Warm Up Use the following words in 2-3 sentences to demonstrate what you remember about Mitosis: division, chromosomes, copying, cells, PMAT

Cell Division – Mitosis (Review) – Division of a somatic (body) cell that results in 2 genetically identical daughter cells Cells must divide for growth, repair of tissues, and asexual reproduction Cell division begins in Interphase when the chromosomes duplicate

Parent cell Chromosomes duplicate 2 new daughter cells identical to parent cell Daughter cells are genetically identical to parent cell – same kind and number of chromosomes Mitosis occurs in somatic or body cells (ex: liver, heart) Every organism has its own unique number of chromosomes. Humans have 46. This is called its diploid number or the total number of chromosomes in a somatic (body) cell. Diploid means “2 sets” and is written as “2N”.

Body cells of adult organisms have 2 sets of homologous (matching) chromosomes – 1 set from female parent and 1 set from male parent

Cell Division –Meiosis – the process in which the number of chromosomes in the original cell is reduced by HALF through the separation of homologous chromosomes Meiosis occurs in sex organs only Males (XY) – sex organs are the testes in humans Females (XX) – sex organs are the ovaries in humans

Parent cell Chromosomes duplicate Division 1 Division 2 Daughter cells have half as many chromosomes as parent cell

Meiosis produces sex cells – cells with ½ the number of chromosomes as the original cell Males – meiosis produces 4 sperm Females – meiosis produces 1 “good” egg The other 3 cells are called polar bodies – they give up their cytoplasm to nourish the 1 good egg. Egg and sperm (sex cells) are also called gametes

Gametes have ½ the number of chromosomes as somatic (body) cells. We call this the haploid number. Haploid means “1 set” and is written as “N”. If human diploid number is 46, what is its haploid number? 23 Diploid # of a dog – 78Haploid # of a dog – 39 Diploid # of a fly – 8Haploid # of a fly – 4

When does meiosis occur in humans? 1.Males beginning at puberty 2.Females before birth – all eggs are produced before birth and at puberty eggs mature

23 46 Zygote develops into embryo and finally adult organism by mitosis Fertilization – process by which an egg and sperm unite Zygote – fertilized egg Embryo – organism in early stage of development Fertilized egg – zygote

Unique events in Meiosis Homologous (matching) chromosomes pair up before 1 st cell division Homologous chromosomes: -look alike -code for same traits -receive one from each parent

During 1 st division, homologous chromosomes exchange genes during process called “crossing over” Crossing over increases genetic variation and is the reason why siblings look different

No crossing over – daughter cells are identical to parent cells Crossing over occurs –causes genetic variation (Daughter cells are NOT identical to parent cell)