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Meiosis - Chapter 10.1 and 11.3 & Chromosomes Chromosome Structure •DNA coils around histones (proteins) to form nucleosomes, •This coils to form chromatin fibers. •The chromatin fibers supercoil to form chromosomes that are visible in the metaphase stage of mitosis. https://www.youtube.com/watch?v=tsVHWbXqum8&t=249s How many chromosomes do we have in our cells? ▪ Human body cells have 46 chromosomes ▪ Each parent contributes 23 chromosomes ▪ Homologous chromosomes - a pair of chromosomes that have the same genes Homologous Chromosomes -Same length •-Same centromere position (middle) •-One maternal and one paternal chromosome Homologous Chromosomes ▪ Carry the same genes that control the same inherited traits ▪ Alleles are the variations of one gene –Ex: eye colors can brown, blue, green, hazel, etc…. ▪ You inherit one chromosome from each parent! DO NOW Label the following structures: Pairs #1-23 23 pairs = 46 total chromosomes These are your chromosomes! 1 chromosome in each pair comes from your mother and father! Unduplicated homologous chromosomes Duplicated homologous chromosomes Karyotyping • A micrograph that shows pairs of homologous chromosomes in decreasing size ● 46 chromosomes, 23 pairs ○ Pairs #1 - 22 are autosomes ○ Pair #23 are the sex chromosomes ■ XX or XY Karyotype Chromosomal Mutations nondisjunction Monosomy – Results in a sex cell with one less chromosome Trisomy – Results in a sex chromosome with one extra chromosome Do Now •1. How many autosomes? •2. How many sex chromosomes? •3. What is the sex of this patient? •4. Monosomy, Trisomy? •5. Is this individual normal? Before we start genetics, let's review the process of what makes us all look different compared to our siblings, family, and classmates! Sexual Reproduction Sexual reproduction creates genetic variability. That is why we all look different and you look different from your siblings! Define the following words… • Meiosis: – Sexual reproduction-production of gametes • Gamete: – Sex Cells (egg = females & sperm = males) – contain half the number of chromosomes (23) • Somatic Cells: – Body Cells – Contain 46 chromsomes • Haploid: – Half the chromosome number (n) – 1 of each pair of chromosomes (23) – Gametes are haploid • Diploid: – 2 of each type of chromosome (2n) – Somatic cells are diploid • Fertilization: – Sperm enters egg • Zygote: – Fertilized egg (diploid) Think - Pair - Share Can you apply terms learned on the past few slides to this picture? Meiosis • The sexual life cycle in animals involves meiosis • Meiosis produces gametes (23 chromosomes). • When gametes combine in fertilization, that creates a zygote. • A zygote has 46 chromosomes Meiosis • 2 Divisions Meiosis I - separation of homologous chromosomes Meiosis II - separation of sister chromatids DNA Replication before Meiosis 1 Before Meiosis begins, all DNA must be replicated Meiosis 1 1. Crossing over occurs 1. Homologous chromosomes separate, randomly! 46 Duplicated 23 duplicated Crossing over = the exchange of genes between homologous chromosomes, resulting in a mixture of parental characteristics in offspring. Crossing over • The same genes “switch”, which allows for unique gametes to form, different from the exact chromosomes given from the mother and father. • Occurs during Meiosis 1, Prophase 1 Meiosis 2 1. Sister chromatids separate into 4 different gametes, randomly. Each gamete has a unique combination of genes! 23 duplicated 23 unduplicated chromosomes in each gamete Meiosis 1 & 2 - Females (Eggs) and Males (Sperm) What creates genetic variability? 1. Crossing over - switching of genes during meiosis 1 creates a unique combination of genes 1. Independent Assortment - chromosomes separate into gametes randomly during meiosis 1 and meiosis 2 1. Random Fertilization - random sperm and egg combine to make a unique gamete