Chromosomes PART 1 Honors Genetics Ms. Gaynor

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 (Mitosis) identical  Makes 2 genetically identical daughter cells from 1 parent cell  Before cells divide They duplicate their genetic material  ensures that 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 (in nucleus of eukaryotes) can be in 2 forms n Chromatin : DNA is n ot t ightly packed together (loosely coiled)  Occurs during interphase Chromosomes : tightly packed together (TIGHTLY coiled)  Occurs during mitosis (cell division)

Tightly coiled? Not tightly coiled?

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 (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.

The DNA molecules in a cell Are packaged into chromosomes 50 µm

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

Another view…

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 have 46 chromosomes  # of chromosomes does NOT necessarily equal complexity

Different Types of Cells  There are 2 types of cells in the human body 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 Cytokinesis,division of cytoplasm Cytokinesis,division of cytoplasm Occurs in somatic cells Occurs in somatic cells  In meiosis Gamete cells are produced after a reduction in chromosome number Gamete cells are produced after a reduction in chromosome number

Chromosome Number haploid.  Gametes (sperm/egg) have 1 copy of each chromosome and are called haploid.  Somatic diploid.  Somatic (body) human cells have 2 copies of each chromosomes and are called diploid. PAIRS Human somatic cells have 23 PAIRS of chromosomes (total 46 chromosomes). All pairs of chromosomes differ in size, shapes, and set of genes.

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

Set of Chromosomes  Each chromosome set contain 2 “homologues.”  Chromosomes have a “twin” or matching pair.  Homologous Chromosomes  Homologous Chromosomes= chromosomes that are similar in shape, size, and genes.  Each homologue in a pair comes from each parent!