Chromosomes Chromosomes A chromosome is a rodshaped structure

Chromosomes

Chromosomes • A chromosome is a rod-shaped structure made of DNA and proteins • In eukaryotes, DNA wraps around proteins called histones to help maintain the compact structure of chromosomes

Chromatids • Chromosomes consist of two halves, called chromatids • Chromatids form when DNA makes a copy of itself before a cell divides • When the cell divides, each new cell gets one chromatid from each chromosome. • The 2 chromatids are attached at one point, called a centromere • When cell division is not occurring, DNA is lesstightly coiled and forms a structure called chromatin

Chromosome Numbers • Each species has a characteristic number of chromosomes in each cell

Chromosome Types • Animal chromosomes are classified as either sex chromosomes or autosomes • Autosomes: all the chromosomes that determine characteristics • A human cell produced by sexual reproduction has 22 pairs of autosomes. • Each pair of autosomes are called homologous pairs • Same size and same shape • Traits for genes in same location

Sex Chromosomes • Sex chromosomes: determine the sex of an organism

Human Chromosomes • Humans have 46 chromosomes • 22 pairs of autosomes • 1 pair of sex chromosomes • They are X and Y – females usually are XX, males are usually XY • 23 total pairs of chromosomes

Karyotype • A karyotype is a picture of the chromosomes in a normal, dividing, human cell

Diploid • Diploid: describes cells with two sets of chromosomes • They have two autosomes for each homologous pair • 1 from mom and 1 from dad • In humans, all non-reproductive cells are diploid • Diploid abbreviated as 2 n (‘di’ = 2)

Haploid • Haploid: describes cells with only 1 set of chromosomes • These are sperm and egg (gametes) cells • They have half the chromosomes of diploids • Haploid cells have only 1 autosome of a homologous pair and 1 sex chromosome • 23 total chromosomes • Come either from mom OR dad • Haploid is usually abbreviated as 1 n (‘hap’ = half or 1).

Meeting of Sperm and Egg • When a sperm cell (1 n) combines with an egg cell (1 n), they create a new organism, whose cell will be diploid (2 n). • If reproduction occurred between 2 diploids, the new cell would have too many chromosomes and not be functional.