test 3

If the genes have common regulatory sequences within promotors (eukaryotes)

they can be regulated with the same transcription factors

**Epigenetic modification**

changes in gene expression caused by altering chromatin structure (without changes in the DNA sequence)

Many eukaryotic promoters contain a sequence called the TATA box

RNAP can only bind to the promoter after general transcription factors bind to the TATA box (RNAP and other proteins in the complex → basal transcription apparatus)

Enhancers and Silencers

stretched of nucleotides outside the promoter region

explain histones and stuff

**Histones (proteins):** Histones bind to DNA and help give chromosomes their shape, and help control the activity of genes

The double helix structure can wrap around all the histones to form a **nucleosome**

**Chromatin** is nucleosomes which are condensed coiled up and organized are 

**Chromosomes** are just a tightly wound and packed bundles of nucleosomes that is an interaction between **DNA** and a **protein**

**How DNA is packaged influences gene expression** 

Expression depends on

1. Modification of histone proteins 


1. Methylation of bases

**Histone Acetylation PROMOTES gene expression**

add acetyl groups to positively charged amino acids, reducing ionic attraction and therefore weakening the association of DNA and histones, opening chromatin

**Histone deacetylases can repress..**

**transcription by removing acetyl groups**

tRNA

* Binds to a specific amino acid


* Anticodon is complementary to mRNA codon
* Noncovalently interacts with ribosomes
* “Charged” by specific enzyme (requires ATP)

ribosome

holds mRNA and charged tRNAs in correct position to allow assembly of polypeptide - 3 sites (A, P, and E site )