Cell Biology
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Invagination definition.
The action or process of being turned inside out or folded back on itself to form a cavity or pouch.
Why have a nucleus?
• Separate fragile chromosomes from cytoskeletal cytoplasmic filaments • Separated RNA transcription (in nucleus) from translation machinery (in cytoplasm)
How does the nucleus protect chromosomes?
The DNA and proteins are highly packed and dense
Why is it important to separate RNA transcription from translation machinery?
• RNA processing (nucleus) - alternative splicing • Regulation of RNA export • Regulate the import/export of other proteins
Briefly describe the possible evolution of the nucleus.
• DNA attached to cell membrane at many points (ancient prokaryote cell) • Membrane invaginate : mesosomes • Repeated invagination forms nucleus (ancient eukaryotic cell)
What is a mesosome?
An organelle of bacteria that appears as an invagination of the plasma membrane
Describe the structure of DNA.
DNA is a double helix with a sugar phosphate backbone and pairs of bases protruding into the middle
Describe the structure of the sugar-phosphate backbone.
Nucleotides linked together by a covalent bond between 5’ phosphate and 3’ -OH group of deoxyribose
Which bases form how many hydrogen bonds with each other?
Thymine forms 2 hydrogen bonds with adenine Cytosine forms 3 hydrogen bonds with guanine
Which base pairs contain a purine and which contain a pyrimidine?
A + G contain one large purine T + C contain one small pyrimidine
Why do the two strands of DNA wind into a double helix?
To maximise the efficiency of base pairing
What is the function of DNA?
Storage of genetic information for replication and gene expression
What is the definition of gene expression?
The process by which the information in a strand of DNA is copied into a new molecule of mRNA
What is splicing?
The process of removing non-coding introns
What is the name of non-sex chromosomes and how many pairs do we have?
Autosomes - 22 pairs
Give some properties of telomeres.
• End of the chromosomes (protective) • 1000 X GGGTTA • Special replication mechanism • Shorten with age
Give some properties of centromeres.
• Hold metaphase chromosomes together • Attaches to mitotic spindle • Repetite sequences; highly packaged
How are autosomes ordered and classified.
Ordered by size Classified by position of centromere
Meaning of metacentric
In the middle
Meaning of submetacentric
Intermediate (slightly closer to one end)
Meaning of acrocentric
Terminal (at the end)
Chromatin structure
Association between DNA and protein
Process of DNA packaging
• Short region of DNA double helix • “Beads on a string” form of chromatin (nucleosome) • Chromatin fiber of packed nucleosomes • Chromatin fiber packed into loops • Entire mitotic chromosome
What is the net result of DNA packaging?
Each DNA molecule has been packaged into a mitotic chromosome that is 1000-fold shorter than its fully extended length.