SB 2: Chromosomes & Inheritance

Normal human chromosomes

• Each human cell carries 23 pairs of chromosomes in the nuclei (46 total)

• Within each pair, chromosomes match

• Exception of male sex chromosomes (XY)

Scientists studying fertilization

• Found that maternal and paternal gamete (cells) equally contribute egg and sperm nuclei

• Implies something in the nucleus contains hereditary material

• More studies revealed smaller, discrete, colored bodies called chromosomes

Mitosis vs. Meiosis

• Mitosis results in two daughter cells that are identical to the parent, somatic

• Meiosis allows for the chromosomes composing each pair to segregate, so the resulting gamete receives only one chromosome from each pair

• Generates egg/sperm cells containing half the number of chromosomes (haploid)

Fertilization

• Chromosomes in a fertilized egg consist of two matching sets, one by maternal gamete, one by paternal

• Maternal and paternal chromosomes form pairs, alike in size and shape

• These cells are diploid (two matching sets)

Chromatin

• A mass of extremely fine tangled string that resembles the chromosome

• Surrounded by the nuclear envelope

• Each chromatin thread is composed of DNA (genetic info) and protein (package and manage info)

Interphase

• Gap 1 (G1) - cell achieves most of its growth, period from birth of a new cell to onset of chromosome replication

• Synthesis (S) - DNA synthesis and chromosome duplication

• Gap 2 (G2) - Cell grows, synthesizes proteins needed for mitosis

Prophase

• Mass of chromatin condenses into discrete, individual chromosomes

• Each condensed chromosome has been duplicated during interphase, so it consists of sister chromatids attached at the centromere (X shape)

• Dark-stained nucleoli breaks down, ceasing the manufacture of ribosomes to focus energy on cell division

Prophase outside the nucleus, in the cytoplasm

• Microtubule network from interphase gets replaced with new set of microtubules that are more dynamic (grow outward and shrink back from centrosomes)

• Duplicated (2) centrosomes from earlier travel outside the nucleus to opposite ends of the cell

• Separation driven by the microtubules within each centrosome interacting and generating force to push apart

Prometaphase

• Breakdown of nuclear envelope, allowing microtubules extending from centrosomes to invade nucleus

• Chromosomes attach to microtubules using kinetochore (structure in centromere region of chromatid)

Metaphase & Anaphase

• Metaphase plate

• Anaphase, separation

• Telophase, prophase in reverse

• Cytokinesis

Cell’s decision to divide

• Conditions within the cell that register being a sufficient size for division

• Signals from environment, hormonal cues/contacts with neighboring cells

• Initiation during G1 period of Interphase

Regulatory checkpoints to ensure correct chromosome separation

• Enzymes monitoring DNA replication to ensure that cells do not begin mitosis until all chromosomes have been copied

Somatic cells

• Cells that grow and divide via mitosis, or stop growing and become arrested in G0

• Make up organism’s tissue

Germ cells

• Set aside during embryonic development for the specialized role in gamete production

• Half the number of chromosomes

Meiosis divisions

• Two successive nuclear divisions (Meiosis 1 and 2) but only one chromosome duplication (1)

• PMAT in each round

• Round 1: Parent nucleus divides into two daughter nuclei, Round 2: Each daughter divides, resulting in four nuclei

• Four nuclei are partitioned in four cells from cytokinesis

Recombination

• Homologous chromosomes pair with each other in Meiosis 1 (same gene, different allele)

• Non-sister chromatids exchange parts to produce new combinations of the alleles of different genes

• Sister chromatids on a chromosome are no longer identical

Prophase 1

• Condensation of chromatin

• Pairing of homologous chromosomes

• Reciprocal exchange of genetic info between paired homologs

Prophase 1 - Zygotene

• Chromosome seeks out its homologous partner

• Homologs are zipped together in a process called synapsis

• Synaptonemal complex: protein structure that aligns the homologs with precision

Prophase 1 - Pachytene (Form Tetrad)

• A synapsed chromosome pair is known as a Bivalent (encompasses two chromosomes) or a Tetrad (four chromatids)

• One side of bivalent is a maternally derived chromosome, one side is paternally derived

Prophase 1 - Pachytene (Recombination)

• Recombination nodule structures appear along the synaptonemal complex

• Exchange between non-sister (M and P) chromatids occur at these nodules

• Crossing over resulting in recombination of genetic material

• Retain size

Prophase 1 - Diplotene

• Dissolution of synaptonemal complex

• Homologs remain merged at intervals along their length called chiasmata (sites where crossing over occurred)

Metaphase 1

• Kinetochores of sister chromatids fuse, each chromosome has one kinetochore

• Kinetochores of homologs attach to microtubules from opposite spindle poles

• Homologs align at the metaphase plate, maternal and paternal versions on either side; however, each bivalents’ orientation is independent of others

Anaphase 1

• Chiasmata dissolves

• In each homolog pair, one chromosome consisting of two sister chromatids segregates to each spindle pole

• Cohesion keeps sister chromatids together

Prophase 2

• If the chromosomes decondensed during the preceding interphase, they recondense during Prophase 2

• Nuclear envelope breaks down and spindle apparatus reforms

Mistakes in Meiosis produce defective gametes

• If homologs of do not segregate during Meiosis 1, they may travel to the same pole together and become part of the same gamete

• Nondisjunction

Meiosis contributes genetic diversity

• Independent alignment of non-homologous chromosomes

• Crossing over

• 2^n

Mitosis vs. Meiosis

• Mitosis = all eukaryotic cells, conservative mechanism (preserves genetic status quo), produces growth/increasing number of cells, promotes continual replacement of organ tissue, plant parts, etc

• Meiosis = sexually reproducing organisms, genetic variation