Chromosomes Lecture #18 Honors Biology Ms. Day

Why is Cell Division Important?  Unicellular organisms Reproduce by cell division  increasing the population. 100 µm (a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM). Figure 12.2 A

Why Do Multicellular Organisms Depend on Cell Division? Development & Growth Repair (ex: tissue renewal) Maintenance 20 µm200 µm (b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM). (c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).

Cell Division (called Mitosis) identical  Makes 2 genetically identical daughter cells from 1 parent cell (think= mi “two” sis)  Before cells divide They duplicate their DNA  so each daughter cell receives an exact copy of the genetic material (DNA)

Organization of the Genetic Material  All of a cell’s DNA (genetic information) is called its genome In prokaryotes In prokaryotes  Genome=single, long DNA molecule in a circle In eukaryotes In eukaryotes  Genome = several DNA molecules grouped in clumps (called chromosomes )

DNA Molecules  DNA can be in 2 forms n Chromatin : DNA is n ot t ightly packed together (loosely coiled; decondensed)  Occurs during interphase  Think = chromatin Chromosomes : tightly packed together (TIGHTLY coiled; condensed)  Occurs during mitosis  Think = chromosomes

How Does DNA Fit into a Small Cell?  Prokaryotic organisms No nucleus; instead have a nucleiod region  DNA = 1 chromosome

How Does DNA Fit into a Small Cell?  Eukaryotic organisms split their DNA into several pieces (called chromosomes) stored within the cell's nucleus histones  DNA is coiled around proteins known as histones nucleosomes Uses nucleosomes to form a “supercoil”

DNA + histones  form nucleosomes (function = help to “super coil” DNA)

Making Chromosomes before mitosis **Occurs right before cell division (mitosis) Steps: DNA Replication 1. DNA copies itself through DNA Replication histone 2. The DNA coils (wraps) around proteins called histone proteins. 3. Chromosomes are made.

Chromosome Structure Two shapes of chromosomes 1.Single chromosome (1 copy of DNA); “V” shaped 2.Double Chromosome (2 copies of DNA); “X” shaped; a.k.a-duplicated chromosome Occurs right before cell division (mitosis)

Chromosomes and Cell Division  In preparation for cell division (mitosis) DNA is replicated (single  double chromosome) V shape  X shape

Each duplicated chromosome has two sister chromatids, which separate during cell division 0.5 µm Chromosome duplication (including DNA synthesis) Centromere Separation of sister chromatids Sister chromatids Centromeres Sister chromatids A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule. Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the DNA molecule. Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells. Figure 12.4

Double Chromosome Structure Kinetochore attaches to spindle fibers Sister

Draw and Label the Pieces of a Duplicated Chromosome

Chromosomes characteristic, unique  Every eukaryotic species has a characteristic, unique # of chromosomes in EACH cell nucleus Ex: Humans = 46 chromosomes Adler Fern = over 2000 C’s  # of chromosomes does NOT necessarily equal complexity

Different Types of Cells  There are 2 types of cells in an organism 1. Somatic cells 1. Somatic cells: all body cells Gametes 2. Gametes: reproductive cells (sperm and egg)

Different cell types  different cell divisions  Eukaryotic cell division consists of Mitosis = division of the nucleus Mitosis = division of the nucleus Called karyokinesis Called karyokinesis Cytokinesis = division of cytoplasm Cytokinesis = division of cytoplasm Occurs in somatic cells Occurs in somatic cells  In meiosis Gamete cells are produced after chromosome number is cut in ½ Gamete cells are produced after chromosome number is cut in ½

Chromosome Number  Gametes (sperm/egg) have 1 copy of each chromosome haploid (1n)  called haploid (1n)  Somatic  Somatic (body) have 2 copies of each chromosomes diploid (2n)  called diploid (2n)

Haploid vs. Diploid  Haploid= when a cell has 1 copy of each chromosome.  Expressed as n = 23  Diploid= when a cell has 2 copy of each chromosome. (a.k.a TOTAL # of C’s in organisms)  Expressed as 2n = 46 Haploid egg cell Haploid sperm cell n n 2n Diploid Zygote cell Fertilization

Pairs of Chromosomes  Chromosomes have a “twin” (matching pair) PAIRS Ex: Human somatic cells have 23 PAIRS of chromosomes (total 46 C’s) All pairs of chromosomes differ in size, shapes, and set of genes. Homologous Chromosomes Homologous Chromosomes = chromosomes that are similar in shape, size, and genes.

Set of Chromosomes  Each chromosome set contain 2 “homologues” or “twins”  Each homologue is also called an autosome  A chromosome that does NOT determine gender  Each homologue in a pair comes from each parent!  Also have a “ sex chromosome ” pair  Chromosomes that DO determine gender fertilization ZYGOTE = 1 st diploid body cell of organism