# How many chromosomes do humans have?

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How many chromosomes do humans have?
Warm-up # /23/15 Draw a chromosome and label the following parts: chromosome, sister chromatid, centromere How many chromosomes do humans have?

Human Chromosomes Humans have 23 pairs of chromosomes (46 total)
22 pairs of autosomes 1 pair of sex chromosomes Karyotype – A picture of all the chromosomes in a somatic cell arranged by size

One chromosome in the pair came from the male parent and one came from the female parent.
Homologous chromosomes – each pair has genes for the same traits

How can 2 cells join without doubling the amount of chromosomes?
Meiosis a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell

Meiosis: Where and Who? For humans meiosis occurs in the ovaries and testes The process of meiosis produces egg and sperm cells gametogenesis (spermatogenesis & oogenesis) Two gametes come together by fertilization

A cell with only one of each kind of chromosome is called haploid (n).
Organisms produce gametes (sex cells) that contain one of each kind of chromosome. A cell with only one of each kind of chromosome is called haploid (n). Sex cells have one of each kind of chromosome so that when they combine the resulting zygote is diploid

Meiosis: Why? Mitosis divides one diploid cell to form two diploid cells For example: A human cell with 46 chromosomes divides to form two cells with 46 chromosomes. If each parent were to pass on a diploid cell to the offspring, that offspring would then have 4 copies of each chromosome 46 chromosomes from each parent would yield a 92 chromosome offspring Meiosis allows for two divisions to divide one diploid cell into four haploid cells.

Label the diagram Homologous chromosomes Sister chromatids Centromere

Warm-up # /26/15 How are homologous chromosomes different from sister chromatids? Sister chromatids are identical Homologous chromosomes have the same genes, but may have different version of the gene What makes a cell haploid (n) or diploid (2n)? Haploid – 1 set of chromosomes Diploid – 2 complete sets of chromosomes

Chromosomes Each chromosome can contain thousands of genes
Genes can have different versions  Alleles

Meiosis Phases Meiosis I - homologous chromosomes separate
Prophase I Metaphase I Anaphase I Telophase I Meiosis II - sister chromatids separate (more similar to mitosis) Prophase II Metaphase II Anaphase II Telophase II

Interphase Before Meiosis (just like before Mitosis) the cell must prepare for division: DNA and organelles are replicated During this phase, chromosomes are not yet visible. chromatin

Law of Segregation Each gamete only gets one allele of each gene

Independent Assortment
Mendel's law of independent assortment: Genes for different traits assort independently of each other Organisms inherit two alleles for each trait when gametes are produced

Variation of Traits Crossing over during Prophase I
Exchange of genetic material between homologous chromosomes

Crossing Over

All Different Humans have 223 different combination for gametes!

2 1 4 2 Function Type of cells # of divisions # of daughter cells
Mitosis Meiosis Function Type of cells # of divisions # of daughter cells Results in Haploid/diploid cells? Growth Repair Reproduction Create new cells for the purpose of reproduction Somatic cells (body cells, anything other than gametes) Germ line cells (gametes/sex cells) 2 1 4 2 Haploid Diploid

Warm-up # /27/15 Name & describe 2 events/processes that occur during meiosis that contribute to genetic variation Crossing over – recombination of alleles Independent assortment – genes of different traits assort independently of each other

Warm-up #10 1/28/15 Copy the diagram in your notebook.
Draw the possible gametes if crossing over DOES NOT occur Draw the gametes if crossing over DOES occur What affect does crossing over have on the variation of gametes?

Without Crossing Over

With crossing over

Law of Independent assortment
2n different combinations If we have 3 different chromosomes, how many different combinations are possible?

n=3 23=8

Chromosomal Mutations
Errors can also occur during Meiosis. Homologous chromosomes do not separate properly –nondisjunction Gametes with either an extra copy of a chromosome or no copy Normal Example Nondisjunction Examples

Identifying Chromosomal Disorders
Karyotype Photograph is taken of the paired chromosomes during metaphase Arranged according to length Easy to see if there are any extra or missing chromosomes This individual has an extra Y chromosome

Monosomy Zygote gets only one chromosome  missing one chromosome
Most zygotes with monosomy do not survive

Trisomy One copy of a chromosome from one parent and two copies from the other parent three copies

Downs Syndrome (Trisomy 21)
3 copies of the 21st chromosome mental retardation, susceptibility to certain illness or diseases, and a shorter life span

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