Chromosomes

Chromosome Structure When a cell is not dividing the DNA is loose, long, and skinny and is called chromatin. During cell division the DNA in a cell’s nucleus is coiled around proteins to form structures called chromosomes.

Chromosome Structure histones

Chromosome Structure in Prokaryotes Prokaryotic organisms like bacteria contain only one chromosome. This chromosome is always circular.

Chromosome Numbers Each species has a characteristic number of chromosomes. Humans for example have 46 chromosomes (23 pairs).

Sex Chromosomes vs. Autosomes There are 2 kinds of chromosomes: Sex chromosomes Sex chromosomes - the chromosomes that determine the sex of the organism. In humans, the sex chromosomes are the X and Y chromosomes. Humans only have 2 sex chromosomes (1 pair) out of the 46 total chromosomes (23 pairs) we have. Females = XX Males = XY Autosomes Autosomes - All other chromosomes that are not sex chromosomes. Humans usually have 44 autosomes (22 pairs).

Homologous Chromosomes Every cell has 2 copies of each autosome. One copy comes from each parent. These pairs are referred to as homologous chromosomes or homologues. (homologue means “same size”)

Boy or Girl??? Boy Karyotypes A karyotype is a picture of the chromosomes in a cell. You can see that in this karyotype there are 22 pairs of autosomes and 1 pair of sex chromosomes. Homologous Pairs Boy or Girl??? Boy

The Y Chromosome Decides Boy or Girl? The Y Chromosome Decides Y - Chromosome X - Chromosome

Most Cells are Diploid In humans, all cells except sex cells are called diploid because there are two copies of each chromosome. The abbreviation for diploid is 2n (= 2 sets of chromosomes in homologous pairs)

Haploid Cells Sex cells - sperm and egg cells All sex cells are called haploid because they have 1 copy of each chromosome; Chromosomes are not in homologous pairs. The abbreviation for haploid is 1n (= 1 set of chromosomes not in homologous pairs)

Fertilization Results In A Diploid Zygote In humans, all sex cells are haploid cells. When sperm (1n) and egg (1n) combine, they create a fertilized egg called a zygote which is a diploid (2n) cell (and will eventually grow into a baby through cell division!)

Fertilization Results In A Diploid Zygote Egg 1n Haploid nucleus Sperm 1n Haploid nucleus

Fertilization Results In A Diploid Zygote Egg 1n Haploid nucleus Sperm 1n Haploid nucleus

Fertilization Results In A Diploid Zygote Egg 1n Haploid nucleus Haploid nucleus Sperm 1n

Fertilization Results In A Diploid Zygote Egg 1n Haploid nucleus Haploid nucleus Sperm 1n

Zygote 2n Zygote 2n