DNA strands run in 1 of 2 directions depending on which end of the strand is free and which end has a phosphate bond. One direction is 5' to 3' () and the other direction is 3' to 5' (downstream). polymerase binds to the DNA at the TATA box and unzips the double helix. Working along the DNA chain, RNA polymerase reads the DNA nitrogenous bases and matches RNA nitrogenous bases from the nucleus (uracil instead of thymine). As it moves, RNA polymerase re-zips the DNA behind it and lets the mRNA separate from the DNA. It continues to do this until it reaches a sequence downstream called a signal. In other words, RNA polymerase copies the DNA sequence downstream of the TATA box into an mRNA strand.

The answers in the blank spaces:

1. Upstream

2. RNA

3. Terminator

Explanation:

This question is describing the process of transcription, which is the process whereby DNA template is used to synthesize an mRNA molecule. Transcription involves three stages viz: initiation, elongation and termination.

- DNA strands run in 1 of 2 directions depending on which end of the strand is free and which end has a phosphate bond. One direction is 5' to 3' (UPSTREAM) and the other direction is 3' to 5' (downstream). An enzyme called RNA polymerase binds to the DNA at the TATA box and unzips the double helix (INITIATION).

- Working along the DNA chain, RNA polymerase reads the DNA nitrogenous bases and matches RNA nitrogenous bases from the nucleus (uracil instead of thymine). As it moves, RNA polymerase re-zips the DNA behind it and lets the mRNA separate from the DNA (ELONGATION).

- It continues to do this until it reaches a sequence downstream called a TERMINATOR signal (TERMINATION). In other words, RNA polymerase copies the DNA sequence downstream of the TATA box into an mRNA strand.

stratosphere: as the altitude increases, the air temperature increases. the stratosphere has a layer of ozone, called the ozone layer. this layer absorbs most of the ultraviolet radiation from sunlight. this results in the stratosphere being warmer.

1) gene flow

- it is described as the transfer of genes from one population to another population so that frequency of gene is increased.

-a zebra migrates to join a different herd of zebras.

2) natural selection

- a mechanism of evolutionary change in which individuals with advantageous traits survive and reproduce whereas the others ( those that can not adapt) eliminate from the population.

-competition for sunlight leads to taller trees.

3) mutation

- permanent change in the nucleotide sequence of dna ( genetic material) that could have harmful or useful consequences.

-the dna of a snake changes to make its venom stronger.

4) genetic drift - it is a random phenomenon ( that occurs by chance such as fire) that results in change in the gene frequency in a small population.    

-a grassfire randomly sweeps through a population of buffalo and kills most of the animals.

explanation: