chromosomes

• Chromosomes are made of chromatin and chromatin consists of DNA and protein
• Chromosome packaging


  1. DNA wound around histones to form nucleosomes

  2. More condensing packaging levels

  3. Results in chromosome level
• Chromosome


  Highly intense packaging of DNA to fit inside the nucleus and enable cell division
• DNA needs to be unwound during DNA replication
• Human chromosomes


  • Can be a single component or replicated
  • Newly made chromosome is bound at the centromere region
• In humans, there are 46 chromosomes that get duplicated during interphase, resulting in 92 chromatids
• During mitosis, the 92 chromatids get pulled apart so each cell has 46 chromosomes
• Gene expression is regulated, so not all genes are active in every cell type
• Karyotype


  Image of all 46 chromosomes, typically from a cell in metaphase
• In a karyotype, the chromosomes are arranged in homologous pairs
• Chromosome types


  • Autosomes (22 pairs)
  • Sex chromosomes (1 pair)
• Autosomes

  Chromosomes not related to biological sex
• Sex chromosomes


  Determine biological sex
• Females have two X chromosomes, males have one X and one Y chromosome
• Gametes (sperm and egg) are haploid, with half the number of chromosomes as body cells
• The fertilized egg (zygote) is diploid, with two sets of chromosomes
• Fathers determine the biological sex of the offspring
• Karyotypes can reveal genetic disorders related to missing or extra chromosomes
• The field of genetics continues to expand, including careers like genetic counseling
• Closed captioning is on. To turn off, click CC button at bottom right. Follow the amoebas on Facebook and Twitter (@AmoebaSisters)!
• If you have ever studied mitosis or meiosis, chances are, the whole chromosome number thing, it's crossed your mind.
• How do you start out with a cell with 46 chromosomes, And then it does mitosis, and then you end with two cells that each have 46 chromosomes? Or in meiosis you end op with 4 cells that each have 23 chromosomes.
• "Here's a chromosome, there's a chromosome, everywhere a chromosome chromosome…" but how?
• This video is going to explain the chromosome numbers in these processes, but please watch our mitosis and meiosis videos first, so that you can understand the actual process first.
• And also out videos are about chromosome numbers in human cells, so please assume that for the entire video.
• Human body cells have 46 chromosomes, and human sex cells, also called gametes, have 23 chromosomes.
• Chromosomes


  Made up of DNA and protein, highly condensed to make DNA portable and organized
• Chromosome


  • Has one chromatid or copy
  • Has two chromatids or copies
• Centromere


  The area in the middle of the chromosomes where the spindle attaches
• Interphase


  1. Chromosomes are duplicated

  2. Start with 46 chromosomes

  3. End up with 46 chromosomes and 92 chromatids
• At anaphase in mitosis, the sister chromatids separate and you could argue there are 92 chromosomes, but they are still referred to as chromatids during cell division.
• At the end of mitosis, the chromatids separate and you end up with 2 cells that each have 46 chromosomes.
• The two daughter cells in mitosis are identical to the starting cell.
• Meiosis


  1. Start with 46 chromosomes and 46 chromatids

  2. End meiosis I with 2 cells, each with 23 chromosomes and 46 chromatids

  3. End meiosis II with 4 cells, each with 23 chromosomes and 23 chromatids
• In mitosis, the daughter cells are identical to the original cell with 46 chromosomes.
• In meiosis, the daughter cells have half the number of chromosomes (23) compared to the original cell (46).
• Chromosome counting depends on the definition of chromatid.