Human Bio Chapter 9- DNA

What is DNA? And where is it found?

DNA short for deoxyribosenucleic acid

Contains the genetic information that determines the structure of the cells and why it functions

MOLECULE

Most found in nucleus (nuclear DNA), small amount found in mitochondria (mtDNA)

What is the structure of DNA and Nucleotides?

Molecule made up of small repeating units called nucleotides

Each Nucleotide is composed of a sugar molecule (deoxyribose in DNA, ribose in RNA), a phosphate group and a nitrogenous base (adenine, thymine/uracil in RNA, cytosine, guanine)

How does DNA twist into a double helix?

suagr molecule of one nucleotide bonds to the phosphate group of another one

Forms a long chain of alternating sugars and phosphates with side chains of bases

Two strands join together by specific bases being attracted to one another by weak hydrogen bonds

(A→ T double bond) (G → C triple bond)

What side is the 5’ and 3’? And, which side is the template and new strand?

What side of the DNA is the leading and lagging strand?

Leading = start from the top

Lagging = starting from the “bottom” up

What does the order of the bases determine?

order in which the nitrogenous bases occur in the DNA molecule determines the genetic code.

Each gene consists of up to 2 million pairs of bases therefore number of possible combinations of base pairs is enormous

What is the structure of a chromosome? (Hint: nucleosomes → chromatin →chromatid → chromosome)

DNA strands wrapped around his tones to form NUCELOSOMES

In a cell that is NOT dividing, the coiled DNA forms a tangled network called CHROMATIN

If a cell is undergoing mitosis or meiosis the chromatin may condense and turn into a CHROMATID

Two chromatids make a CHROMOSOME

Chromatins can become chromosomes as well

What is a gene?

a gene is a section of DNA that codes for a particular protein

Each chromosome is made up of these sections/genes

There are 46 chromosomes in a normal human cell

What is the mitochondrial DNA?

Mitochondrial DNA is a circular molecule not bound to histones

Plays an important role in coding for tRNA and enzymes needed for cellular respiration

How does DNA replication work? step 1

1- two strands of DNA is separated by enzyme HELICASE “unzips”

→ this is possible because the hydrogen bonds between the bases are weak

→ each strand has half the original information

→ each strand serves as a template for the nucleotides that will form the new strand

How does DNA replication work? Step 2

2- Because each base can only pair with their complementary base, the new strand formed is identical to the original.

DNA polymerase adds new nucleotides to the new strand

DNA ligase joins short sections of DNA together

What does the genetic code in DNA provide instructions for?

Protein Synthesis

What are proteins produced from?

Amino acids being joined tgt by peptide bonds

What is RNA?

Ribonucleic acid

Composed of a chain of nucleotides

What is the difference between RNA and DNA?

RNA

• sugar molecule is ribose (one more oxygen atom than deoxyribose)

• Single stranded

• Uracil instead of thymine

• RNA strand folds onto itself

DNA

• sugar molecule os deoxyribese

• Double stranded

• Thymine not uracil

What is mRNA?

Messenger RNA

Made in the nucleus and takes geneti code into the cytoplasm, allowing the genetic code to be ‘read’ by ribosomes

What is rRNA?

Ribosomal RNA

Makes up ~60% of ribosome’s mass (other 40% protein)

Ensures the correct alignment of mRNA, tRNA, and ribosome

Has enzymatic roles in formation of peptide bonds between amino acids

What is tRNA?

Transfer RNA

Small molecule of RNA (Only 70-90 nucleotides)

Each tRNA molecule is able to carry a specific amino acid and therefore plays a vital roles in protein synthesis

How are proteins made?

Protein synthesis

Through Transcription and Translation

Sequence of bases determine the protein that is produced

What is transcription?

Process by which the genetic instructions are copied (or transcribed) from the DNA to the mrNA molecule

Triggered by chemical messengers that enter the nucleus from the cytosol and bid to the DNA

What is the first step to Transcription?

Occurs in the nucleus

Helicase “unzips”/ makes the double-stranded DNA molecule come apart by breaking the weak hydrogen bonds between the bases

What is the 2nd stage of Transcription?

RNA polymerase transcribes the bases on one strand of the DNA to make complementary molecules of mRNA

* thymine is replaced by uracil when adenine shows up *

Strand that is copied = template strand

Other strand = coding strand

What is the third stage of trancription?

A sequence of bases tells RNA polymerase to stop copying

mRNA molecule is released and leaves the nucleus through the pores to the site of protein production (cytosol)

What is translation?

Production of a protein using the information that is coded in the mRNA molecule

What is the first stage of translation?

Occurs in the cytoplasm

Ribosome attaches to one end of the mRNA molecule at the start codon, AUG

This is to ensure the ribosomes attach to the correct end of the mRNA

What is the second stage of translation?

Ribosome moves along the mRNA 3 bases at a time (or 1 codon)

Each codon corresponds to a specific amino acid (AUG = methionine)

Therefore every protein beings with methionine, but it can be removed later

What is the third stage of Translation

As the ribosome reads the codons on the mRNA, tRNA molecules join the mRNA with complementary bases. Each bond formed between the amino acids require energy from the breakdown of 1 ATP molecule

Sequence of 3 bases matching the mRNA codon is called an anticodon

Amino acids joined in the correct sequence so the protein is assembled correctly

What is the fourth stage of translation?

Once tRNA has delivered its amino acid, it detaches from the ribosome and can then pick up another amino acid from the cytosol

Creates a chain of amino acids that bind together to become a protein

What is gene expression?

process of copying information from DNA in to mRNA and the translating the message into a series of amino acids to from a protein

What do genes contain?

Instructions for making mRNA

Any given time a cell is making mRNA from only a fraction of its genes

Genes being used to make mRNA said to be “switched on”

Gene not being used = “switched off”

What are the factors that determine wether a gene is being expressed?

Age of the cell

Time of day

Signals from other cells

Environment of the cells

Wether the cel is dividing

What is lipid and carbohydrate synthesis?

There are no genes that carry instructions for the manufacture of lipids or carbohydrates

Synthesis of these substances require enzymes

Enzymes are proteins

DNA in genes carries the code for protein manufacture, therefore genes control synthesis of lipids and carbohydrates indirectly

What are epigenetics?

Factors that make genes more or less likely to be expressed that is inherited

EPI = on top of

Therefore epigenetics refer to changes in gene expression that result from mechanisms other than changes in the genes

Growing list of environmental factors that can cause epigenetic changes that are inherited by offspring

What is the genome?

Hereditary information that is encoded in a person’s DNA

What is the epigenome?

Sum of all the factors that determine: when, where, and which genes are expressed

What does the epigenome do?

Helps control which genes are active in a particular cell, and therefore which proteins are produced

If the epigenome is abnormal, certain cells may be abnormal and disease may result

One way genes are regulated epigenetic all is through changes in chromatin

What do epigenetic factors do?

Tell muscle cells to behave like muscles cells, nerve like nerve cells and so on

Tells each type of cell to behave the way they need to

What is chromatin?

DNA and the histone proteins associated with it

Gene expression may change if the way in which the DNA is wrapped around the histone changes

What happens of the amino acids that are in the histone proteins changes?

Changes the shape of the histone

This modified shape may be copied each time a new DNA molecule os formed

What does this modified histone do?

Ensures that a stem cell that differentiated into a specialised cell remains a specialised cells and doesn’t revert back into a stem cell *like a liver cell*

What is acetylation?

Another way of histone modification

Is the addition of an acetylene group to the histone protein

Reduces the attraction between histones and DNA, relaxing the structure of the chromatin

RROMOTES TRANSCRIPTION by allowing RNA polymerase access, therefore ENHANCES GENE EXPRESSION

What is methylation?

Addition of methyl group to the DNA molecule or histone protein

Occurs at sites on the DNA molecules where a cytosine nucleotide is adjacent to a guanine nucleotide (CpG sites)

INHIBITS gene expression by restricting access to RNA polymerase

Generally decreases transcription of genes, but it depends where the methyl groups attach and how many become attached

If chromatin relaxes, transcription increase

How does ones environment effect their epigenome?

Person’s epigenome can be changed by exposure to certain environmental stimuli

Some environmental agents include (but are not limited to) severe stress, nutritional factors and toxins or drugs that may enter the cell

How does the environmental factors affect?

Does not change DNA, but interferes with transcription and translation process involved in protein production

Epigenetic factors can influence any step in gene expression