4.1 DNA structure and function / 4.2 Genes

DNA

Most humans have 46 molecules of DNA, each abut 2in long

Essential function is carry instructions called GENES for synthesis of proteins

Humans have an estimated 22,300 genes, which is about 2% of DNA, 98% of DNA is regulating gene activity and chromosome structure

DNA usually occurs as 6 ft of DNA in the first half of a cells life cycle and two as much as it’s replicated through it’s cell cycle

DNA is superbly coiled and supercoiled as to not break, get tangled, etc

DNA bases

Cytosine - sugar, phosphate, and single ring carbon-nitrogen ring - pyrimidines

Thymine - sugar, phosphate, and single ring carbon-nitrogen ring - pyrimidines

Guanine - sugar, phosphate, and double ring carbon-nitrogen ring - purines

Adenine - sugar, phosphate, and double ring carbon-nitrogen ring - purines

Base Pairs + Law of Complementary Base Pairings

Base pairs are A - T and C - G

One strand governing what comes after it is known as law of complementary base pairings, allowing use to predict the base sequence if you know what is happening on the other strand

Chromatin / chromosomes

Added alongside DNA, is a protein that forms fine filamentous material

In most cells chromatin occur as 46 long filaments called chromosomes

Ribonucleic Acid

Many forms of RNA with diverse functions but 3 most important

Messanger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA ( tRNA)

RNA is much shorter ranging from 70 to 10,000 bases in comparison to 100 million base pairs in DNA

Only one nucleic chain

Has nitrogenous bases: adenine, cytosine, and guanine, and uracil takes thymine’s place

RNA’s job is to interprets DNA’s code and then use those instruction to synthesize proteins

RNA is considered disposable and is found in the cyptoplasm

Genes

Information containing segment of DNA that codes for production of a molecule of RNA, which usually goes out to play a role in the synthesis of one or more proteins

The amino acid sequence of a protein is based on the nucleotide sequence in DNA

There are about 3,000 nucleotides in a gene, but can go up to 2.4 million

Most genes are identical in every human being, 99.99% identical

How many chromosomes?

There are 46 human chromosomes, they come in two sets of 23 - one from each parent

Genome

All of the DNA coding and noncoding in one 23 chromosome set is a genome

The genome total is about 3.1 nucleotide pairs

Genetic Code

System that enables ATCG nucleotides to code for amino acid sequences of all proteins

Base Triplet / Codon

Sequence of 3 dna nucleotides codes for 1 amino acid

When mRNA is producted, it carries a coded message based on the DNA triplet

A codon is 3-base sequence

Stop Codon

UAG, UGA, and UAA are all stop codons that message to signal end of message, enabling cell’s protein synthesizing machinery to sense that it has reach the end of the instructions for that particular protein

Start Codon

AUG is a start codon

Do cells use all of their genes?

No, they only use what is needed in that exact situation - only 1/3 to 2/3 of genes are used at any given time

For example, genes for digestive enzymes would be used in stomach cells but not in muscle cells

What does mRNA do?

When a gene is activated, mRNA is created to mirror the gene. It migrates from the nucleus to the cytoplasm, and it acts as code for assembling amino acids in the right order to make a particular protein

Protein Synthesis Order

DNA → mRNA → Protein

DNA → mRNA is transcription, occurring in the nucleus

mRNA → protein is translation occurring in the cytoplasm

Transcription

Copying genetic instructions from DNA to RNA

RNA polymerase binds to DNA and assembles the RNA, opening 17 DNA base pairs at a time, reading the bases from one strand of the DNA and making the corresponding RNA, rewinding the DNA behind it. Several RNA polymerase can be creating multiple RNA strands at the same time

What RNA participates in translation?

Converts language of nucleotides into language of amino acids

1) mRNA carries genetic code from nucleus to cytoplasm, during synthesis in nucleus it acquires a protein cap that allows it to pass through nuclear pore into the cytosol

2) tRNA is small and binds to free amnio acids in the cytosol and delivers it to the ribosome to be added to protein chain. single stranded molecule that turns back and coils on itself to form L Shape

3) ribosome are reading machines in cytosol and outside of rough ER, and the nuclear envelope. Inactive ribosomes occur in cytosol in two pieces small subunit and large subunit. Each has several enzymes and ribosomal RNA rRNA. The two units come together only when translating mRNA. They have 3 pockets to bind to tRNA. A site is first, accepting amino acids, P site is next for growing proteins, and finally E site for exiting

What is the first step of translation

1) initiation: mRNA passes through the nuclear pores into cytosol and forms a loop, ribosomal subunits bind to leaders sequence of the bases on the mRNA near the cap and slides along until it recognizes the start codon AUG. Then initiator tRNA pairs with start codon and settles into P site of the ribosome. Large subunits of ribosomes then join, assembled ribosomes now embrace the mRNA in a groove between the subunits and begins sliding along, reading the bases

What is the second step of translation

Elongation: Next tRNA arrives, carries another amino acid, binds to A site of ribosome and it’s anticodon pairs w/ 2nd codon of mRNA, enzymes in the ribosome creates a peptide bond between the proteins being created. The ribosome then slides down to read the next codon, the initiator tRNA then shifts to E site where it exits the ribosome. The second tRNA shifts to the P site, and the vacant A site binds a third tRNA. The process of moving from A, P, and E sites, we continue the act of elongation, gaining a long strand of proteins

Third step of translations

3) termination - ribosomes reach a stop codon, it’s A site binds a protein “release factor” instead of tRNA. The release factor causes the finished protein to break away from the ribosome. The ribosome then dissociates into its two subunits, but they can reassemble to begin the process again

Fourth set in translation

Making proteins for packing or export - if a protein is going to be packaged to become a lysosome or secreted from the cell, the ribosome will dock on the rough ER and a new protein will spool into the cistern of the ER instead of into cytosol.

ER will modify the protein and package it into transport vesicles

Polyribosome

Multiple ribosomes working together to help in translating the mRNA

Protein Processing

After protein is synthesize, it must go through further processing

1) proteins are assembled on the ER surface. It threads through the pores of the ER membrane and into the cistern. Enzymes than modify the new protein in posttranslational modification.

2) rough ER is finished with proteins, it pinches of into a bubble like transport vesicle coated with protein clathrin. It helps to select proteins to be transported into the vesicles and mold the forming vesicles, after vesicles detach from ER they fuse into irregularly shaped clusters that carry their cargo to golgi complex

3) At the complex, the clusters fuse and form new golgi cisterns

4) new cistern migrates through the golgi complex toward the opposite side, maturing as it travels and new enzymes are produced to modify the cargo.

5) finally the trans cistern, farthest away from ER, breaking up into vesicles laden with cell product

6) some of the golgi vesicles become lysosomes and others become secretory vesicles that migrate the plasma membrane and fuse with it, releasing cell products by exocytosis.

How is something besides protein created?

This such as fat, steroids, etc are created by enzymes which are proteins encoded by genes

For example, testosterone - a cell of the testis take in a cholesterol and enzymatically coverts it to testosterone