cell bio chapter 5 - DNA and chromosomes

streptococcus pneumoniae

heat killed bacteria that can transform living cells

s strain

Smooth colonies and pathogenic (lethal)

r strain

Rough colonies and harmless

Fred Griffith

found that a substance present in the S strain could transform the
nonlethal R strain into the deadly S strain

Avery, Macleod, and McCarty

prepared an extract from the disease-causing S strain and fractionated it into
different fractions (RNA, protein, DNA, lipids, and carbohydrates)

DNA’s transformation of R strain into S strain

first evidence that DNA can serve as the genetic material

Hershey and Chase

used bacteriophages to demonstrate that DNA rather than protein
carries the genetic material

Genes

store the genetic information which determines the characteristics of a species as a whole and the individuals within it

What are the 3 parts of a nucleic acid?

Phosphate group, 5 carbon sugar, nitrogenous base

Why is the sugar-phosphate backbone important for DNA?

It forms the stable outer structure

histones

structural proteins that act like a molecular spool, wrapping and organizing long strands of DNA so they can fit inside a cell nucleus

core histones

H2A, H2B, H3, H4

30 nm fiber

H1 condenses the loose "beads-on-a-string" DNA chain by pulling individual nucleosomes together into a thicker, helical structure

Why is complementary base pairing important for DNA?

allows each strand to serve as a template for making a new strand, enabling accurate DNA replication

Why is hydrogen bonding between bases important for DNA?

holds the two strands together but allows them to separate when needed

Nucleotide

a sugar-phosphate covalently linked to a base

Polynucleotide chains

Covalently linked nucleotides

Deoxyribose vs ribose

The presence of the OH group on 2’ C of ribose makes the RNA chain less stable than the DNA chain.

What bonds connect the sugar-phosphate backbone of DNA?

phosphodiester bonds

When is the phosphate group of a nucleotide negatively charged?

At intracellular pH

DNA strands

AKA polynucleotide chains

How many hydrogen bonds form between A & T?

2

How many hydrogen bonds form between G & C?

3

Base pair

A purine-pyrimidine pair

What does the 3’ end carry?

the unlinked –OH group attached to the 3’ position on the sugar

What does the 5’ end carry?

a free phosphate group

Purine

2 ring base

Pyrimidine

single ring base

Chromosome painting

exposure to a collection of fluorescent dye-labeled DNA

Chromosomal translocation

when a segment of 1 chromosome has been swapped with a segment from the other

• a frequent event in cancer cells

What is a characteristic of budding yeast?

It has a high density of genes

noncoding DNA

sequences of DNA that do not encode protein

30 nm fiber

What are some functions of noncoding DNA?

The regulation of protein-coding sequences, scaffold attachment regions,origins of DNA replication, centromeres and telomeres.

Interphase

the cell is actively expressing its genes and the DNA is replicated

Mitosis

the division of the nucleus

M phase

chromosomes condense, gene expression largely ceases, and the mitotic spindle forms from microtubules and other proteins

What does the mitotic spindle do?

It captures condensed chromosomes and relocates them to each end of the cell

What does the nuclear envelope do?

It forms around each chromosome set and the cell divides to produce two daughter cells in the final step of M phase

DNA sequence elements

required for chromosome replication and later segregation in eukaryotes

S phase

DNA is replicated beginning at the origins and proceeding bidirectionally from them

Centromere

helps to hold the duplicated chromosomes together until they are ready to be separated

Telomeres

contain repeated nucleotide sequences that cap the end of the chromosome preventing it being mistaken as broken DNA

highly compact

A typical duplicated mitotic chromosome is ___?

distinct territories

Interphase chromosomes occupy their own ___ within the nucleus

nuclear pores

openings in the nuclear envelope that allow things to enter and exit the nucleus

the nuclear lamina

What is the nuclear envelope supported by?

What is the nuclear lamina?

a network of protein filaments that forms a thin layer underlying the inner nuclear membrane

Nucleolus

the most prominent structure in the interphase nucleus

How do chromatin appear in the nucleus?

as a diffuse speckled mass with especially dense chromosomal regions called heterochromatin (dark staining)

heterochromatin

gene-poor and are located mainly around the periphery immediately under the nuclear envelope

the genes for ribosomal RNA which are located on multiple chromosomes

What does the large dark region in the nucleolus contain?

All 4 histones that make up the nucleosome core are ___

relatively small proteins with a high proportion of positively charged amino acids (K and R)

Nucleosomes contain DNA wrapped around a ___

core of 8 histone molecules

What are the proteins on eukaryotic chromosomes?

Histones and Nonhistone chromosome proteins

Chromatin

Nuclear DNA + associated proteins (histone and non-histone proteins)

Nuclease

Can digest linker DNA which releases the nucleosome core particle from chromatin

What is tightly wound DNA immune to?

Degradation by nuclease

H1

acts as the structural "glue" for chromatin condensation

What are histones made of?

They are rich in the positively charged amino acids lysine (K) and arginine (R)

H1’s structure

consists of a globular region plus a pair of long tails at its C-terminal and N-terminal ends

10 nm fiber

DNA first wraps around core histone proteins to form individual nucleosomes

looped domains

folded chromatin in human chromosomes

What do non histone chromosomal proteins do?

They bind to specific DNA sequences which creates a clamp at the base of each looped domain

Difference between heterochromatin and euchromatin

heterochromatin is more condensed than euchromatin

What do chromatin-remodeling complexes do?

they reposition the DNA wrapped around nucleosomes

Chromatin remodeling complexes

protein machines that use the energy of ATP hydrolysis to change the position of the DNA wrapped around the nucleosome.

What groups can modify histones by covalently bonding to them?

• Acetyl group (A)

• Methyl group (M)

• Phosphate (P)

Where do most histone modifications occur?

on the N-terminal tails

DNA barrier sequence

prevents the spread of heterochromatin

Position effect

the activity of a gene depends on its position along a chromosome

How do genes relate to proteins?

Genes contain the DNA instructions for making functional products, often proteins.

How do we visualize chromosomes?

as a karyotype, where chromosomes are condensed, stained, and arranged into homologous pairs

How does heterochromatin affect gene expression?

It makes genes harder for transcription machinery to access, so gene expression decreases or is turned off.

How do Barr bodies relate to gene expression?

Because it is condensed into heterochromatin, most of the genes on the Barr body are not expressed