Intro to Genetics- Exam 1 (Mitosis and Meiosis)

Chromosomes exist in ___ pairs in ___ organsims

Homologous; Diploid

Mitosis partitions chromosomes into ___

Dividing cells

Meiosis creates ___ gametes and spores

Haploid

Meiosis enhances ___ in species

Genetic variation

Meiosis is critical to the successful ___ of all diploid organisms

Sexual reporoduction

Chromosomes are long stretches of ___

DNA

Chromosomes are composed of thousands of ___

Nucleotides

Chromosomes contain hundreds of ___

Genes

Where do we find DNA?

Depends on the cell type

Terms to describe cells

• Eukaryotic

• Prokaryotic

• Diploid

• Haploid

Chromosomes replicate and divide through ___

Mitosis and meiosis

Genes are the fundamental unit of ___

Heredity

Chromosomes carry a version of the gene called ___

An allele

Prokaryotic Cells

• Bacteria, archaea

• Unicellular, no membrane bound organelles

Eukaryotic cells

• Protists, plants, fungi, animals

• Unicellular and multicellular organisms with membrane-bound organelles

• Genetic material is surrounded in a nuclear envelope to form a nucleus

All cells share common features:

Plasma membrane, DNA, ribosomes

Nucleus

• Found in eukaryotes

• Porous structure

• Membrane bound (double-membrane structure)

• Houses genetic material (DNA)

Nucleolus

Where ribosomal RNA (rRNA) is synthesized

Nucleoid

• Found in prokaryotes

• Not membrane bound

Chromatin

• In eukaryotic cells

• Found in the granular region of the nucleoplasm

• DNA is in this form when the nucleus is not dividing

Eukaryotic chromosomes

• Chromosomes are found in a compact form only when the cell is dividing

• Chromosomes take on a relaxed form when not dividing (spaghetti)

Centromere

• Constricted regions on the chromosomes

• Link sister chromatids

Diploid cells

(2N), carry two sets of genetic information

Haploid cells

(N), carry one set of genetic information

Ploidy

Sets of genetic information

Some plants are ___ and have multiple sets of chromosomes

Polyploids

A diploid organism has two sets of chromosomes organized as ___

Homologous pairs

Homologous Pairs

• Not identical copies

• 2 different sets of instructions

• Will always have alleles for genes on matching chromosomes (hair color is on chr. 11 for both versions)

  • Mom vs. dad’s allele may be different, but the instructions are in the same location

Criteria for classifying two chromosomes as homologous pairs

• Both are the same size and exhibit identical centromere locations

  • Excludes X and Y chromosomes in mammals

• Form pairs or synapse (“come together”) during the stages of meiosis

• Contain identical linear order of the gene loci (“address”)

• One member of each pair is derived from the maternal parent and one from the paternal parent

Each cell has a characteristic number of chromosomes with a distinct ___

Size and shape

Sister chromatids

• Exact copies

• Only seen when the cell is dividing

Telomeres

• Region of repetitive nucleotide sequences at each end of a chromatid, which protects the end of the chromosome from deterioration

• Every time your cell divides, they should get a little smaller

  • There for protection

  • Don’t shrink in cancer cells, allowing for unlimited reproduction

• When they get too short, the cell should die

Gene location: 21q3

• ___ chromosome

• ___ arm

• distance from centromere (bigger number = farther away)

Metacentric

• Centromere is in the middle

• Large p arms

Submetacentric

• Between middle and end

• Medium p arms

Acrocentric

• Close to end

• Small p arms

Telocentric

• At end (“at the telomeres”)

• P arms not visible

Mitosis

• Responsible for cell growth (replication) and repair

• Maintains the ploidy of cells

• Produces somatic cells

• 2N → 2N + 2N

Somatic cells

Any body cells (skin, organs, etc.)

Meiosis

• 2 phases, 2 divisions

• Reduces ploidy by half (haploid cells - 1 diploid → 4 haploid)

• Produces haploid sex cells (gametes) in diploid organisms

• 2N → N + N

Gametes

Sperm and eggs (only 1 set of chromosomes)

The cell cycle

• Cyclical process of cell growth

• Includes both nuclear division (karyokinesis) and cell division (cytokinesis)

• Regulated process!!!

Interphase

• Period between mitoses, prepping for division

• Chromosomes are relaxed and difficult to see with a light microscope (in spaghetti form)

• Divided into several stages (G1, S, G2)

S Phase

DNA is synthesized

G1, G2

Gap phases, phases of cell growth

G0

• Not dividing

• Still alive and metabolic, just no longer cycling

• Can come out of this phase

Mitotic cycle

Cell division

Interphase

• 3 subphases

• Absence of visible chromosomes

• Chromosomes are replicated (where sister chromatids appear)

• Centrosome is duplicated

Centrosome

• Outside the nucleus (NOT the centromere)

• Organelle that serves as a main microtubule organizing center

• Composed of centrioles - found in centrosome of animal and plant cells

• Organize spindle fibers (microtubules) needed for movement of chromosomes during mitosis and meiosis

• Replicates and moves to opposite end of the cell (creates “poles”)

M Phase

• 5 discrete subphases

  • Prophase

  • Prometaphase

  • Metaphase

  • Anaphase

  • Telophase

Prophase

• Nuclear envelope breaks down

• Chromosomes condense

• Centrosomes start migrating to establish opposite poles

• Sister chromatids connected at the centromere

  • Held together by multi-subunit protein complex called cohesin

• Chromosomes start to become visible

Prometaphase

• Centrioles reach opposite poles (poles exist, no longer moving)

• Chromosomes are fully condensed (not continuing to condense)

• Nuclear envelope completes disassembly

• Chromosomes attach to spindle by their kinetochores (at centromere)

  • Need all this attachment to control and direct movement

• Chromosomes move towards metaphase plate

Metaphase

• Chromosomes align on the metaphase plate - this is not an actual structure

  • Move to the center of the chromosome

• “Something lining up at the middle of the cell”

Kinetochore

• Found at centromere

• Where spindle fibers attach

• Protein structure on chromatids where the spindle fibers attach during cell division to move chromosomes and pull sister chromatids apart

Kinetochore Mocrotubules

• One end near the centrosome region and the other end anchored to the kinetochore

• Contraction of kinetochore microtubules separates sister chromatids

Anaphase

• Sister chromatids are separate (releasing copy made)

• Daughter chromosomes migrate towards opposite poles (spindle fibers break down and get shorter)

Telophase

• Migration of chromosomes is complete (no longer moving, at poles)

• Two nuclear membranes form (rebuilds protection)

• Chromosomes relax (spaghetti form)

• Cytokinesis occurs (physically splitting cell into 2, new cell)

Results of mitosis

• Produces two daughter cells that are genetically identical to each other and the parent cell

  • Same number of chromosomes and same ploidy

• Daughter cells contain a full set of chromosomes

• Each daughter cell contains approximately half the cytoplasm and organelle content of the parent cell

Meiosis DNA synthesis

• Occurs before the beginning of meiosis I

• DOES NOT occur again before meiosis II

Meiosis Interphase

DNA synthesis and chromosome replication phase

Meiosis I

• Reductional division (converts diploid into haploid)

  • Prophase:

    • Pairing of homologs by synapsis

    • Tetrad = homologous pair

    • Crossing over occurs via the chiasmata

  • Metaphase:

    • Tetrads migrate to the metaphase plate

  • Anaphase:

    • Tetrads separate and migrate to the cell poles

Meiosis II

• Equational division - divides chromosomes into chromatids

• Results in 4 genetically distinct daughter cells

Meiosis I and II each have:

Prophase, metaphase, anaphase, and telophase stages

Prophase I

• Nuclear envelope breaks down

• Chromosomes condense

• Centrosomes migrate to opposite poles

• Spindles attach to kinetochores

• Crossover occurs between homologs (gives up space to share a little bit of genetic information between non-sister chromatids - creates genetic variation)

Crossing over

• Meiotic event

• Genetic exchange between non-sister chromatids

• The point of exchange during chromosomal crossover is termed the chiasma (plural: chiasata)

Metaphase I

• Terminal chiasmata holding non-sister chromatids together

• Binding to spindle fibers moves chromatids to metaphase plate

• Tetrads are at the center of the cell

Anaphase I

• Disjunction occurs

• Homologs move to opposite poles

• Separating tetrads, they move to the poles

Telophase I

• Sister chromatids remain attached at the centromere

• Nuclear membrane may reform

• Cells may enter a short interphase or go directly to prophase II

• Cytokinesis: Two haploid daughter cells are forms (1 → 2)

• NO DNA replication between meiosis I and II

Meiosis II

Second meiotic division (metaphase II, anaphase II, telophase II, cytokinesis)

Prophase II

• Nuclear envelope disintegrates if formed during telophase I

• Each dyad (“sister chromatids”) is composed of one pair of sister chromatids attached at the centromere

• Spindle forms and attaches to kinetochore

Metaphase II

Chromosomes are arranged at the equator (“something moving to the center of the cell via cpindle fibers”)

Anaphase II

• Sister chromatids separate

• Chromatids (single chromosome) migrate towards opposite poles

Telophase II

• Monads (new name for chromosomes, =1) at opposite poles

• Nuclear membrane forms (definitely happens this time)

• Cytokinesis (2 → 4)

Results of meiosis

• Produces haploid gametes or spores, each containing one member of a homologous pair of chromosomes

• 4 haploid nuclei are formed from a single diploid

• Chromosome number in each new haploid cell is reduced by half

• Newly formed haploid cells are genetically different from one another and from the parental cell

Spermatogenesis

• Sperm

• 1 diploid → 4 haploid

Oogenesis

• Eggs

• 1 diploid → 1 haploid

  • One of the two diploids are lost after meiosis I

  • One of the two haploids (that form from the remaining diploid) is lost

Crossing over produces chromosomes containing both ___

Maternal and paternal information

Independent assortment

A mixture of maternal and paternal chromosomes in each cell

Genetic recombination

The process of exchanging genetic material between different organisms, leading to offspring with new combinations of traits

Nonhomologous chromosomes

• aka heterologous chromosomes

• Pairs of chromosomes that carry different genes and do not share the same genetic sequence or traits (ex. X and Y)

Sister chromatids vs. homologous chromosomes

• Sister chromatids: identical copies of a single chromosome formed during DNA replication

• Homologous chromosomes: pairs of chromosomes from each parent that may carry different alleles for the same gene

Tetrad

The four spores produced after meiosis