Unit 4: The Cell Cycle, Mitosis, Meiosis (BIOL1410 Lecture Notes - Fall 2024) PDF

BIOL1410 Lecture Notes – Fall 2024 Unit 4: The Cell Cycle, Mitosis, Meiosis UNIT 4: THE CELL CYCLE, MITOSIS, MEIOSIS A) 2 Cell Types Based on Chromosome Content: 1) Gametes: — 23 different chromosom...

BIOL1410 Lecture Notes – Fall 2024 Unit 4: The Cell Cycle, Mitosis, Meiosis UNIT 4: THE CELL CYCLE, MITOSIS, MEIOSIS A) 2 Cell Types Based on Chromosome Content: 1) Gametes: — 23 different chromosomes = haploid (n) — includes ova (eggs) and sperm 2) Somatic Cells — includes all cells of body except cells undergoing/resulting from meiosis ¦ e.g. muscle cell, nerve cell, etc. — 46 chromosomes = diploid (2n) ¦ = 23 homologous pairs ¦ Homologous Chromosomes = a chromosome pair (1 from each parent) that are highly similar in length, centromere position, and have genes for the same trait (e.g. eye colour) in the same location (locus) o Gene = a unit of heredity = a region of DNA which contains information for synthesis of a protein ¦ BUT the homologous chromosomes may have different versions of that gene (alleles) that code for proteins that produce alternate versions of that trait. § e.g. blue eyes (chromosome from one parent), brown eyes (chromosome from the second parent) — each somatic cell contains 23 different chromosome pairs — 23 chromosomes from an ovum paired with 23 chromosomes from a sperm = 46 chromosomes (diploid/2n somatic cell) — chromosomes and chromosome pairs are numbered 1-23: a) 1 to 22 = autosomal chromosomes (autosomes) ¦ contain genes for somatic characteristics ¦ e.g. hair and eye colour, height ¦ each autosomal chromosome pair is not identical, but equivalent= homologous chromosomes b) 23 = sex chromosome ¦ contain genes that determine biological sex, XX=Female XY=Male ¦ X or Y from sperm ¦ X from ovum B) Cell Cycle – for growth and repair of tissues — Diploid somatic cell (2n) à 2 genetically identical somatic diploid cells (2n) — 2 stages: 1) Interphase 2) Mitotic (M)/Cell Division phase 1. Interphase (G1, S, G2 phases) — chromosomes are present as long thread-like chromatin strands 1  BIOL1410 Lecture Notes – Fall 2024 Unit 4: The Cell Cycle, Mitosis, Meiosis — normal cellular metabolic activities occurring e.g. protein synthesis — phases of interphase: a) G1 Phase — growth, metabolism — centrosome replication begins — Note: cells that do not divide again once mature, (i.e. remain in G1) are said to be in the G0 phase ¦ e.g. nerve and some muscle cells b) S Phase — chromosomes replicate, but are still present as long thread-like chromatin strands (not individually visible) — ALWAYS occurs before cell division (mitosis and meiosis) — replicates are called sister chromatids ¦ attached to each other at an area of DNA called the centromere ¦ kinetochores (proteins) form on each centromere (1/chromatid) — kinetochore = site of attachment of spindle microtubules c) G2 Phase — growth, metabolism — production of enzymes & other proteins needed for cell division — centrosome replication is completed 2. Mitotic Phase — involves: a) mitosis = division of nuclear material (chromosomes) — 4 phases, but continuous cycle! i. Prophase ii. Metaphase iii. Anaphase iv. Telophase b) cytokinesis = division of the cytoplasm — when mitotic phase ends: — mitosis & cytokinesis complete — resulting diploid cells go into interphase (G1) — cycle starts over C) Somatic Cell Formation: Mitosis 1. Prophase — chromosomes condense (thicken, coil) - become individually visible — nucleoli disappear and nuclear envelope breaks up — centrosomes move to opposites sides (poles) of the cell — spindle microtubules grow out of centrosomes and attach to kinetochore proteins of each sister chromatid, forming the spindle apparatus — Spindle begins to move chromosomes toward cell equator 2  BIOL1410 Lecture Notes – Fall 2024 Unit 4: The Cell Cycle, Mitosis, Meiosis 2. Metaphase — 46 replicated chromosomes line up on cell equator 3. Anaphase — spindle microtubules shorten and pull the kinetochores away from each other – causes separation of centromeres, which results in separation of the sister chromatids into 92 individual chromosomes — 46 chromosomes migrate to each pole — cytokinesis (division of cytoplasm) begins 4. Telophase — chromosomes uncoil to long thread-like strands of chromatin — nucleoli, nuclear envelope reappear — spindle disassembles — cytokinesis completes (or may end after telophase ends) D) Gamete Formation: Meiosis = reproductive nuclear division — meiosis produces gametes (ovum and sperm) — 1 cell (diploid, 2n) produces 4 genetically unique gametes (each haploid, n) — involves: interphase (G1, S, G2) followed by 2 cell divisions: 1) Meiosis I, immediately followed by 2) Meiosis II (no DNA replication occurs between I and II) 1. Meiosis I (reduction division in which chromosome number changes from 2n to n) — events same as in mitosis except: a) Prophase I — homologous chromosomes attach together to form tetrads (4 chromatids in a row) b) Metaphase I — 23 tetrads line up along cell equator c) Anaphase I — tetrads (homologous chromosome pairs) separate and migrate to opposite poles i.e. 1 homologous chromosome (with 2 sister chromatids) goes to each pole — sister chromatids DON’T separate d) Telophase I Identical steps to Telophase of mitosis but with 23 replicated chromosomes instead of 46. — After Meiosis I and cytokinesis: — each new cell has 23 different chromosomes (half the number of chromosomes) = haploid (n) o these cells DO NOT have homologous chromosomes o each cell has a single copy of each autosomal chromosome and a single sex chromosome 2. Meiosis II – is the same as mitosis except it starts with 23 chromosomes instead of 46 3  BIOL1410 Lecture Notes – Fall 2024 Unit 4: The Cell Cycle, Mitosis, Meiosis a) prophase II b) metaphase II – 23 replicated chromosomes line up at equator c) anaphase II – sister chromatids split and 23 chromosomes migrate to each pole d) telophase II ¦ cytokinesis – is the same as for mitosis/meiosis I (begins in anaphase/anaphase I/anaphase II, and continues through telophase/telophase I/telophase II) Mitosis Meiosis I Meiosis II Prophase PI PII Tetrads form Metaphase MI MII 46 chromosomes line up at 23 tetrads line up at equator 23 chromosomes line up at equator equator Anaphase AI AII kinetochores split & Tetrads split & kinetochores split, sister 46 chromosomes migrate to each 23 replicated chromosomes migrate chromatids separate and pole to each pole 23 chromosomes migrate after cytokinesis after cytokinesis after cytokinesis = interphase 2 haploid cells = gametes 2 identical diploid cells 2 nearly identical 4 haploid cells 23 pairs of chromosomes in copies of each of 23 1 copy of each of 23 each = 46 in total chromosomes with sister chromatids chromosomes E) Why Meiosis? — Need to turn diploid germ cells into haploid gametes (ova/sperm) — 23 Chromosomes from haploid (n) sperm cell combine with 23 chromosomes from haploid ovum (n) to form a single 46 chromosome cell called a zygote (2n) 4