Cell Division – Meiosis Chromosomes are DNA! Chromosomes contain genetic information

Cell Division – Mitosis (Review) – Division of a somatic 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

Mitosis occurs in somatic or body cells Chromosomes duplicate Parent cell 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, skin, stomach 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 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 Meiosis also occurs in the sex organs of other animals, plants, fungi, etc…

Meiosis Mitosis

Meiosis Division 2 Division 1 Chromosomes duplicate Daughter cells have half as many chromosomes as parent cell 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 (viable) 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 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 – 78 Haploid # of a dog – 39 Diploid # of a fly – 8 Haploid # of a fly – 4

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

Chromosome Number Remember, chromosome number is unique to each kind of organism and all cells (except sex cells) in an organism have the same kind and number of chromosomes. Ex: All humans have 46 chromosomes and all cells in the human body (except sperm and egg) have 46 chromosomes. This is why the chromosome number in sex cells must be reduced in half by meiosis Ex: Humans have 46 chromosomes in their somatic cells, but 23 chromosomes in their sex cells (egg and sperm)

WRONG!!!

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

Fertilized egg – zygote Without meiosis ………… Fertilized egg – zygote 46 92

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

During 1st division, homologous chromosomes exchange genes during process called “crossing over” These homologous chromosomes separate during 2nd division of meiosis – so chromosomes in gametes are different from each other due to 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)

Comparing Mitosis and Meiosis

Female (XX) = before birth # of Divisions (Draw picture) 1 2 Mitosis Meiosis What kind of cells? Somatic cells Male (XY) = Sperm Female (XX) = Egg When does this occur? Any time Male (XY) = puberty Female (XX) = before birth # of Divisions (Draw picture) 1 2 # of Daughter cells Male (XY) = 4 sperm Female (XX) = 1 viable egg # of Chromosomes Same as parent cell diploid or 2N In humans 46 Half as many as parent cell haploid or N In humans 23 Type of Reproduction Asexual Sexual Genetic Composition Daughter cells identical / not identical to parent cell Genetic variation Pairing of Homologous Chromosomes YES / NO Crossing over of genes Function/Importance Growth, repair; development of adult from zygote Production of gametes: eggs and sperm Sex Cells