Cell Division and Specialisation

Question 1

G1

Answer 1

-First growth phase.
-Cells increase in size.
-Genes are transcribed to make RNA.
-Organelles duplicate.
-Biosynthesis of proteins needed to replicate DNA.

Question 2

S phase

Answer 2

-Replication phase.
-DNA replicates in a specific order with genes active in all cells being made first (housekeeping genes).
-Afterwards chromosomes consist of two identical sister chromatids.
-Occurs rapidly to reduce the chance of mutations occurring when DNA base pairs are exposed.

Question 3

G2

Answer 3

-Second growth phase.
-Cells grow.
-Chemicals stimulate proteins involved in making chromosomes condense and spindles form.

Question 4

Restriction point

Answer 4

-Between G1 and S.
-Where chromosomes are checked for damage.
-If detected they are destroyed or delayed.
-Also checks for uncontrolled division that could lead to cancer.
-Similar check between G2 and mitosis.

Question 5

G0

Answer 5

-Triggered by differentiating cells or damage detected at a checkpoint in early G1.
-Cells undergo programmed cell death (apoptosis) or are left to die away (senescence).

Question 6

Homologous chromosomes

Answer 6

-Code for the same gene but may have different versions of the gene (alleles).
-One from father, one from mother per pair.
-23 pairs of homologous chromosomes (46 total chromosomes).

Question 7

Prophase

Answer 7

-Chromosomes (two identical sister chromatids) shorten and thicken due to DNA supercoiling.
-Nuclear membrane breaks down and nucleolus disintegrates.
-Centrioles duplicate and move to each pole of the cell. They are joined by tubulin threads.

Question 8

Metaphase

Answer 8

-Pairs of chromatids lined up along the metaphase plane (equator) of a cell.
-Attached to spindle fibres at their centromere.

Question 9

Anaphase

Answer 9

-Centromeres divide and sister chromosomes are pulled apart.
-Moved apart by motor proteins on tubulin threads to each opposite pole.
-Chromosomes now known as chromatids, assume a V shape.

Question 10

Telophase

Answer 10

-Separated chromosomes reach the poles.
-Nuclear envelope forms around each of the two clusters of chromosomes making two identical nuclei.
-Chromosomes uncoil.
-Spindle fibres disappear.

Question 11

Cytokinesis

Answer 11

-Cell divides into two identical daughter cells.
-In animals the plasma membrane folds inwards.
-In plants an end plate forms across the equator, and membrane and cell wall material is laid down on either side.

Question 12

Centrioles

Answer 12

-Parts of the cytoskeleton.
-Arranged in 9 groups of 3 microtubules

Question 13

Meiosis

Answer 13

-Combining of gametes (mother and father chromosomes) to get a random selection of characteristics.
-Two haploid gametes fuse to produce a diploid zygote.

Question 13

Prophase 1

Answer 13

-Chromosomes thicken (supercoil) and become visible due to chromatin condensing.
-Nuclear envelope breaks down.
-Threads of tubulin form in the centriole.
-Chromosomes from the mother and father come together in homologous pairs.
-Crossing over occurs between them where alleles of non-sister chromatids are wrapped around each other and are shuffled by the swapping of sections.

Question 14

Metaphase 1

Answer 14

-Pair of crossed (bivalent) homologous chromosomes attach along the equator.
-Attaches to a spindle thread by its centromere.
-Pairs arranged randomly and face opposite ends of the cell.
-The way they line up determines how they will separate during anaphase.
-This is known as independent assortment.

Question 15

Anaphase 1

Answer 15

-Each pair of homologous chromosomes are pulled apart by motor proteins along tubulin threads of the spindle.
-Each chromosome consists still of two chromatids, and the centromere does not divide.
-The crossed-over areas separated, resulting in swapped areas and allele shuffling.

Question 16

Telophase 1

Answer 16

-Two nuclear envelopes form around each set of chromatids.
-The cell divided by cytokinesis, chromosomes uncoil during short interphase.
-Each new nucleus contains half the original number of chromosomes (haploid).
-In plants this stage is skipped.

Question 17

Prophase 2

Answer 17

-Nuclear envelope breaks down again and spindles form.
-Chromosomes coil and condense.
-Due to crossing over the chromatids of each chromosome are no longer identical.

Question 18

Metaphase 2

Answer 18

-Chromosomes attach to the equator of the spindle by their centromere.
-Chromatids are randomly arranged in chromosomes to further mix alleles. This is known as random assortment, and determines how they will separate during anaphase.

Question 19

Anaphase 2

Answer 19

-Centromeres divide and chromatids are dragged to each pole by motor proteins along tubulin threads of the spindle.
-Chromatids are randomly segregated.

Question 20

Telophase 2

Answer 20

-Nuclear envelopes form around the four haploid nuclei.
-Four haploid cells are produced (gametes).

Question 21

Variation in meiosis occurs due to

Answer 21

-Crossing over shuffling alleles during prophase 1.
-Independent assortment of alleles randomly distributes mother and father in anaphase 1.
-Independent assortment of chromatids during anaphase 2.
-Random combinations of gametes during fertilisation.
-Random mutations in parts of chromosomes.

Question 22

Significance of mitosis

Answer 22

-Involves the production of genetically identical daughter cells.
-In asexual reproduction (protoctists), new individuals are produced that are identical to the original.
-Multicellular organisms grow by producing more identical cells.
-Tissues are repaired when growth factors (secreted by platelets, WBCs and damaged cells) stimulate the proliferation of endothelial and smooth muscle cells to repair damaged blood vessels.

Question 23

Cell differentiation

Answer 23

-Multicellular organisms start as a single undifferentiated cell called a zygote.
-This is unspecialised, and all genes in its genome are able to be expressed. It is known as a stem cell.
-After mitotic divisions an embryo forms, containing many undifferentiated embryonic stem cells.

Question 24

Effects of differentiation

Answer 24

-Proportions of different organelles can differ. -Shape of the cell changes. -Some contents of the cell change.

Question 25

Adaptations of sperm cells

Answer 25

-Many mitochondria to carry out aerobic respiration and provide energy for movement. -Undulipodium (tail) that propels cell. -Small, long and thin so can move easily. -Acrosome releases enzymes to digest the outer protective covering of the ovum, allowing the sperm head to enter. -Head contains haploid male gametes.

Question 26

Adaptations of palisade cells

Answer 26

-Palisade cells pack together closely, but allow air spaces for CO2 to diffuse into the cells. -They have a large vacuole to position chloroplasts near to the edge of the cell, reducing CO2s diffusion distance. -Contain cytoskeleton threads and motor proteins to move the chloroplasts in accordance to the energy of sunlight.

Question 27

Epithelial tissue

Answer 27

-Lines free surfaces of the body (eg skin, blood vessels, heart chambers and organ walls). -Cells very thin and close to form continuous sheets that minimise diffusion distance. -Can have cilia or microvilli to increase SA. -Short cell cycles, divide two or three times a day. -Epidermal tissue in plants.

Question 28

Connective tissue

Answer 28

-Consists of a nonliving extracellular matrix containing collagen, elastin proteins and polysaccharides. -Separates living cells within the tissue and enables it to withstand forces. -Eg in tendons and ligaments.

Question 29

Cartilage

Answer 29

-Example of connective tissue. -Immature cells are called chondroblasts, and when they mature they become chondrocytes. -Three different types of cartilage. -Hyaline cartilage forms in embryo, covers ends of bones, joins ribs to sternum and keep trachea open. -Fibrous cartilage in discs between vertebrae in spine and knees. -Elastic cartilage makes up outer ear and epiglottis.

Question 30

Muscle tissue

Answer 30

-Many blood vessels. -Cells are elongated and contain organelles called myofilaments made of actin and myosin that allow for tissue to contract. -Skeletal muscles are joined to bones by tendons and allow bones to move. -Cardiac muscle allows the heart to beat and pump blood. -Smooth muscle in walls of intestine, blood vessels and uterus, propels substances along.

Question 31

Stem cells

Answer 31

-Undifferentiated cells (can become any cell) -Pluripotent. -Can divide by mitosis to provide cells that can differentiate into specialised cells for repair.

Question 32

Sources of stem cells

Answer 32

-Embryonic stem cells, in early embryo formation. -Stem cells in umblical cord blood. -Adult stem cells in developed tissues, such as bone marrow. -Induced pluripotent stem cells, developed in labs by reprogramming differentiated cells to swich on certain key genes.