Chapter 11: Cell Cycle & Division

Cell division importance

Growth, repair, and reproduction (asexual = clones, sexual = gametes fuse to form zygote)

Four events of cell division

Signal to divide; DNA replication; DNA segregation; Cytokinesis

Binary fission

Process in prokaryotes where one cell divides into two identical cells

Steps of binary fission

Signal → DNA replication → DNA segregation → Cytokinesis

Chromatin

DNA + histone proteins in a loose, accessible form (interphase)

Histones

Positively charged proteins (lysine-rich) that DNA wraps around

DNA structure

Double helix with antiparallel strands held by hydrogen bonds

DNA bases

Adenine, Thymine, Cytosine, Guanine (A, T, C, G)

DNA length in humans

~3.2 billion base pairs; ~2 meters long if stretched

Nucleosome

DNA wrapped around histones ("beads on a string")

Chromosome

Highly condensed chromatin (not accessible for transcription)

DNA packaging levels

DNA → nucleosome → chromatin fiber → loop domains → condensed chromosome

Epigenetics

Regulation of gene expression without changing DNA sequence

HATs (Histone Acetyltransferases)

Open chromatin → increase gene expression

HDACs (Histone Deacetylases)

Close chromatin → decrease gene expression

Cell cycle

G1 → S → G2 → M (mitosis)

G1 phase

Cell growth and normal function

S phase

DNA replication

G2 phase

Preparation for mitosis

M phase

Mitosis + cytokinesis

G0 phase

Non-dividing state (cells exit cycle)

Diploid (2n)

Two copies of each chromosome

Haploid (n)

One copy of each chromosome (gametes)

Human chromosome number

46 chromosomes (23 pairs)

Autosomes

22 non-sex chromosomes

Sex chromosomes

X and Y chromosomes

Centromere

Region where sister chromatids are joined

Kinetochore

Protein structure on centromere where spindle fibers attach

Centrosome

Microtubule organizing center (spindle poles)

Spindle fibers

Microtubules that move chromosomes during division

Mitosis

Division of nucleus producing two identical diploid cells

Purpose of mitosis

Growth, repair, and maintenance

Prophase (mitosis)

Chromosomes condense; spindle forms

Prometaphase

Nuclear envelope breaks; microtubules attach to kinetochores

Metaphase

Chromosomes align at the cell equator

Anaphase

Sister chromatids separate and move to opposite poles

Telophase

Nuclei reform; chromosomes decondense

Cytokinesis

Division of cytoplasm into two cells

Cohesin

Protein that holds sister chromatids together

Cyclin

Regulatory protein that fluctuates during the cell cycle

CDK (Cyclin-dependent kinase)

Enzyme always present; activated by cyclin to drive cell cycle

Cyclin-CDK complex

Active complex that phosphorylates proteins to progress cell cycle

Cell cycle checkpoints

G1 (DNA damage), S (replication issues), G2 (DNA complete), M (spindle attachment)

Meiosis

Cell division that produces four haploid, genetically different cells

Purpose of meiosis

Produce gametes and increase genetic variation

Meiosis I

Separation of homologous chromosomes (reduction division)

Meiosis II

Separation of sister chromatids (similar to mitosis)

Synapsis

Pairing of homologous chromosomes in prophase I

Tetrad

Four chromatids (paired homologous chromosomes)

Crossing over

Exchange of DNA between homologous chromosomes (prophase I)

Chiasma

Site where crossing over occurs

Independent assortment

Random alignment of homologous chromosomes (metaphase I)

Segregation

Separation of chromosomes into different gametes

Genetic variation sources

Crossing over, independent assortment, and random fertilization

Number of possible gametes

2^23 combinations (~8.4 million)

Spermatogenesis

Production of 4 equal haploid sperm cells

Oogenesis

Production of 1 egg + 3 polar bodies (unequal division)

Polar bodies

Nonfunctional cells produced during oogenesis

Nondisjunction

Failure of chromosomes to separate properly

Nondisjunction result

Aneuploidy (abnormal chromosome number)

Aneuploidy

Too many or too few chromosomes

Causes of nondisjunction

Cohesin failure or kinetochore attachment issues

Geriatric pregnancy

Pregnancy at age 35+ with higher risk of nondisjunction

Trisomy 21

Three copies of chromosome 21 (Down syndrome)

Down syndrome effects

Developmental delay, epicanthal folds, short stature, heart defects

Down syndrome frequency

~1 in 700 births; higher with maternal age

Translocation

Chromosome segment attaches to a different chromosome

Reciprocal translocation

Exchange of segments between two chromosomes

Philadelphia chromosome

Translocation between chromosomes 9 and 22

BCR-ABL

Fusion gene formed from translocation → causes cancer

Insertion mutation

Addition of extra DNA segment

Deletion mutation

Loss of DNA segment

Karyotype

Visual display of chromosomes arranged by size

FISH (Fluorescence in situ hybridization)

Technique to visualize chromosomes using fluorescent probes

Chromosome arms

p arm (short arm) and q arm (long arm)

Prophase I importance

Only phase where crossing over occurs

Difference: mitosis vs meiosis

Mitosis = identical cells; Meiosis = genetic variation + haploid cells

Somatic cells

Body cells (diploid)

Germ line cells

Cells that produce gametes (haploid)

Zygote

Diploid cell formed from fertilization

Fertilization

Fusion of two haploid gametes to form diploid zygote

Differentiation

Process where cells become specialized despite same DNA

Why different cells exist

Different gene expression (epigenetics)

Condensed chromosomes

Not accessible for transcription

Interphase chromatin

Accessible for transcription and replication