SNAB Topic 3

Edexcel SNAB AS Level Biology, Unit 2, Topic 3 - In the beginning

  • Created by: Jess
  • Created on: 08-05-11 08:05



- bacteria and cyanobacteria

- cells do not have a nucleus or other membrane-bound organelles

- smaller than eukaryotic cells

- all cells have a cell wall


- all other living organisms (including animals and plants)

- cells contain a nucleus and membrane-bound organelles

- larger than prokaryotic cells

- not all eukaryotic cells have a cell wall

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Animal cell

NUCLEUS - contains chromosomes and a nucleolus

NUCLEOLUS - a dense body inside the nucleus where ribosomes are made

rER - proteins are transported through here to other parts of the cell

RIBOSOMES - the site of protein synthesis

CELL SURFACE MEMBRANE - phospholipid bilayer, partially permeable barrier

sER - doesn't have attatched ribsomes, makes lipids and steroids

GOLGI - modifies proteins and packages them in vesicles for transport

LYSOSOME - involved in the breakdown of unwanted structures in the cell

CENTRIOLES - involved in the formation of the spindle in nuclear division

MITOCHONDRION - site of the later stages of aerobic respiration

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Protein traffiking

1.) transcription of DNA to mRNA

2.) mRNA leaves the nucleus

3.) protein made on ribosomes enter through the rough ER

4.) protein moves through the ER assuming a 3D shape en route

5.) vesicles pinched off the rough ER contain the protein

6.) vesicles from rough ER fuse to form the Golgi apparatus

7.) proteins are modified within the Golgi apparatus

8.) vesicles pinched off the Golgi contain the modified protein

9.) vesicle fuses with the cell surface membrane releasing the protein

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- the ovum is wafted along one of the oviducts from the ovary to the uterus

- the sperm are attracted to the ovum by chemicals released from it

- the acrosome releases digestive enzymes which break down the zona pellucida

- the acrosome is a type of lysosome (enzyme-filled sacs in cytoplasm)

- once a sperm fuses with the egg, chemicals are released

- this causes the zona pellucida to thicken, preventing more sperm entering

- the sperm nucleus fuses with the egg nucleus

- a fertilized egg is produced

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- meiosis is a type of cell division that creates haploid gametes

- haploid cells in humans contain 23 chromosomes (half normal number)

- chromosomes replicate before division

- after replication each chromosome is made up of 2 strands of genetic material (chromatids)

- homologous chromosomes pair up and then separate

- chromatids separate and gametes are formed, each with half the number of origingal chromosomes

- the arrangement of each chromosome pair is completely random

- this results in genetic variation (independent assortment)

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Crossing over

- during the 1st meiotic division, homologous chromosomes come together as pairs and all 4 chromatids come into contact

- at these contact points chromatids break and rejoin, exchanging pieces of DNA

- the point where chromatids break is called a chiasma

- several of these often occur along the length of each pair of chromosomes

- this gives rise to a large amount of variation

- crossing over produces chromosomes that contain new combinations of alleles from both parents

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Fertilization in plants

- takes place in the embryo sac within the ovule

- the pollen grain germinates on the style & a pollen tube grows down to the ovary

- the pollen tube growth is controlled by the tube nucleus

- the pollen grain has two nuclei, the tube nucleus and the generative nucleus

- on germination of the pollen, the generative nucleus forms two gamete nuclei

- these move down the pollen tube

- the tube grows through a microscopic pore into the embryo sac

- the two gamete nuclei enter the sac

- one fuses with the egg cell and forms a diploid zygote (forms the embryo)

- the second fuses with the two polar nuclei to form a triploid cell (endosperm)

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The cell cycle - Interphase

- during interphase the cell synthesises new components (organelles, DNA, etc.)

- the formation of new proteins occurs thoughout interphase

- the formation of new DNA occurs during the S phase

- the length of interphase differs depending on the role of the cell

- the interphase nucleus is a uniform, featureless structure

- it has one or two darker regions called nucleoli, where ribosomes are formed

- the rest of the nucleus contains the chromosomes

- these are unravelled to allow access to the genetic material

- in preparation for division, new proteins, organelles and DNA are made

- the DNA is made by DNA replication so that the DNA is identical

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INTERPHASE - new organelles are synthesised, DNA replication occurs

PROPHASE - the chromosomes become shorter and thicker, spindle forms

METAPHASE - chromosomes move to equator, centromeres attach to spindles

ANAPHASE - spindle fibres shorten, pulling chromatids towards the poles

TELOPHASE - nuclear envelope forms (to form 2 nuclei), chromosomes unravel

CYTOPLASMIC DIVISION - cell surface membrane contracts until the cell divides into two new cells

- mitosis is used for growth & repair so that the organism can replace damaged cells, some organisms can even regrow body parts

- also used for asexual reproduction, where organisms use mitosis to grow copies of themselves, producing genetically identical offspring

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Stem cells

- stem cells are unspecialised cells

- stem cells can be found in 3 places only: embryos, umbilical cord, bone marrow

- there are different types of stem cell:

TOTIPOTENT - embryos are totipotent cells, can develop into a complete human

PLURIPOTENT - can give rise to most cell types but not all

MULTIPOTENT - can give rise to some cell types but not all

- stem cells offer great hope to medicine

- scientists hope to produce universal human donor cells which would provide new organs and tissues for transplantation, but there is a risk of rejection

- therapeutic cloning - a patient needing transplant would provide a cell, which would be turned into a zygote, then a blastocyst full of their own stem cells

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Gene expression

- as an embryo develops, cells become specialised for one function

- the structure and function is dependent on the proteins that it synthesises

- cells become specialised because only some genes are switched on

- RNA polymerase needs to bind to a section of DNA next to the gene

- this is called the promoter region

- only when RNA polymerase has attached will transcription proceed

- transcription can be prevented by protein repressor molecules attaching to the promoter region, blocking the attachment site for RNA polymerase

- in this case, the gene is switched off

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How are cells organized into tissues?

- specialised cells can group themselves into clusters

- cells have recognition proteins, known as adhesion molecules

- these are found on the cell surface membrane

- adhesion molecules recognize other cells like themselves and stick to them

- the exposed part of the protein binds to complementary proteins on the other cell

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Polygenic inheritance

PHENOTYPE - the characteristics shown in an organism, e.g. height

GENOTYPE - genetic make-up

DISCONTINOUS VARIATION  - e.g. human blood groups

CONTINUOUS VARIATION - e.g. height in humans

POLYGENIC - when a number of genes are involved in the inheritance of a characteristic, rather than just one

MULITFACTORIAL - several genetic and environmental factors are involved

- eye colour in an example of polygenic inheritance

- alleles at several loci control eye colour, e.g. BB BB bb

- B adds pigment to the iris, b does not

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Gene and environment interactions

HEIGHT - height is determined by both genes and the environment (e.g. diet)

HAIR COLOUR - hair colour is determined mainly by genes, however the pigment in the hair that makes it dark can be destroyed by UV light, making it lighter

MAOA - this is an enzyme in the brain that affects the regulation of behaviour, a mutation in this can cause aggresive behaviour, as well as environmental factors

CANCER - cancers are often caused by damage to DNA, this can be caused by UV light or asbestos, etc.

ONCOGENS - these help to control the cell cycle, and a mutation may mean that the cell cycle is continually active, leading to a tumour

TUMOUR SUPPRESSOR GENES - these produce proteins that stop the cycle, and mutations mean that the cycle does not stop, leading to a tumour

- most common cancers occur more frequently in relatives of cancer patients

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