Biology 12: Genetics CHROMOSOMES!

CELL REVIEW All living things are made up of cells (skin, muscle, blood, nerve) Work like a factory to keep us functioning Major parts of a human cell Nucleus: Contains genetic information

What is a chromosome? Chromosomes are the rod- shaped, filamentous bodies present in the nucleus. They become visible during cell division. They are the carriers of the gene or unit of heredity. (Chromo = colour; Soma = body) due to their marked affinity for basic dyes.

What is a chromosome? Spool and Thread – What does the thread represent? – What does the “spool” represent?

What is a chromosome? Reflecting on our spool and thread…. – When DNA (thread) is being transferred, it must be “wound up” – When it’s being used, it’s no longer “wound up” Think about when you travel… – Suitcase vs. pile of clothes

Coiling Think about storing a garden hose: – Easier if you coil it, but it requires work/energy – In the nuclues, proteins provide the energy to coil the DNA Video- We will pause to allow you to take notes Video- We will pause to allow you to take notes

Components of Chromosomes Histones- proteins that package and order DNA (the “spools” of in-class example) Nucleosomes- a segment of DNA wrapped around 8 histones (the “thread” of in-class example)

Chromatin: “Beads on a string” – Primary functions of chromatin: Prevent DNA damage Organize DNA into smaller size so that it fits into cell’s nucleus Supports DNA and strengthen it for mitosis Control DNA replication and gene expression Solenoid: the coiled fiber of chromatin

What is chromatin? Combination of DNA and proteins within the nucleus Primary functions of chromatin: – Prevent DNA damage – Organize DNA into smaller size so that it fits into cell’s nucleus – Supports DNA and strengthen it for mitosis – Control DNA replication and gene expression Chromatin is only found in eukaryotic cells – Eukaryote (true nucleus) vs prokaryote (unicellular)

Labeling a Chromosome

Parts of the chromosome Chromatid: one copy of a duplicated chromosome Centromere: Holds the two identical sister chromatids together (middle of chromosome) – When chromosomes are stained, they’ll be darker at centromere – The centromere is an important component of chromosome structure and segregation (cell division)

Kinetochore: protein structure on chromatids where the spindle fibers attach during cell division to pull the sister chromatids apart Microtubules: what the spindle fibers are made of Telomere: a region of organic molecules (nucleotides) at each end of a chromatid – Protects the end of the chromosome & helps maintain its structure

“Ploidiness” Ploidy refers to the number of sets of chromosomes in the nucleus of a cell Euploidy: having chromosomes present in an exact multiple of the haploid number Gametes (sperm or egg) normally contain only one set of chromosome – this number is called Haploid (n) Somatic cells usually contain two sets of chromosome - Diploid number (2n) 3n – triploid 4n – tetraploid A cell/organism that has more than two paired sets of chromosomes are Polyploidy – Many plants are polyploidy

“Ploidiness” A change in the chromosome number that does not involve entire sets of chromosomes, but only a few of the chromosomes - is Aneuploidy. -Usually leads to an abnormal condition Monosomics (2n-1) Trisomics (2n+1) Nullisomics (2n-2) Tetrasomics (2n+2)

“Ploidiness” THE NUMBER OF CHROMOSOMES IS NOT RELATED TO COMPLEXITY OF THE ORGANISM!

Review what we have learned so far: Interactive video

Why do ferns have more chromosomes than us?? Let’s let Hank tell us!

Chromosome Size In contrast to other cell organelles, the size of chromosomes shows a remarkable variation depending upon the stages of cell division. Interphase: chromosome are longest & thinnest Prophase: there is a progressive decrease in their length accompanied with an increase in thickness Anaphase: chromosomes are smallest. Metaphase: Chromosomes are the most easily observed and studied during metaphase when they are very thick, quite short and well spread in the cell. Therefore, chromosomes measurements are generally taken during mitotic metaphase.

Chromosomes may differ in the position of the Centromere, the place on the chromosome where spindle fibers are attached during cell division. In general, if the centromere is near the middle, the chromosome is metacentric If the centromere is toward one end, the chromosome is acrocentric or submetacentric If the centromere is very near the end, the chromosome is telocentric.

The centromere divides the chromosome into two arms, so that, for example, an acrocentric chromosome has one short and one long arm, While, a metacentric chromosome has arms of equal length. All house mouse chromosomes are telocentric, while human chromosomes include both metacentric and acrocentric, but no telocentric.

Autosomal pair Sex chromosome Diploid No. of No. of X Y (2n) metacentrics acrocentric or telocentric Cat M M Dog M A Pig M M Goat A M Sheep A M Cow M M Horse M A M – Metacentric; A – Acrocentric

Bill Nye Cell Cycle