition from metaphase to anaphase in mitosis.
• Helps destroy cyclins to end a phase.
In short:
Cyclins, CDKs, CDK inhibitors, p53, ATM/ATR, Chk1/2, and APC/C are the key specialized proteins that act at checkpoints to control cell division.
Let me know if you want a memory tip or visual to help recall these!
When do cells know to stop dividing?
When do cells know to stop dividing
1. Mainly by contact inhibition when they contact neighbouring cells.
2. External cues that initiate cell division now initiates its inhibition
3. Withdrawal or break down of these external cues
4. Lack of nutrients
Of interest:
1. What is amazing during cell division is that enzymes and proteins selectively access, activate and silence information in all DNA.
2. They make base by base copies of all the bases in every DNA molecule all during the G1 phase.
3. Once S begins, DNA replication proceeds at about the same rate for all cells of a species.
4. The cell cycle lasts about the same time for cells of the same type.
Great thoughts — let’s go through each part and clarify where needed:
When do cells know to stop dividing?
Yes — you’re absolutely right:
1. Mainly by contact inhibition — when cells touch neighboring cells, they receive signals to stop dividing.
Other ways cells know when to stop dividing include:
• DNA damage detection (via p53).
• Loss of growth signals (like growth factors).
• Telomere shortening (in aging cells).
• Hormonal or environmental cues.
Interesting Point 1:
“What is amazing during cell division is that enzymes and proteins selectively access, activate and silence information in all DNA.”
Yes! This refers to epigenetic control:
• Some genes are turned on, others off, depending on the cell’s needs.
• This regulation ensures only the right genes are expressed during each stage.
• It’s done by transcription factors, histone modifiers, and DNA methylation.
Interesting Point 2 (Correction):
“They make base by base copies of all the bases in every DNA molecule all during the G1 phase.”
This needs a correction:
• The actual copying (replication) of DNA happens in the S phase, not G1.
Let’s fix it:
DNA replication (copying all bases) occurs during the S phase of the cell cycle. In G1, the cell just grows and prepares materials needed for DNA replication.
Summary:
• Cells stop dividing due to contact inhibition, signals, or DNA damage.
• DNA is carefully regulated by proteins that turn genes on or off.
• DNA copying happens in S phase, not G1.
Let me know if you want a timeline of G1, S, G2, and M with key events!
Great question! Here’s a simple explanation of how touching neighboring cells (contact inhibition) tells a cell to stop dividing:
What is Contact Inhibition?
Contact inhibition is a process where normal cells stop dividing when they bump into each other or fill up the space.
How does it work?
1. Cells have surface proteins (like cadherins and integrins) that can sense when another cell is nearby or touching them.
2. When cells touch each other, these surface proteins send signals inside the cell that say:
“We’re surrounded — no more space — stop dividing.”
3. These signals turn off the cyclins/CDKs or activate inhibitors (like p27) that block the cell cycle.
What’s the result?
• Cells pause in the G1 phase.
• No DNA replication or division happens.
• The tissue maintains a proper size and shape.
Extra note:
• Cancer cells often lose contact inhibition, so they keep dividing even when crowded, which is one reason tumors form.
Let me know if you want a one-line summary to memorize it!
Difference between sister chromatids and non sister chromatids
What is a homologous pair
Sister chromatids is two chromatids from one parent joined by a centromere
Non sister is two chromatids from two different parents joined by a centromere
Homologous pair is one sister chromatid and one non sister chromatid
You’re almost there — you’re thinking in the right direction! Let’s clarify it step by step:
- Sister Chromatids:
• These are two identical copies of one chromosome made during DNA replication.
• They are joined together at the centromere.
• Same genes, same alleles (since they’re copies of each other).
• From the same parent.Example: After DNA replication, the chromosome from your mom becomes two identical sister chromatids. - Non-sister Chromatids:
• These are chromatids from homologous chromosomes — one from mom, one from dad.
• They carry the same genes but may have different alleles.
• They are not joined by a centromere.
• During meiosis, crossing over happens between non-sister chromatids. - Homologous Pair:
• A homologous pair consists of two chromosomes:
One from your mom + one from your dad.
• Each chromosome in the pair may be duplicated, meaning:
• One homologous chromosome = 2 sister chromatids
• So, a homologous pair can include 4 chromatids in total:
• 2 sister chromatids from mom
• 2 sister chromatids from dad
• In meiosis, these 4 chromatids line up together.
In short:
Term What It Is From Same Parent? Joined by Centromere?
Sister Chromatids Identical copies of one chromosome Yes Yes
Non-sister Chromatids From homologous chromosomes (mom + dad) No No
Homologous Pair 1 chromosome from mom + 1 from dad No No (but each has its own centromere)
Let me know if you want a drawing or visual summary to help!
Great question! Here’s a simple breakdown of the difference between a chromatid and a chromosome:
Chromosome:
• A complete DNA molecule with proteins.
• Carries genetic information.
• In humans, we have 46 chromosomes (23 pairs).
• A chromosome can exist as:
• One chromatid (before replication)
• Two chromatids (after replication) — still counted as one chromosome.
Chromatid:
• A single copy or strand of a replicated chromosome.
• After DNA replication, each chromosome has two sister chromatids.
• Sister chromatids are joined at the centromere.
• Once they separate (during cell division), each becomes its own chromosome.
Quick Summary:
Term Before DNA Replication After DNA Replication
Chromosome 1 chromatid 2 sister chromatids = 1 chromosome
Chromatid A single DNA strand One half of a replicated chromosome
Analogy:
Imagine a book:
• 1 book = 1 chromosome
• When you photocopy it (replication), you now have two identical pages bound together — those are sister chromatids, still counted as one book/chromosome until they separate.
Want a one-liner to memorize this?
Where do the centrosomes divide before mitosis begins ?
Function of centrosome
What is mitosis?
Importance of mitosis
The centrosome, a microtubule divides in the G, phase before mitosis begins.
They organize the spindle, the structure, that brings about chromosome movement during division.
Mitosis
This division by the somatic or body cells is responsible for the creation of new identical diploid daughter cells from a single diploid cell. It thus includes cytokinesis.
Mitosis serves the following
• Development of an individual’s over 70 trillion cells from fertilization
• Growth of all tissues and organs after birth
• Replacement of cells that die Repair of damaged tissues
Difference between eu and heterochromatin
Which of them condenses in cell division
The chromatin that is condensed and can be easily viewed under the microscope is called heterochromatin.
Explanation:
• Heterochromatin is tightly packed and inactive, meaning it’s usually not involved in gene expression.
• It appears as dense, dark-staining regions under a microscope during cell division.
In contrast:
• Euchromatin is less condensed, actively involved in gene expression, and is lighter under a microscope.
Key Points:
• Heterochromatin: Tightly packed, visible under the microscope, inactive.
• Euchromatin: Loosely packed, active, less visible under the microscope.
Let me know if you want more details on this!
Ah, I see! In terms of cell division, the chromatin that condenses and becomes visible under the microscope is euchromatin, not heterochromatin.
Here’s why:
• Euchromatin is the active chromatin that is loosened and uncoiled during interphase. It is where gene expression occurs.
• During cell division, euchromatin condenses into chromosomes, which can then be easily observed under the microscope.
Heterochromatin:
• Heterochromatin is already in a tightly condensed form, so it’s not typically involved in gene expression during interphase. It remains condensed and dark-staining throughout most of the cell cycle, but it doesn’t further condense in preparation for division.
Summary:
• Euchromatin condenses during cell division and becomes visible as chromosomes.
• Heterochromatin is already condensed and generally remains inactive.
Let me know if you need further clarification!
It seems like there might have been a typo in your question. I believe you meant to ask about “heterochromatin” instead of “heywrochromqtin.”
Heterochromatin:
Heterochromatin is a form of highly condensed chromatin that plays several important roles in the cell:
Roles of Heterochromatin:
1. Gene Regulation:
• Heterochromatin is generally gene-inactive (not expressed), which helps silence certain genes. It plays a role in regulating gene activity by keeping specific regions of the genome tightly packed and inaccessible to the transcription machinery.
2. Maintaining Chromosome Structure:
• Heterochromatin helps in maintaining the structural integrity of chromosomes. It is found around the centromeres and telomeres, regions essential for the proper organization and movement of chromosomes during cell division.
3. Genomic Stability:
• By keeping certain regions of DNA tightly packed, heterochromatin helps in protecting the genome from damage and maintaining genomic stability. It can prevent transposable elements from being activated.
4. X-Chromosome Inactivation:
• In females, one of the X chromosomes in each cell is inactivated and remains as heterochromatin. This ensures that the two X chromosomes in females do not result in double the gene dosage, which is necessary for proper development.
Summary:
Heterochromatin is mainly used in gene silencing, maintaining chromosomal structure, and ensuring genomic stability. It helps keep certain regions of the genome inactive and tightly packaged.
Let me know if you’d like more details!
Explain the phases from interphase to mitosis in detail.
G2 of Interphase
1. Chromosomes yet
to be condensed
2. Centrosomes duplicates into centrioles
3. Nucleolus still visible
Witterdule
Chromosome, consisting
onwe sater chromatos
Prophase
1. Chromosomes shorten and thicken into compact rods of sister chromatids
2. Mitotic spindle begins to form
3. Nuclear membrane breaks down and nucleolus disappears
Kinetochore|
Kinetochore
microtoule
Prometaphase
1. Nuclear envelope fragments
2. Microtubules from centrosome invade nuclear area
Chromosomes now have a kinetochore
Metaphase
1. Centrosomes at opposite poles
Chromosome line along midline of the cell. metaphase plate
Fibres of aster attach to plasma membrane
Longest stage of mitosis
(20 mins)
Daughter chromosomes
Anaphase
1. An enzyme causes centromeres to divide and sister chromatids of each chromosome pair separate and move to the end of the poles
2. The centromere leads with the arms trailing behind.
3. Sister chromatids are identical and each daughter cell receives one chromatid from each chromosome making the daughter cells genetically identical
Nue earl
forming
Telophase
1. Chromatids cluster on each side of the cell.
2. RER produces a new
nuclear envelope
3. Two new daughter nuclei form in the cell
4. Nucleoli reappear
Interphase (G1, S & G2):
The cell replicates its cytoplasmic components and DNA to get ready for the next cycle. Chromosomes are not clearly discerned in the nucleus.
Prophase:
Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes. The nucleolus disappears and nuclear envelope fragments. The sister chromatids of a chromosome are attached at the centromere. Centrioles begin moving to opposite ends of the cell and the spindle fibers extend and spindles attach to the centromeres (prometaphase)
Metaphase:
Spindle fibers align the chromosomes along the middle of the cell nucleus. This line is referred to as the metaphase plate. This organization helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome.
Chromosomes are easier to visualize here by light microscope. Thus clinical diagnosis of chromosome disorders is based on metaphase chromosomes
Anaphase:
The two sister chromatids of each duplicated chromosome separate at the centromere and move to opposite sides of the cell. Motion results from a combination of movement along the spindle microtubules and through the physical interaction of polar microtubules.
Anaphase is delayed until all chromosomes are positioned at the metaphase plate.
Telophase:
Chromatids arrive at opposite poles of cell, and new membranes form around the daughter nuclei. The chromosomes are no longer visible under the light microscope. The spindle fibers disperse, and cytokinesis or the partitioning of the cell may also begin during this stage.
Cytokinesis:
In animal cells, cytokinesis results when a fiber ring composed of a protein called actin around the center of the cell contracts pinching the cell into two daughter cells, each with one nucleus.
clinical diagnosis of chromosome disorders is based on the chromosomes in which part of mitosis
Explain
Yes, that’s correct! Metaphase chromosomes are the easiest to visualize under a light microscope and are therefore used for clinical diagnosis of chromosome disorders.
Here’s why:
Why Metaphase Chromosomes Are Easier to See:
• During metaphase (a stage of mitosis), the chromatin (DNA) is fully condensed into distinct, thick, and compact structures known as chromosomes.
• These condensed chromosomes are highly visible under a light microscope because they are shortened and thicker compared to their uncondensed form in interphase (when they exist as chromatin).
• Each chromosome consists of two sister chromatids, which are joined at the centromere.
Clinical Diagnosis of Chromosome Disorders:
• Chromosome disorders (such as Down syndrome, Turner syndrome, and Klinefelter syndrome) often involve structural changes to the chromosomes (e.g., extra chromosomes, deletions, or duplications).
• Karyotyping is a technique used to examine metaphase chromosomes. It involves:
1. Collecting cells (e.g., from a blood sample).
2. Inducing the cells to divide until they reach metaphase.
3. Staining and visualizing the chromosomes under a microscope.
4. Arranging the chromosomes into a karyotype, which can reveal:
• Chromosome number (e.g., extra or missing chromosomes).
• Chromosome structure (e.g., translocations, deletions, or duplications).
Summary:
• Metaphase chromosomes are condensed and distinct during metaphase, making them easier to visualize with a light microscope.
• This makes them the ideal stage for identifying and diagnosing chromosome disorders through techniques like karyotyping.
Let me know if you need more details or examples of chromosome disorders!
In the context of visualizing chromosomes under a light microscope during metaphase, the chromatin is condensed and forms chromosomes that can be easily seen. This condensed chromatin primarily consists of euchromatin, although heterochromatin can also be part of the chromosomes.
Key Differences:
1. Euchromatin:
• Active chromatin that is loosely packed during interphase.
• Becomes condensed during cell division (particularly during metaphase) to form visible chromosomes.
• It contains genes that are actively transcribed and expressed.
2. Heterochromatin:
• Tightly packed and inactive chromatin that remains condensed even during interphase.
• Found near the centromere and telomere regions of chromosomes.
• Non-active in terms of gene expression and doesn’t participate in transcription.
During Metaphase:
• Both euchromatin and heterochromatin are part of the chromosomes.
• Euchromatin becomes more tightly condensed and forms the easily visible chromosome structures under the microscope.
• Heterochromatin remains condensed but is generally not involved in gene expression during this time.
Summary:
• Euchromatin is the chromatin that undergoes condensation during cell division to form visible chromosomes during metaphase. While heterochromatin is also part of the chromosome structure, it remains more condensed and inactive.
Let me know if you’d like further clarification!
Cell division is all about efficiency and timing
1. A mechanism for checking completion of stages before progression e.g. entry
S phase can only happen if events in G1 are completed. - check points
2. A mechanism that initiates events in the correct order by turning each event on at a specific time, thus providing a fix amount of time for the completion of each event. - flow of production
3. A mechanism to ensure that each event is triggered only once per cycle - efficiency
4. On/off switches that trigger events in a complete irreversible fashion. It will be disastrous if chromosome condensation or nuclear envelope breaks down but not completed - automation
5. Backup mechanisms to ensure that the cycle can work properly even when parts of the system malfunction
6. Adaptability so that the systems behaviour can be modified to suit specific cell type or environmental conditions
A sperm fertilizes an egg and a new individual begins life
1. The sperm and egg must have half the number of chromosomes or else the chromosome number will double with every generation.
2. For now, combination of sperm and egg is the only way to propagate and ensure continuity of the next generation.
3. For variation, there is the need for chromosomes to be able to swap segments or mix to produce different combinations of genes.
4. After gene combination, there is the need for independent assortment of the swapped genes to ensure that further variation is achieved.
5. Thus sexual reproduction was designed to introduce genetic variability to enable us survive the changing environment.
6. In sexually reproducing organisms, meiosis is the type of nuclear division that makes all this possible
What phase in meiosis is similar to mitosis?
Why is the prophase of meiosis I divided into five stages? State those stages
Meiosis of eukaryotes
Meiosis requires two nuclear divisions - Meiosis I and Meiosis Il to produce four daughter haploid gamates each having one of each kind of chromosome. Therefore each daughter cell will have half the total number of chromosomes present in the diploid parent nucleus.
Interphase in meiosis is similar to that of mitosis. The Prophase of Meiosis I is divided into 5 stages to ensure genetic arrangement.
Leptotene
Chromosomes, each consisting of two sister chromatids from each parent start condensing into visible strands within the nucleus. At this point, the two sister chromatids are still so tightly bound that they are indistinguishable from one another.
Zygotene
Chromosomes from each parent line up and homologous chromosomes pair up (synapsis) - mechanism unknown. Pairing is highly specific, exact and in perfect alignment. In males the X and Y chromosomes being non homologous line up end to end. Each pair is called bivalent (two chromosomes) or tetrad (four chromatids)
Pachytene (most unique in cell division)
Chromosomal crossing over occurs between non-sister chromatids resulting in the exchange of segments over regions of homology a process called genetic recombination. This tightly controlled regional exchange results in recombination of information. The sites where the chromatids are attached is the chiasmata.
Diplotene
There is a little separation of homologous chromosomes, as they are held tight at the chiasmata points. This enables the homologous chromosomes to locate onto the same spindle.
Diakinesis
This is the point in meiosis where the homologous chromosomes are visible under the scope especially the chiasmata. The events here are similar to the pro-metaphase stage of mitosis - The nuclear membrane disappears and the spindle begins to form.
In female human development, all developing oocytes stop at diplotene till puberty.
