DNA and protein synthesis

Year 11 human bio ATAR

What does DNA stand for? 

Deoxyribonucleic acid

What are nucleotides? 

The building blocks of DNA, consisting of a sugar-phosphate backbone and a nitrogen base

What are the four nitrogen bases in DNA?

Adenine (A), Thymine (T), Guanine (G), and Cytosine (C)

What is the backbone of DNA made of?

Sugar (deoxyribose) and phosphate groups

What are genes?

Segments of DNA that contain the code for specific proteins

What are chromatins? 

Thread-like structures of loosely coiled DNA found in non-dividing cells

What are chromosomes? 

Tightly coiled chromatin structures visible during cell division

What is the overall structure of DNA? 

A double helix - two strands that twist around each other

Which bases pair together in DNA?

Adenine with Thymine (A-T) and Guanine with Cytosine (G-C) (ATCG)

What type of bonds hold the base pairs together?

Weak hydrogen bonds

How many hydrogen bonds form between A-T pairs vs G-C pairs?

A-T pairs have 2 hydrogen bonds, G-C pairs have 3 hydrogen bonds

What are the four structures of DNA that enable its functions?

1) Heritable, 2) Stores a code, 3) Used to compare organisms, 4) Can replicate

What bonds connect nucleotides in the sugar-phosphate backbone?

Covalent bonds (phosphodiester bonds)

What does it mean that DNA is directional? 

DNA has a 5' (five prime) end and a 3' (three prime) end that are different

What is found at the 5' end of DNA? 

A phosphate group

What is found at the 3' end of DNA?

A hydroxyl (-OH) group

In which direction is DNA always read?

5' to 3' direction

What are histones? 

Proteins that DNA wraps around for packaging

What is a nucleosome?

A structure formed by 8 histones with DNA wrapped around them

What is the DNA packaging hierarchy? 

DNA → Histones → 8 Histones → Nucleosome → Chromosome

How is mitochondrial DNA different from nuclear DNA?

It's circular (not linear), smaller, not wrapped around histones, and contains only 37 genes

What do mitochondrial genes code for? 

tRNA (for protein building) and enzymes used in cellular respiration

How does DNA control cell processes?

DNA provides instructions for protein synthesis, and proteins control most body processes

What are proteins made of?

Strings of amino acids

How many bases code for one amino acid?

Three bases (called a triplet or codon)

Give examples of base triplets and their amino acids.

CAG codes for Valine, TTA codes for Leucine, CCC codes for Proline

What is DNA replication?

The production of a new DNA double helix with one parental strand and one daughter strand

What energy source is required for DNA replication?

ATP

What enzyme breaks the hydrogen bonds between DNA strands? 

DNA helicase

What does DNA helicase create when it separates DNA strands?

Two replication forks

What enzyme joins nucleotides together during replication?

DNA polymerase

What is the semi-conservative model of DNA replication? 

Each new DNA molecule consists of one original (parent) strand and one newly synthesized (daughter) strand

What is DNA ligase and why is it needed?

An enzyme that joins DNA fragments together on the lagging strand during replication

Why are there fragments on the lagging strand? 

Because DNA replication only occurs in the 5' to 3' direction, creating discontinuous synthesis on one strand

What does RNA stand for? 

Ribonucleic acid

How does RNA differ from DNA in structure?

RNA has ribose sugar (not deoxyribose), is single-stranded, and contains Uracil instead of Thymine

What are the four bases in RNA?

Adenine (A), Uracil (U), Cytosine (C), and Guanine (G)

What are the three types of RNA?

Ribosomal RNA (rRNA), Messenger RNA (mRNA), and Transfer RNA (tRNA)

What is the function of rRNA? 

Ensures correct alignment of mRNA, tRNA, and ribosome; helps form peptide bonds

What is the function of mRNA?

Carries genetic code from nucleus to cytoplasm for protein synthesis

What is the function of tRNA?

Carries specific amino acids to ribosomes during protein synthesis

What is a triplet? 

Three bases on DNA

What is a codon? 

Three bases on mRNA

What is an anticodon?

Three bases on tRNA that are complementary to a codon

What are the two main processes of protein synthesis?

Transcription and Translation

Where does transcription occur? 

In the nucleus

What is the purpose of transcription?

To copy DNA code into mRNA that can leave the nucleus

What enzyme is responsible for making mRNA?

RNA polymerase

What is a promoter sequence?

The DNA sequence where RNA polymerase attaches to begin transcription

What are the template strand and coding strand? 

The template strand is used to make mRNA; the coding strand has the same sequence as mRNA (except T instead of U)

What is the start signal for transcription?

Chemical messengers that bind to DNA at the relevant gene

What is post-transcriptional modification?

Addition of a 5' cap and 3' poly-A tail to protect the mRNA transcript

What are introns and exons? 

Introns are non-coding sections removed from mRNA; exons are coding sections that remain

What is splicing? 

The removal of introns from the mRNA transcript

Where does translation occur? 

In the cytoplasm at ribosomes

What is the start codon and what does it code for?

AUG, which codes for methionine

How does the ribosome read mRNA?

Three bases at a time (codons)

What is the structure of tRNA?

It has a section that binds to amino acids and an anticodon loop with three bases

How are amino acids joined together during translation?

By peptide bonds, which require energy

What happens to tRNA after it delivers its amino acid?

It detaches from the ribosome and picks up another amino acid from the cytoplasm

How fast can protein synthesis occur? 

One ribosome can make a 400-amino acid protein in 20 seconds

What is gene expression? 

The process of copying information from DNA to mRNA and translating it into amino acids

Why don't all genes get expressed in every cell?

Cells only use genes they need; genes must be "switched on" to be expressed

What factors affect gene expression? 

Age of cell, time of day, signals from other cells, cell environment, and whether the cell is dividing

How do genes control lipid and carbohydrate synthesis?

Through producing enzymes (proteins) that catalyze their synthesis

What is epigenetics? 

Changes in gene expression that result from mechanisms other than changes in DNA sequence

What are the two main types of epigenetic modifications?

Histone modification and DNA methylation

What is histone modification?

Chemical changes to histone tails that affect how tightly DNA is coiled

What is acetylation and its effect?

Addition of acetyl groups to histones, which opens up DNA and increases likelihood of gene expression

What is deacetylation?

Removal of acetyl groups, which tightens DNA coiling and decreases gene expression

What is methylation and its effect?

What are CpG sites? 

Regions of DNA where cytosine is next to guanine

What environmental factors can affect the epigenome

Severe stress, nutritional factors, toxins, and drugs

Do epigenetic changes alter the DNA sequence? 

No, they change gene expression without altering the actual DNA sequence