Chromosomes

What is a Chromosome?• Chromosomes are

rod-shaped structures made of DNA and proteins

• DNA is a long, thin molecule that stores genetic information

When and how do chromosomes form?

• Chromosomes form during cell division when the DNA is compacted

• DNA wraps tightly around proteins called histones, which maintain the shape of the chromosome and assist in the tight packing of DNA.

DNA

Histone

Chromosomes• Each half of a duplicated

chromosome is identical to the other, and each half is called a chromatid.– The constricted area of each

chromatid is the centromere, which holds the 2 chromatids together until separation during cell division.

– The centromere is important in moving the chromosomes during cell division

– The ends of each chromosome has a similar sequence those ends are called telomeres

Duplicated chromosome

½ of duplicated chromosome =

chromatid

Prokaryotes

• Chromosomes are much simpler • DNA is made of only 1 chromosome, which is

circular-shaped• Very compact in order to fit into the cell.

Eukaryotes

• Each species has a characteristic chromosome number in each cell.

• For Example:

Gorilla48

Cat32

Dog78

Fern1,262

Human46

2 Major Types of Chromosomes1. Sex Chromosomes determine the gender of the

organismi. For humans they are X or Y:

a. Females =b. Males =

ii. 2 of the 46 chromosomes are this type in humans

2. Autosomes consist of all the other chromosomes in an organism.

i. 44 of the 46 are this type in humansii. The 44 = 2 copies of the same 22 chromosomes; 1 came

from the mom and 1 came from the dad. Therefore 22 autosomes came from mom and 22 came from dad.

iii. Two copies of each autosome are called homologous chromosomes or homologues. They are the same size and shape and carry the genes for the same traits.

XXXY

Homologous Chromosomes or Homologues Two Copies of the

same chromosome

• Homologues do not have identical coding but have the same genes that code for the same traits

• For Example: This gene might code for brown eyes (the trait being eye color)

Where as this gene might code for blue eyes

KaryotypeA Karyotype is a visual representation in an

organized form of chromosomes in an organism.

Arranged by size of autosomes from

longest to shortest, leaving the sex

chromosomes last

Chromosomes• When a cell has two sets of chromosomes, it is

diploid, which means the cell has both chromosomes for each homologous pair– Ex. All normal body cells (somatic cells) are diploid.– Commonly abbreviated as 2n.– For humans, the diploid number 46.

• When a cell only has one set of the chromosomes, it is haploid, which means the cell only has one copy of each chromosome for each homologous pair.– Ex. Sperm and Egg cells (Gametes / Sex Cells)– Commonly abbreviated as n.– For humans, the haploid number is 23.

Organism Diploid (2n) Haploid (n)HumanFruit Fly

Garden Pea?????

46

82017

23

410

Will Not Happen

The Diploid number of chromosomes must be _____, but the haploid number can be _____ or ____

(For a normal healthy organism barring chromosomal disorders)

EVENEVEN ODD

Why do the reproductive cells need to be haploid and not diploid?

Using Humans as the example if you had mom give 46 chromosomes and dad give 46 chromosomes then the baby would have 92 chromosomes which is not the correct number of chromosomes for a human. Therefore the reproductive cells must be half the number of the normal cells.

46 46

92 46

23 23