8-1 Chromosomes The DNA in a human cell is estimated to consist of 6 billion pairs of nucleotides! Chromosomes are rod-shaped structures made of DNA and proteins. Histones: DNA in eukaryotic cells wraps tightly around these proteins, which help pack DNA & maintain the shape of chromosomes. Non-histone proteins: control the activity of specific regions of DNA.

As cells prepare to divide, DNA coils around proteins (histones) and twists into rod-shaped chromosomes. *

Chromosomes consist of two identical halves. Each half is a chromatid, which are constricted by a centromere. Chromatids form as the DNA makes a copy of itself before cell division. When cells divide, each of the two new cells receives one chromatid from each chromosome. The constricted area is a centromere, which holds the chromatids together until they separate during cell division.

Sex Chromosomes and Autosomes Human & animal chromosomes are either sex chromosomes or autosomes. Sex chromosomes (2 out of 46) determine the sex of an organism, and they may also carry genes for other characteristics. Karyotype

Chromosome Numbers Cells produced by sexual reproduction have two copies of each autosome, one from each parent. These two copies are homologous chromosomes. They are the same size and shape and carry genes for the same traits (like eye color). See pg. 146 in text for a bigger table.

Diploid and Haploid Cells Cells having two sets of chromosomes are said to be diploid. Diploid cells also have two sex chromosomes in animals. All normal human cells , except reproductive cells, are diploid, or 2n. In humans, the diploid, or 2n is 46 (22 pairs) and 2 sex chromosomes. Human sperm and egg cells are haploid cells. Haploid cells contain only one set of chromosomes, and are abbreviated as 1n.

What are homologous chromosomes? Section 8-1 Review What are homologous chromosomes? Describe the differences between a chromo-some and a DNA molecule. Compare the structure of prokaryotic chromo-somes with that of eukaryotic chromosomes. Contrast sex chromosomes with autosomes. Using Table 8-1, list the haploid and diploid number of chromosomes for each organism.

Attachments Chromatin__Chromosomes__and_DNA_Subunits.asf