Unit 2 Genetics

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why do cells divide in unicellular organisms?

for reproduction of the species.

why do cells divide in multicellular organisms?

for regeneration, to replace damaged/dead cells, and for growth.

why do cells have to divide for an organism to grow?

Because of the surface area to volume ratio. Cells can only grow to a certain size, then they divide or die. As it increases, the amount of cytoplasm & organelles increase, but the cell membrane doesn’t grow at the same rate.

Why is cell division important?

It allows living things to grow, repair damaged tissues, replace old cells and reproduce.

What does cell division maintain?

It maintains the surface area to volume ratio, allowing a constant amount of material to diffuse through the cell membrane.

How do cells divide?

Mitosis (duplication of the nucleus), Cytokinesis (division of the cytoplasm n organelle contents) and cell division.

Interphase — Cell Cycle “The preparation phase”

• The cell gets ready to divide

• Cell gets bigger and makes proteins

• Copies its DNA (synthesis)

Miotic phase — Cell Cycle

• Cell divides into 2 new cells

• Mitosis occurs. DNA splits into 4 steps (PMAT)

• Cytokinesis

What does PMAT stand for?

Prophase, Metaphase, Anaphase, Telophase

Cytokinesis (definition)

The physical process where the cytoplasm splits, dividing one cell into two separate daughter cells at the end of cell division

Asexual reproduction

• Offspring genetically identical to the parent

• New cells produced by mitosis

• Only 1 parent required

• parent & daughter cells have the same # of chromosomes

• Ensures genetic continuity

Sexual reproduction

• 2 parents required

• Gametes come tgt in fertilization to form a zygote

• Each gamete has half # of chromosomes

• ensures genetic variation

Human gametes

They’re haploid and contain 23 chromosomes. (n=23). This is half the number of chromosomes in somatic cells.

Human somatic cells

They’re diploid and contain all 46 chromosomes. 2n = 46

Chromatin

• Found during interphase.

• Loosely organized, complex DNA & protein in the nucleus.

• DNA is loosely unravelled, so cells can easily read its genes to make proteins.

Homologous chromosome

• Only found in diploid cells

• Chromosomes of the same shape, size, & gene organization

• Duplicated before mitosis/meiosis

• Separated during meiosis

• Reunited during fertilization

Sister Chromatids

• Two identical copies of chromosomes joined tgt

• Formed during DNA replication

• Connected by a centromere

• 2 sister chromatids are considered one chromosome

Centromere

The central structure that holds sister chromatids together

Chromosome

Thread like structures found in the nucleus of a eukaryotic cell that carry genetic information in the form of genes.

Structure of Chromosomes

Two sister chromatids connected by a centromere. P arm (shorter arm), Q arm (longer arm), telomeres, and histones.

What are telomeres?

structures on a chromosome that protect the very ends of its arms.

What are histones?

The type of protein DNA wraps around to compact itself.

How are nucleosome beads formed?

DNA double helix is wound around a core of 8 histones, forming the bead.

What are the phases of the cell cycle?

G1 (Gap 1), S (DNA synthesis), G2 (Gap 2), M (Mitosis)

What happens during Gap 1 (G1) phase of the cell cycle?

Cells grow and organelles double

What happens during DNA Synthesis (S) phase of the cell cycle?

DNA replication (Chromosomes duplicate)

What happens during Gap 2 (G2) phase of the cell cycle?

growth continues as cell prepares to divide

What happens during Mitosis (M) phase of the cell cycle?

Mitosis; cell division (PMAT)

Prophase (First phase)

• Chromosomes shorten, thicken, n become more visible

• Centrioles move apart n grow spindle fibres

• Nuclear membrane breaks down

Metaphase (middle phase)

• sister chromatids line up down the middle (equatorial plate)

• spindle fibres attatch to centromeres. Chromosomes r most visible here n look dark/thick

Anaphase (apart phase)

• Centromeres split n sister chromatids are pulled apart to opposite poles

• Once separated, each chromatid is considered an individual chromosome again.

Telophase (Two new nuclei)

• Chromosomes reach the poles n begin to unravel back into loose chromatin

• spindle fibres dissolve, and new nuclear membranes form around each mass of chromatin

Cytokinesis (splitting cytoplasm)

• Final step where the cell physically divides into 2 identical daughter cells

• new cells r created

Cytokinesis in animal cells

pinch inward to form a cleavage furrow that will form the cell membranes

Cytokinesis in plant cells

forms a new cell plate down the middle, which builds into a new cell wall

Karyotype

• A picture of an individual’s organized chromosomes laid out by size and shape.

• Numbered 1 through 22, plus sex chromosomes X and Y

• Used to identify chromosomal disorders

What is meiosis?

The process by which gametes (sex cells) are formed in the gonads (reproductive glands) in humans

where does oogenesis occur and what does it produce?

Occurs in the ovaries, producing oocytes (egg cells)

where does Spermatogenesis occur and what does it produce?

occurs in the testes, producing spermatoeoa (sperm)

What happens during meiosis?

4 haploid cells (n) are produced by a single diploid (2n) cell during two consecutive divisions.

why do chromosomes have to duplicate before meiosis ?

Before meiosis occurs, chromosomes duplicate at the end of interphase. This results in a diploid cell with 46 chromosomes (2n) and 92 pc of DNA. They need to duplicate because they go through two consecutive divisions. This gives enough genetic material to successfully produce four haploid cells without running out.

Prophase I

• Nuclear membrane dissolves

• Synapsis occurs

• Chromosomes condense

• Crossing over can occur during this time

• Spindle fibres begin to form from centrioles at opposite ends (poles) of the cell.

• 2n(2) chromosomes present

Metaphase I

• Tetrads line up randomly at the equatorial plate (independent assortment)

What is synapsis?

The process where two homologous chromosomes form pairs called tetrads.

What is recombination/crossing over?

The random swapping of DNA between homologous chromosomes, creating unique new gene combinations.

What are tetrads?

4-part chromosome structure that forms during Prophase I.

Anaphase I

• Tetrads separate and move to opposite poles of the cell

• 23 chromosomes —> n(2)

• sister chromatid pairs will be seen at each end of the cell

Telophase I & Cytokinesis

• Division of cytoplasm occurs

• forms 2 cells

• each new cell contains n(2) chromosomes

• sister chromatids are still attached at the centromere

• goes from a diploid cell to a haploid cell

Prophase II

• Each cell contains n(2) chromosomes

• 23 nonhomologous chromosomes, consisting of 2 chromatids each, are seen

• Centrioles move to opposite poles

• new spindle fibres form

Metaphase II

• n(2) chromosomes line up at the equatorial plate

• Attach to the spindle fibres.

Anaphase II

• n(2) chromosomes

• Chromatids separate at the centromere

• Move to opposite poles

• n chromosomes at each pole

• Nuclear membrane begins to re form

Telophase II and cytokinesis

• Cytoplasm separates

• 4 cells w/ n chromosomes each remain

• The 4 cells are gametes

What phase does DNA duplication occur?

Interphase

How many chromosomes are “n” (in humans)

23

How many complete sets of chromosomes do haploid cells have?

only have one

How many complete sets of chromosomes do diploid cells have?

they have two

What does spermatogenesis mean?

Spermato means sperm (atozoa), genesis means formation.

what does oogenesis mean?

oo means oocyte, genesis means formation.

What are the characteristics of spermatogenesis (meiosis in males)

• all 4 haploid cells produced are viable

• cells lose all organelles

What is the exception with miosis in males regarding losing their organelles?

An exception is the mitochondria. It becomes concentrated at the neck of a “flagellum” to travel through female system.

What are the characteristics of oogenesis (meiosis in females)

• Only 1 haploid cell they produce is viable (ovum)

• All its organelles become concentrated in the single viable cell

• ovum must supply all organelles for the zygote

• other 3 cells (polar bodies) are discarded.

How does fertilization occur?

Spermatozoa (male haploid gamete) unites w/ a ovum (haploid female gamete) to form a diploid zygote.

Where does a zygote get its genetic info from?

They get from both of its parents gametes.

what do the letters on chromosomes represent

They represent alleles, which is another term for genes.