Eukaryotic Chromosomes
The DNA in a eukaryotic cell nucleus is organized as one or more chromosomes that differ in length and shape
• During most of the cell’s life, each chromosome consists of one DNA strand (and associated proteins).
• When the cell prepares to divide, it duplicates all of its chromosomes, so that both offspring get a full set
• Each duplicated chromosome has two DNA strands (sister chromatids) attached to one another at the centromere centromere one chromatid its sister chromatid a chromosome
(unduplicated)

a chromosome
(duplicated)

Chromosome Structure
• A duplicated, condensed chromosome consists of two long filaments bunched into a characteristic X shape
• Each filament consists of a coil of DNA wrapped around
“spools” of proteins called histones • Each DNA-histone spools is a nucleosome, the smallest unit of chromosomal organization in eukaryotes
• The DNA molecule consists of two strands twisted into a double helix

Stretched out end to end, the
DNA molecules in a human cell would be about 2 meters (6.5 feet) long. That is a lot of DNA to fit into a nucleus that is less than 10 micrometers in diameter! Proteins structurally organize the DNA and help it pack tightly into a small nucleus. DNA molecule

Figure 8-2b p134

Chromosome Number
• The total number of chromosomes in a eukaryotic cell (chromosome number) is characteristic of the species – human body cells have 46 chromosomes • Human body cells have two of each type of chromosome – their chromosome number is diploid (2n)
• A karyotype shows how many chromosomes are in an individual cell, and reveals major structural abnormalities

This human karyotype shows 22 pairs of autosomes and a pair of X chromosomes. Autosomes and Sex Chromosomes
• In a diploid organism, one chromosome in a chromosome pair is inherited from the mother and one from the father
• Most chromosomes are autosomes – pairs of chromosomes with the same length, shape, and centromere location • Pairs of sex chromosomes differ between females and males From
Mom

From
Dad

diploid cell in female diploid cell in male Which one came from dad?

diploid reproductive cell in female

diploid reproductive cell in male

eggs

sperm

XX

XY

XX

XY