Cell Division, Cell Diversity & Cellular Organisation (OCR A Level Biology): Flashcards

New cells must contain accurate DNA to function correctly, but errors can occur during replication, so the cycle is regulated to ensure daughter cells receive correct genetic information.

• G₁ checkpoint: chromosomes checked for damage; the cell does not enter S phase until any repairs are made\n- S phase checkpoint: chromosomes checked to ensure they have all been replicated; the cycle stops if not\n- G₂ checkpoint: a further check for DNA damage after replication; the cycle is delayed until repairs are made\n- Metaphase checkpoint: checks the chromosomes are correctly attached to the spindle fibres before anaphase

• Animal cells: the cytoplasm constricts between the two nuclei

• Plant cells: a new cell wall is formed

• Checkpoints throughout the cycle check the replicated DNA for errors

• Proof-reading and repair enzymes carry out this checking

• Where possible the error is repaired; otherwise the cell may destroy itself to prevent passing on harmful mutations

True

False — cell growth stops during the M (mitosis) phase.

The process of nuclear division producing two genetically identical daughter nuclei, each with the same number of chromosomes as the parent cell nucleus.

• Prophase

• Metaphase

• Anaphase

• Telophase

Remembered by the mnemonic 'PMAT'.

• Chromosomes condense and become visible when stained

• Each chromosome consists of two identical sister chromatids joined at the centromere

• The two centrosomes move towards opposite poles

• Spindle fibres begin to emerge from the centrosomes

• The nuclear envelope breaks down and the nucleolus disappears

• Centrosomes reach opposite poles

• Spindle fibres continue to extend from the centrosomes

• Chromosomes line up at the equator of the spindle (the metaphase plate)

• Spindle fibres attach to the centromeres, with each sister chromatid attached to a fibre from an opposite pole

• The sister chromatids separate at the centromere (the centromere divides in two)

• Spindle fibres begin to shorten

• The separated chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres

• Chromosomes arrive at opposite poles and begin to decondense

• Nuclear envelopes reform around each set of chromosomes

• The spindle fibres break down

• New nucleoli form within each nucleus

The two identical chromatids that make up a chromosome after DNA replication. Each contains one DNA molecule and they are joined at the centromere.

Specific proteins at the centromere to which the spindle fibres attach during metaphase, allowing chromosomes to be moved.

Spindle fibres are protein microtubules.

They emerge from the centrosomes, which in animal cells consist of two centrioles.

During anaphase, the sister chromatids separate at the centromere and are pulled to opposite poles.

Metaphase is the stage in which chromosomes line up at the equator of the spindle.

Each daughter nucleus has the same number of chromosomes as the parent cell and is genetically identical to it. This is ensured by DNA replication producing identical sister chromatids, which are then separated equally.

False — chromosomes line up at the equator during metaphase; during anaphase the sister chromatids are pulled to opposite poles.

True

A specific region of a plant where growth occurs, containing actively dividing cells.

The root tip meristem.

Just behind the protective root cap.

A zone of cell division, containing cells undergoing mitosis.

A method of preparing a temporary slide in which stained root tips are gently squashed, spreading the cells into a thin sheet so that individual cells undergoing mitosis can be seen clearly.

Garlic or onion (Allium cepa).

The stain (e.g. acetic orcein) stains the chromosomes a deep purple, making them visible so the stages of mitosis can be identified.

To spread the cells out into a thin sheet, so that individual cells undergoing mitosis can be seen clearly under the microscope.

Growth in plants occurs in specific regions called meristems.

The two stages can look very similar.

By counting the number of nuclei in one cell: multiple nuclei within a single cell indicates telophase.

• It can be difficult to distinguish between prophase and telophase

• The size of cells or tissue structures may appear inconsistent between specimen slides

• Optical microscopes have lower magnification than other microscopes, so some structures cannot be seen

• Preparing the slides can alter the structure of the cells

If there are multiple nuclei within one cell, then the stage of mitosis occurring is telophase.

True

False — optical microscopes have a lower magnification power, so some structures cannot be seen.

The process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent nucleus.

It is fundamental to many biological processes, including:

• Growth of multicellular organisms

• Replacement of cells and repair of tissues

• Asexual reproduction

A unicellular zygote divides repeatedly by mitosis, producing genetically identical daughter cells with the same number of chromosomes as the parent cell. This allows the zygote to grow into a multicellular organism.

They are genetically identical to one another (clones) and have the same number of chromosomes as the parent cell.

In plants, growth by mitosis may be confined to certain regions known as the meristems (growing points).

Cells are constantly dying and must be continually replaced by genetically identical cells produced by mitosis. Damaged tissues can also be repaired by mitosis followed by cell division.

• The skin

• The lining of the gut

Mitosis produces genetically identical cells that can rebuild lost structures.

For example, zebrafish can regenerate fins, and axolotls can regenerate their legs and tail.

The production of new individuals of a species by a single parent organism, where the offspring are genetically identical to the parent.

Asexual reproduction relies on mitosis, which produces genetically identical daughter cells. As only a single parent is involved, the offspring are clones of that parent.

• Binary fission in unicellular organisms such as Amoeba

• Budding in Hydra and yeast

• Runners from strawberry plants

The two genetically identical daughter cells produced by mitosis can be described as clones.

False — mitosis produces daughter cells that are genetically identical to the parent cell.

True

A form of nuclear division that produces haploid cells from a diploid cell. It generates the gametes used in sexual reproduction in plants and animals.

Meiosis involves two divisions:

• Meiosis I

• Meiosis II

Each division has the stages prophase, metaphase, anaphase and telophase.

• DNA condenses and becomes visible as chromosomes (each already made of two sister chromatids joined at a centromere)

• Chromosomes arrange side by side in homologous pairs (each pair is a bivalent)

• Crossing over of non-sister chromatids may occur at points called chiasmata

• Centrioles migrate to opposite poles and the spindle forms

• The nuclear envelope breaks down and the nucleolus disintegrates

A pair of homologous chromosomes lying side by side, as seen during prophase I of meiosis.

The point at which crossing over occurs between non-sister chromatids of homologous chromosomes during prophase I.

• Bivalents line up along the equator of the spindle

• Spindle fibres attach to the centromeres

During metaphase I, the maternal and paternal chromosomes of each pair position themselves independently of the other pairs.

Which side of the equator each chromosome ends up on is therefore due to chance.

• The homologous pairs are separated, with microtubules pulling whole chromosomes to opposite ends of the spindle

• The centromeres do not divide, so each chromosome still consists of two chromatids

• The chromosomes arrive at opposite poles

• Spindle fibres start to break down

• Nuclear envelopes form around the two groups of chromosomes and nucleoli reform

• Some plant cells go straight into meiosis II without the nucleus reforming

Animal cells: the cell surface membrane pinches inwards, forming a cleavage furrow that contracts to divide the cytoplasm in half.

Plant cells: Golgi vesicles gather along the equator (cell plate) and merge to form the new cell surface membrane, secreting calcium pectate to form the middle lamella; cellulose is then laid down to build the cell walls.

Because the DNA does not need to be replicated again — replication already occurred before meiosis I. The chromosomes entering meiosis II still consist of two sister chromatids.

A spindle forms at a right angle to the old one. The nuclear envelope breaks down and the chromosomes condense.

In metaphase II, the chromosomes line up in a single file along the equator of the spindle.

• The centromeres divide and individual chromatids are pulled to opposite poles

• This creates four groups of chromosomes, each with half the number of chromosomes of the original parent cell

The final products of meiosis are four haploid cells.

Chromosomes are counted by the number of centromeres present.

When 46 chromosomes duplicate during interphase, the amount of DNA doubles but there are still only 46 centromeres, so there are still 46 chromosomes — now made of 92 chromatids.

False — in anaphase I the centromeres do not divide; whole homologous chromosomes are separated. Centromeres only divide in anaphase II.

True

Genetic variation between offspring is advantageous for natural selection, as it increases the chance that some individuals will have alleles suited to a changing environment.

• Crossing over (during meiosis I)

• Independent assortment (random orientation of homologous pairs)

• Random fusion of gametes at fertilisation

The process during meiosis I by which non-sister chromatids of homologous chromosomes exchange alleles, producing new combinations of alleles on the chromosomes.

The points at which non-sister chromatids cross over and become entangled during meiosis I. (Singular: chiasma.)

• During meiosis I, homologous chromosomes pair up in close proximity

• The non-sister chromatids cross over and become entangled, forming chiasmata

• The entanglement places stress on the DNA, so a section of chromatid may break and rejoin with the chromatid of the other chromosome

• This swaps alleles, producing a new combination of alleles on the two chromosomes

Crossing over is more likely to occur further down the chromosome, away from the centromere.

The production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I.

• In metaphase I, homologous pairs line up at the equator with either chromosome on top — this is completely random

• The orientation of one homologous pair is independent of any other pair

• The pairs are then separated to different poles, so the combination of alleles in each daughter cell depends on how the pairs lined up, producing many different combinations

2ⁿ, where n is the number of chromosomes in a haploid cell.

For humans (n = 23) this gives 2²³ combinations.

Independent assortment results from the random alignment of homologous pairs along the equator of the spindle during metaphase I of meiosis I.

During fertilisation, any male gamete can fuse with any female gamete. Because each gamete already carries a different combination of alleles, this random fusion means each zygote has a unique combination of alleles.

Mutation — a random mutation during DNA replication can produce new alleles, increasing genetic variation on a scale smaller than whole chromosomes.

False — crossing over occurs between non-sister chromatids of homologous chromosomes.

True

A eukaryotic cell that has developed specific adaptations to its structure so that it can carry out a particular function within a multicellular organism.

Because the structure of each cell is adapted to suit its specific function. Different functions require different structural adaptations, such as:\n\n- The shape of the cell\n\n- The organelles the cell contains (or does not contain)

Ribosomes are the organelle responsible for protein synthesis, so a cell adapted to make large quantities of protein will contain many ribosomes to carry out this function.

• Biconcave shape increases the surface area over which oxygen can be absorbed\n\n- Cytoplasm contains high amounts of haemoglobin, which readily binds to oxygen\n\n- No nucleus, making more space for haemoglobin and maximising oxygen-carrying capacity\n\n- Elastic membrane allows the cell to change shape as it squeezes through narrow capillaries

The biconcave shape of an erythrocyte increases the surface area available for oxygen absorption.

• Very flexible shape allows it to squeeze through cell junctions in the capillary wall\n\n- Flexibility lets it form pseudopodia (cytoplasmic projections) that engulf microorganisms\n\n- Large number of lysosomes containing digestive enzymes to digest and destroy invading cells\n\n- Flexible nuclear membrane helps it penetrate cell junctions, thought to cause its characteristic lobed nucleus

• Head contains a haploid nucleus (half the normal number of chromosomes)\n\n- Acrosome in the head contains digestive enzymes to break down the outer layer of the egg cell so the nucleus can enter\n\n- Mid-piece is packed with mitochondria to release energy (via respiration) for tail movement\n\n- Tail rotates to propel the sperm forwards towards the egg

The acrosome contains digestive enzymes that break down the outer layer of the egg cell, allowing the sperm's haploid nucleus to enter and fuse with the egg's nucleus.

• Root hair projection increases the surface area in contact with the soil, increasing the rate of water uptake by osmosis\n\n- Thinner walls than other plant cells, giving a shorter diffusion distance so water passes through easily\n\n- Permanent vacuole contains concentrated cell sap, maintaining a water potential gradient with the soil water\n\n- Mitochondria provide energy for the active transport of mineral ions

Root hair cells are found underground where there is no light, so photosynthesis cannot occur and chloroplasts would serve no purpose.

• Cells have cilia (hair-like structures) that beat in a coordinated way to shift material along the surface of the tissue\n\n- Goblet cells secrete mucus which traps dust, dirt and microorganisms, preventing them from reaching vital organs

• Consists of a single layer of flattened cells on a basement membrane\n\n- The thin cross-section shortens the diffusion pathway, reducing the distance substances must travel\n\n- It is permeable, allowing easy diffusion of gases

• Contains a large number of chloroplasts in the cytoplasm to maximise absorption of light for photosynthesis\n\n- Tall and thin shape allows light to penetrate deeper before meeting a cell wall, and lets many cells be densely packed together

• Inner cell walls are thicker and outer cell walls are thinner, so the cell bends when turgid, opening the stoma\n\n- When turgid the stoma opens; when flaccid the stoma closes\n\n- High density of chloroplasts and mitochondria, thought to play a role in stomatal opening

True — the absence of a nucleus maximises the cell's oxygen-carrying capacity.

False — when guard cells are turgid the stoma opens; it closes only when the guard cells become flaccid.

A group of specialised cells of the same type that work together to perform a particular function.

A structure made up of different tissues that work together to perform a particular function (e.g. the heart contains cardiac muscle tissue, blood vessel tissue and connective tissue).

• Specialised cell

• Tissue

• Organ

• Organ system

Different tissues work together to form organs, and different ones of these work together to form organ systems.

• Epithelial cells in the small intestine are specialised to absorb food efficiently

• Red blood cells are specialised to transport oxygen

Transport of water and dissolved ions up the plant, towards the leaves.

Transport of dissolved sugars and amino acids (by translocation).

Xylem is made of dead cells (no cytoplasm or organelles), whereas phloem is made of living cells supported by companion cells.

• Contain layers of protein filaments that slide over each other to cause contraction

• Have a high density of mitochondria to provide energy (via respiration) for contraction

• Skeletal muscle cells fuse during development to form multinucleated cells that contract in unison

• Has cilia (hair-like structures) that beat in a coordinated way to shift material along the surface

• Goblet cells secrete mucus which traps dust, dirt and microorganisms, preventing them from reaching vital organs

• Consists of a single layer of flattened cells on a basement membrane

• Forms a thin cross-section that shortens the diffusion pathway

• Is permeable, allowing easy diffusion of gases

To provide support. Cartilage is a strong, flexible tissue found in various places around the body.

The outer walls of xylem vessels are thickened with lignin, which strengthens the tubes and helps support the plant.

• No top and bottom walls between cells, forming continuous hollow tubes for water to move up towards the leaves

• Cells are dead, without organelles or cytoplasm, allowing free movement of water

• Outer walls thickened with lignin, strengthening the tubes and supporting the plant

• Made of living cells, supported by companion cells

• Cells joined end-to-end with holes in the end walls (sieve plates) forming tubes that let sugars and amino acids flow easily

• Have very few subcellular structures to aid the flow of materials

In the trachea it forms tracheal rings, which support the trachea and keep it open while allowing it to move and flex during breathing.

True — xylem cells lose their organelles and cytoplasm, allowing free movement of water.

False — it is a single layer of flattened cells, which shortens the diffusion pathway.

A cell that can divide (by mitosis) an unlimited number of times, and whose daughter cells can either remain stem cells or develop into specialised cells.

The process by which a stem cell develops into a specialised cell, such as a blood cell or a muscle cell.

The ability of a stem cell to differentiate into more specialised cell types.

• Totipotency

• Pluripotency

• Multipotency

Any cell type found in an embryo, as well as extra-embryonic cells (the cells that make up the placenta).

The zygote (formed when a sperm cell fertilises an egg cell), as well as embryonic cells up to the 16-cell stage of human development.

Pluripotent (embryonic) stem cells can differentiate into any cell type found in an embryo, but cannot form extra-embryonic cells (placental cells).

Totipotent stem cells can form embryonic cell types and extra-embryonic cells.

An adult stem cell that has lost some of the potency of embryonic stem cells (it is no longer pluripotent) and can produce only a limited range of cell types.

Because they can still divide (by mitosis) an unlimited number of times.

However, having already partially differentiated, they can only produce a limited range of specialised cell types.

They produce new cells for the essential processes of:

• Growth

• Cell replacement

• Tissue repair

Stem cells found in bone marrow are multipotent adult stem cells.

Only blood cells (red blood cells, monocytes, neutrophils and lymphocytes).

The introduction of adult stem cells into damaged tissue to treat diseases (eg. leukaemia) and injuries (eg. skin burns).

In adults, multipotent stem cells can be found throughout the body, for example in the bone marrow, skin, gut, heart and brain.

True

False — only totipotent stem cells can form extra-embryonic cells.

A region of a plant where growth takes place and where plant stem cells are found.

A meristem located between regions of xylem and phloem tissue in plant stems and roots, containing the stem cells that give rise to xylem and phloem cells.

Multipotent adult stem cells.

• Erythrocytes (red blood cells)

• Neutrophils (a type of white blood cell)

Mature erythrocytes lack a nucleus, so they cannot carry out mitosis to produce new cells.

New erythrocytes must therefore develop from bone marrow stem cells to maintain the red blood cell count in the blood.

• Loss of the nucleus

• A shape change to form a biconcave disc

• Increased production of haemoglobin

• An increase in membrane flexibility

• Indentations form in the nucleus, giving it a lobed structure

• Production of hydrolytic enzymes increases

• Lysosomes containing hydrolytic enzymes accumulate

• Membrane flexibility increases

Neutrophils are a type of white blood cell specialised to destroy pathogens by phagocytosis.

Plant stem cells are found in meristems (regions of growth).

• Deposition of lignin in the cell walls

• Loss of cytoplasm

• Loss of the end walls

• A reduction in cytoplasm volume

• Loss of some organelles

• End walls develop into sieve plates

Stem cells in bone marrow are multipotent adult stem cells.

• Xylem

• Phloem

False — erythrocytes lose their nucleus, so new ones must develop from bone marrow stem cells.

True

Because of their ability to differentiate into multiple different cell types.

• Repair damaged tissue

• Treat neurological conditions

• Research developmental biology

• Ethical concerns around using embryonic stem cells, which have the potential to develop into an adult human

• Adult stem cells used in treatment could trigger an immune response unless they are a close tissue match

• Stem cells can divide indefinitely; if this division becomes uncontrolled it can lead to cancers

Embryonic stem cells have the potential to develop into an adult human, raising ethical objections to their use.

They could cause an immune response unless they are a close tissue match to the patient.

Stem cells have the ability to divide indefinitely; if this division becomes uncontrolled it can lead to cancers.

• Accidental damage

• Degenerative disease

• Autoimmune condition

They could be encouraged to differentiate into the damaged cell type and then used to repair the damaged tissue.

• Skin cells to treat burn patients

• Neurones to repair a damaged spinal cord

• Pancreas cells to treat type 1 diabetes

• Retina cells to treat macular degeneration in the eye

Stem cells could be used to generate new neurones to treat the symptoms of these conditions, e.g. replacing damaged brain cells in Alzheimer's and Parkinson's disease.

They have the ability to differentiate into embryos, allowing scientists to study the developmental stages of the early embryo.

• Information about developmental problems

• The effects of medicines on embryos

The process by which a stem cell becomes specialised into a particular cell type, giving stem cells their potential to repair or replace many different tissues.

True — this is the basis of the ethical concerns around their use.

False — stem cells can divide indefinitely, which if uncontrolled can lead to cancers.