1. Cell Biology
Unit Six

2. The Nucleus A prominent, spherical organelle found only in eukaryotes
It is the repository for the cell’s genetic information (DNA)

3. The Nucleus
It compartmentalizes the activities of the genome (DNA replication & RNA transcription) from the rest of the cell
It is enclosed by a double layered nuclear envelope made up of an inner & an outer membrane, separated by a perinuclear space

4. The Nucleus
The nuclear envelope is embedded with numerous nuclear pores which allow for exchange between the nucleus & cytoplasm
Each nuclear pore has a protein diaphragm stretched across it, regulating what goes through the pore

5. The Nucleus

6. The Nucleus
The nucleolus is a spherical organelle without a membrane found inside the nucleus that functions as the ribosome factory for the cell
Each cell has one or two
Proteins & rRNA are combined here to form ribsomes

7. The Nucleus The nucleolus consists of fibrils and granules
The fibrils are DNA transcribing rRNA
The granules are rRNA being combined with proteins to form the two ribosomal subunits

8. The Nucleus

9. The Nucleus

10. Chromosomes
Chromosomes are found in the nucleus and are comprised of approximately 50% DNA & 50% protein
They occur in pairs, one coming from each parent (sexual) or both coming from one parent (asexual)

11. Chromosomes
Most of the proteins are histones, which determine the molecular anatomy of the chromosome
The non-histone proteins also determine the molecular anatomy and also determine gene activity

12. Chromosomes
Nucleosomes

13. Chromosome Packing

14. Chromosomes
During mitosis the DNA & proteins condense and are referred to as chromosomes
During non-division periods the structures relax and are referred to as chromatin

15. Chromosomes
Since chromosomes contain the cell DNA their functions are in heredity, RNA template and protein synthesis

16. DNA & RNA Functions

17. DNA & RNA Structure

18. DNA & RNA Structure

19. DNA & RNA Base Pairing DNA replication
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A T G C A T T G A A G C T G G T A G
T A C G T A A C T T C G A C C A T C
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RNA transcription
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A U G C A U U G A A G C U G G U A G
T A C G T A A C T T C G A C C A T C
| | | | | | | | | | | | | | | | | |

20. RNA Synthesis
RNA synthesis (transcription) occurs at all times with all three types being produced by the same process
In general, DNA is separated and one strand is used as a template, with the sequence of bases determining the sequence of RNA bases

21. RNA Synthesis RNA polymerase will initiate the process
~ one in prokaryotes
~ three in eukaryotes
Occurs in two stages
~ synthesis of RNA nucleotides
~ polymerization of RNA nucleotides
on the DNA template

22. RNA Synthesis
The four RNA nucleotides are synthesized from triphosphate forms by way of pyrocleavage
They are synthesized in the cytoplasm and transported into the nucleus

23. RNA Synthesis Binding of RNA polymerase to DNA promoter
temporarily
separates DNA

24. RNA Synthesis 3. DNA bases attract RNA pairing partners
4. RNA polymerase
binds RNA
nucleotides
together into a
RNA polymer

25. RNA Synthesis 5. DNA terminator sites eject RNA polymerase 6. RNA post
transcriptional
processing

26. Ribosomes
Ribosomes are non-membrane bound organelles found in all cells and function as the manufacturing sites of proteins

27. Ribosomes
Each ribosome is composed of three types of rRNA & fifty different ribosomal proteins and are produced in the nucleolus

28. Ribosomes
Ribosomes made up of two subunits - one small, one large

29. Ribosomes Small subunit binds mRNA
Large subunit has an A site to bind tRNA carrying amino acids, a P site for the tRNA holding the growing protein and an E site for tRNA exiting

30. Ribosomes

31. Protein Synthesis - Translation
mRNA carries a series of codons that code for specific amino acids
A codon is a sequence of three nucleotide bases on mRNA
Since there are only four RNA bases and there are three in each codon, there are 64 condon possibilities

32. Protein Synthesis - Translation
The genetic code is the use of these 64 codons coding for 20 amino acids
Remember - the synthesis of a specific protein requires a specific sequence of amino acids
Therefore the sequence of codons on mRNA must be sequentially specific

33. Protein Synthesis - Translation
The genetic code is commaless and non-overlapping, meaning it is read in frames of three without gaps or overlaps
There is one initiation codon - AUG
There are three stop codons - UAA, UAG & UGA

34. Protein Synthesis - Translation
Translation is the process by which RNA produces proteins in ribosomes
- mRNA carries the code from DNA
- tRNA carries amino acids to the
ribosome
- rRNA is a component of the ribosome
By way of the genetic code, RNA “translates” from the language of nucleic acids to that of proteins

35. Protein Synthesis - The Genetic Code

36. Protein Synthesis - Translation

37. Protein Synthesis - Translation
There are four steps in adding amino acids to a protein
+ amino acid activation
+ transfer of amino acids to the
ribosome
+ peptide bond formation
+ ribosome shift

38. Protein Synthesis - Translation
Amino acid activation occurs with the binding of a particular amino acid to a specific tRNA
This process requires a specific enzyme for each amino acid & tRNA

39. Protein Synthesis – Amino Acid Activation
A group of 20 aminoacyl-tRNA synthetases along with pyrophosphated ATP create a two step process that activates the amino acid

40. Protein Synthesis – Amino Acid Activation
The activation results in the formation of an ester bond between the amino acid and the tRNA

41. Protein Synthesis - tRNA

42. Protein Synthesis – Transfer of Activated Amino Acid
The transfer of the activated amino acid begins with initiation

43. Protein Synthesis – Transfer of Activated Amino Acid

44. Protein Synthesis – Peptide Bond Formation

45. Protein Synthesis – Ribosome Shift

46. Protein Synthesis – Termination of Translation

47. Protein Synthesis – Overview

48. Protein Synthesis – Post Translational Processing

49. Genes, Genetic Code & Genetic Disease