Revision Notes Topic Three 'Voice of the Genome'

Condensed notes for the whole of edexcel AS Biology, 'Voice of the Genome' 

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Topic Three: Voice of the Genome
Feature Prokaryotic Cell Eukaryotic Cell
Animal Cell Plant Cell
Nucleus with envelope Absent Present Present
Membrane-bound organelles Absent Present Present
DNA found as A loop Separate strands Separate strands
Slime capsule Sometimes present Never present Never present
Flagellum Simple (if present) Complex (if present) Absent
Cell wall Present (bacterial) Absent Present (cellulose)
Relative size of cell Small Medium Large
Cellular organisation:
Multicellular organisms made up of many organised cells
Cells ­ tissues ­ organs ­ systems
Cells ­ eukaryotic
Tissues ­ one or a few different cell types working together to perform one function
Organs ­ various tissues grouped to work together and perform their function efficiently
Systems ­ various organs that work together to perform a large-scale function
Protein Synthesis:
1. Transcription of DNA to mRNA (nucleus)
2. mRNA leaves the nucleus through nuclear envelope
3. Protein made on ribosomes enter rough ER
4. Protein moves through the ER assuming its 3-D shape
5. Vesicles pinched off the rough ER contain the protein
6. Vesicles fused to form the flattened sacs of Golgi
7. Proteins are modified within the Golgi apparatus
8. Vesicles pinched off the Golgi apparatus contain
modified protein
9. Vesicles fuse with the cell surface membrane releasing
the protein (e.g. Extracellular enzymes)
All eukaryotes share the same basic structure and function, as multicellular organisms
develop, these cells take on specialised roles
Gametes fundamental difference from other cells is the number of chromosomes
(23 ­ made up of one of each homologous pair and one sex chromosome)
Haploid gametes come together to form a diploid zygote
Mammalian Gametes:
Sperm and ovum

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Fertilisation of the ovum:
1. Sperm reaches the ovum
2. Chemicals are released from the cells surrounding the
ovum, triggering to acrosome reaction
3. Acrosome swells and fuses with sperm cell surface
4. Digestive enzymes in acrosome released
5. Enzymes digest through follicle cells and zona
6. Sperm fuses with the ovum membrane
7. Sperm nucleus enters the ovum
8. Enzymes released from the lysosomes in the ovum
thicken the zona pellucida and stop entry of other
9.…read more

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Female plants - only one of the four nuclei forms an ovum at the end of meiosis (others
Male plants ­ four haploid cells produced by meiosis undergo another division, mitosis.…read more

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Crossing over:
first meiotic division ­ homologous chromosomes come together as pairs and all four
chromatids come into contact
at contact the chromatids break and re-join, exchanging sections of DNA
the point where chromatids break is called a chiasma
several chiasmata occur along the length of each pair of chromosomes = variation…read more

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Fertilisation in mammals:
nuclei from the gametes combine
nucleus from one sperm enters the ovum and their genetic material combines forming a
zygote (containing genetic material from both parents)
Fertilisation in flowering plants:
nuclei from the gametes combine
takes place in the embryo sac within the ovule
pollen grain germinates on the style and a pollen tube grows down through the style
towards the ovary (growth controlled by tube nucleus)
pollen grain contains two nuclei (tube nucleus and generative nucleus)
germination of the pollen ­…read more

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­ endosperm)
Cell Cycle
New cell organelles are synthesised and DNA replication occurs ­ preparation for division
A time of intense and organised activity
Cell synthesises new cell components such as DNA/organelles and membranes
Formation of cellular proteins throughout the whole phase
DNA synthesis = stage `S'
Stages: G1, S, G2
Length of interphase can differ depending upon the role of the cell
Human embryo: no interphase for the first few divisions because the zygote already contains
enough material for first 16ish cells.…read more

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Individual chromosomes unravel
Allows access to the genetic material
enabling new proteins to be synthesised
Cell synthesises additional
Cytoplasmic proteins and organelles
Cell must also produce copies of the DNA
for the two new cells (DNA must be identical)
Cell Division:
1. Chromosomes condense and become visible as chromatids
2. Microtubules form spindles
3. Centrioles move to opposite ends of the cell
4. Forms two poles of the spindle and organise spindle fibres
5. Nucleolus breaks down
6. Nuclear envelope disappears
1.…read more

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Fibres pull chromatids apart with the centromere leading
1. Chromosomes de-condense (become invisible)
2. Nuclear envelope reforms
3. Nucleolus reappears
4.…read more

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Sequence Reason Safety
Cut final 5mm of a root tip from This is where mitosis occurs Care when cutting
a plant (onion)…read more

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Add stain (orcein) and an acidThe acid softens the tissues Take care with the stain and
whilst the stain highlights acid (skin and eyes)
Gently break open on a Spreads out the cells and Careful not to break the glass
microscope slide and squash makes them one layer thick in slide
carefully order to be easily seen
Add more stain and warm, then Warm to intensify the stain.…read more


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