3.9 - 3.16 Reproduction and cell division

• Chromsomes

  • every cell excepts gametes has two of each chromosome one from the mom and one from the dad and it is known as a homologous pair
  • a locus is the position of a gene on a chromosome
  • homologous chromosomes have the same genes and loci, however the alleles possesed is different on each chromosome
• Chromatid - when dna replicates it forms chromosomes made of two identical sister chromatids containing the same copy of genes for that chromosome
• Gamete - haploid sex cell
• Zygote - the diploid cell formed when two gametes fuse
• Haploid - a cell containing half the usual amount of DNA
• Diploid - Describes a cell containing a complete set of DNA
• Meiosis steps

  • Prophase I
  • Metaphase I
  • Anaphase I
  • Telophase I
  • Cytokinesis I
  • Prophase II
  • Metaphase II
  • Anaphase II
  • Telophase II
  • Cytokinesis II
• Chromatin - DNA wrapped around histones to form chromosomes.
• Meiosis prophase I

  • chromatin begins to condense and shorten, forming chromosomes
  • nuclear envolope surrounding DNA breaks down so chromosomes are free in the cytoplasm
• Spindles

  • protein strands that move the chromosomes
  • made by centrioles
• Meiosis metaphase I

  • chromosomes are pushed to the centre of the cell by spindle fibres
  • line up in homologous pairs
• Meiosis anaphase I

  • spindle fibres contract and shorten
  • moves chromosomes to opposite poles of the cell
  • so one chromosome from each pair is at either end
• Meiosis telophase I

  • two nuclear envelopes form around each set of chromosomes

  Cytokinesis I

  • cytoplasm divides to form two cells
• Meiosis II

  • process repeats except in metaphase II there is only one chromosome
  • so the two sister chromatids are split in anaphase II
  • results in 4 non identical haploid cells
• Genetic variation

  • Independent assortment
  • Crossing over
• Independant variation

  • occurs during metaphase I
  • the order the chromosomes line up in is random
  • meaning the combinations of chromosomes going into the daughter cells is random
• Crossing over

  • occurs during prophase I
  • homologous chromosomes swap portions of their chromatids
  • resulting in mix of parental genetic information and new alelle combinations
  • structure formed by the homologous chromsomes formed during crossing over is known as bivalent
• Homologous pair - two chromosomes that are identical in length and have the same number of chromosomes (one from mother one from father)
• locus - position of a gene on a chromosome
• Egg cell

  • contains zona pellucida which is a protective coating
  • main purpose is to stop more than one sperm fertilizing the egg
  • haploid nucleus
  • cortical granules are organelles that release substances which cause the zona pellucida to harden
  • follicle cells form a protective coating around the egg
• Sperm cell

  • contain lots of mitochondria to provide energy for rotation of the flagellum
  • acrosome contains digestive enzymes to break down the zona pellucida and allow sperm to penetrate the egg
  • motile cell
• Fertilisation in mammals

  1. sperm head meets the zona pellucida and acrosome reaction occurs
  2. sperm head fuses with the cell membrane of the egg cell thus allowing the sperm nucleus to enter the egg cell
  3. cortical reaction occurs preventing other sperm from entering the cell
  4. the nuclei fuse and a full set of chromosomes is restored thus creating a diploid zygote
• acrosomal reaction - enzymes digest the zona pellucida in order to enable sperm to reach the egg
• cortical reaction - cortical granules release chemicals that harden the zona pellucida preventing polyspermy
• somatic cells - non sex cells
• Cell cycle

  • Interphase
  • Growth 1
  • S phase
  • Growth 2
  • Mitosis
• Growth phase 1

  • cell grows
  • synthesises proteins
  • carries out its functions
• S phase

  • cell carries out usual functions
  • replicates its DNA in preparation for mitosis
• Growth phase 2

  • continues to grow and synthesise proteins
  • makes proteins needed for cell divisions such as spindle fibres
• Mitotic index

  (Number of cells with visible chromosomes / total number of cells observed) x 100
• Structure of flowers

  A) petal

  B) stigma

  C) style

  D) ovary

  E) carpel

  F) ovule

  G) stem

  H) receptacle

  I) sepal

  J) pollen

  K) anther

  L) filament

  M) stamen

  
• stamen

  • male parts
  • involved in production of male gametes in the form of pollen grains
• carpel

  • female parts
  • the site of ovule development
• double fertilisation

  1. pollen grain composed of the pollen tube cell and generative cell adheres to stigma where it germinates to produce a pollen tube
• double fertilisation

  2) pollen tube grows down the stigma, secreting digestive enzymes which digests surrounding tissue and use it as a source of nutrients
• double fertilisation

  3) pollen tube grows through micropyle into embryo sac
• double fertilisation

  4) generative cell divides to produce two sperm cells which enter embryo sac
• double fertilisation

  5) one of the gametes fuses with the female nucleus to form zygote
• double fertilisation

  6) other male gamete fuses with two polar nuclei to form an endosperm nucleus which is a source of nutrients for embryo
• double fertilisation

  7) fertilised ovule divides by mitosis to form embryo consisting of plumule and radicle and one or two cotyledons

  • the integuments becomes the seed coat
  • ovule becomes seed
  • ovary becomes the fruit