Chemistry to BiologyCells, RNA, & DNA

ASTR 1420

Lecture 5

Sections 5.2 & 5.4

RNA based world

• In the beginning of Earth life, the first life could not be based on DNA because DNA is too complicated to be created by mere “lucky” chemical reactions.

early life must have used a simpler molecule (e.g., RNA)or, DNA was introduced externally?!?

Organic Building Blocks

Six essential chemical elements

for Earth Life

• Carbohydrates (= carbs) : providing energy to cells and make cellular structures sugars, starches, cellulose, etc.

• Lipids (aka fats) : store energy, make cell membranes

• Proteins : versatile, essential molecules of cells

• Nucleic acids : handling heritable information of cells – DNA and RNA

Cells

• All living organisms are made of cells

• Adult human body have 10-50 trillion cells!

Typical Eukaryotic Cell

Monomer/Polymer• Glycogen: the carbohydrate that animals use

to store energy is a polymer made of glucose molecule contains 22 atoms

Most life forms consist of small number of simple molecule types called monomers, which can join together in a repetitive sequence to become larger and more complex molecules called polymers.

glucose

Carboxyl group

Amino group

Amino Acids

• Most important monomer!• Basic building blocks of proteins, nucleotides, and sugars

• Different side chain (R) makes a different amino acid.• There are about 70 different amino acids in Nature

Amino Acids : peptide bond to make a long chain

• Basic building blocks of proteins, nucleotides, and sugars

R

• Typically, few hundred amino acids form a protein polymer.• Out of ~70 amino acids, only 20 are used in Earth life!!

A strand of amino acids, part of a protein

Amino Acids : peptide bond to make a long chain

Chirality of Amino Acids

• Chirality (handedness)

For a given amino acid, there are two geometrical types:

left-handed & right-handed

• In non-biological environ: two types are equally found

• In living organisms: only left-handed amino acids are found! Why?

All Earth life forms were originated from a single ancestral life using only left-handed amino acids!?!

DNA Proteins

• All Earth life have DNA, and DNA is the backbone of heredity.o Heredity DNA replicationo function of DNA (genes) protein synthesis

• What’s protein? o organic compound made of amino acids in a folded chaino essential part of organisms and participate in every process within cells

enzymes involved in matabolism make cell stuctures cell signaling, immune responses, etc.

DNA structure http://www.youtube.com/watch?v=qy8dk5iS1f0

DNA replication http://www.youtube.com/watch?v=hfZ8o9D1tus

Protein synthesis http://www.youtube.com/watch?v=XO-2Z933AzM

Genes• Part of DNA strand representing any single

function (e.g., instruction for one protein)

Species No. of genes

Virus 10

Mitochondria 40

Microbe 1,000

E coli 4,400

Human 27,000

Mouse 29,000

Rice 50,000

Genetic code

Codes for most amino acids really depend on just two letters

in the past, genetic codes were two letter based…

• The set of rules for reading DNA.

• Genetic “words” consist of three DNA bases in a row. For the purpose of protein building, each word represents either a particular amino acid or “start reading” or “stop reading”.

• Genetic code is same for nearly ALL living organisms on Earth! common ancestor of all life on Earth!

Mutation• Replicating the whole ~3 billion bases for

human DNA takes only a few hours• About 1 replication error in billion chanceso wrong baseo deleted baseo extra base o etc.

• Most of these errors have no effect : Why?o about 95% of human DNA bases are “non-

coding DNA”o Most significant change is from “deleted base”.

• Due to this “error”, every individual living organism differ each other.

• Mutation = molecular engine of evolution!

• Any change in the base sequence of DNA

Stumpy, a four-legged duckling at Warrawee Duck Farm, Copythorne, Hampshire, England (2007)

Sickle-cell disease

• Just one base change in one gene (Adenine Thymine)

• Most mutations are harmful, but occasional good mutations are being picked up by “natural selection”

Another evidence of the common ancestor

• Cells need Energy to function• The ATP molecule gives up energy when it splits into ADP and a

phosphate group. • ATP/ADP are like batteryo ATP : fully chargedo ADP : fully discharged

Adenosine diphosphate

Another evidence of the common ancestor

• Cells recyle ATP. • There are many other possibilities of different molecular pairs (i.e.,

different batteries), but all Earth life use ADP/ATP as the energy reservoir.

common ancestor of all Earth life!

In summary…

Important Concepts• Common ancestor of all Earth Life

(chirality of amino acid, common genetic code, and use of ATP among all living organisms on Earth)

• Some non-DNA based life forms in the beginning?

• DNA replication

Important Terms• amino acids / proteins• chirality• genes, genetic code• mutation

Chapter/sections covered in this lecture : 5.2 & 5.4